LED Tail Lights: Why not just use LED bulbs?

[Redsquall]

Hey,
Curious - why are these boards needed over just putting LED bulbs in?
https://store.delorean.com/p-10649-tail-light-circuit-board-pair-led-bulbs-wflasher.aspx

[Patrick C]

Because the original boards are garbage and work intermittently at best.

[Peripatetic]

I've been putting in LEDs into the old board. Best as I can tell this is just conveniently assembled with all the parts instead of you having to track each individual bulb yourself. Plus, new board.

[Gregadeth]

I'm planning on changing my bulbs to LEDs this weekend. I only bought the bulbs, not the circuit boards. Don't think I need them since my current ones are working fine. What I will do is go through the boards and tighten up the connections on the bulb sockets as outlined in the link here:
http://www.dmcnews.com/Techsection/tailights.htm

[DMC-81]

Hi there,

If your old circuit boards are still in good shape (no corrosion) but experiencing an intermittent connection, you can try tightening the rivets that hold the bulb holders on, before buying new boards. Just use a pointed punch and tap lightly on the top while securing the back of the rivet on a flat, solid surface. Be careful as the boards are fragile.

That's what I did before installing my LED bulbs. It fixed a couple of off-and-on connections.

39652

39653

Edit: or the procedure outlined in Gregadeth's post above ^.

[Nicholas R]

You can make the old boards work, but its still a good idea to change them out. I'm still running the old boards, but have large mounds of solder at every rivet, helping with continuity. Years ago I had continuity problems, but so far the solder has fixed it for now. One of these days I'm going to change out the boards, it just has not been a priority yet. Other projects keep getting in the way.

[mr_maxime]

It's just a convenient kit for those who need to change boards and want LEDs. My boards were very corroded and I purchased the bigger LED kit that included this one to replace all of my exterior lights.

[bfloyd]

Same here..
I purchased the new "LED exterior Lighting Kit with new circuit boards" from DMCH and it was a breeze to put in. It did find I had a corroded socket on my left front side marker light, so I'll have to replace that soon. The existing incandescent bulb is solidly corroded in there. My existing circuit boards worked "most of the time", but I have been pulled over twice for not having operating tail lights. A simple walk to the back of the car and bang it with the palm of my hand got me out of tickets both times. It was more a matter of convenience for me to just replace everything at once. Still don't have reverse lights, but have found out that's not a circuit board issue since I don't have 12 volts coming back to the circuit board connector when the car is in reverse. Tried cleaning the bulk head connectors and still got nothing.

[DMCVegas]

Same here..
I purchased the new "LED exterior Lighting Kit with new circuit boards" from DMCH and it was a breeze to put in. It did find I had a corroded socket on my left front side marker light, so I'll have to replace that soon. The existing incandescent bulb is solidly corroded in there.

I'm not one to over react. In fact, I try calm things as much as possible. So please believe me when I tell you that replacing that socket needs to be a high-priority for you.

Corrosion on a connection creates resistance and leads to an additional load on a circuit. That means that you're wires are going to heat up. In my case I had the original headlight switch, and the entire thing just melted because of the corrosion inside of my left front marker light on the fender. Turns out that the rubber gasket had a crack in it, water was filling the lens during rain and carwashes, and that created the corrosion. Sure, the light bulb still worked, but the socket and the wires running through the switch were hot as all hell. I know this because when my headlight switch melted, I inserted a jumper wire to work the lights (daily driver), and that got hot enough to burn my fingertips. Once I ordered a replacement socket and spliced it in, the jumper in the headlight switch socket ran much cooler.

All systems in your car are important to maintain. At the top of that list are Fuel, Electrical, Hydraulic, and Safety. If you've got an electrical problem like that, don't delay. Get that fixed as soon as possible.

[dtavres]

I purchased the new "LED exterior Lighting Kit with new circuit boards" from DMCH and it was a breeze to put in.

I've had 10515 for 2 months now and just noticed that I had only a single tail light and NO brake lights, so I bought the same exterior kit from DMC.

Tonight, I took both tail lights out, but noticed that the screws are old and a few of them have corrosion on the threads. A couple weeks ago at a local car show, another D owner said he replaced all the screws with stainless screws.

Did you use the same screws? Did you replace to new? Did you use SS?

AND, any other tips for doing the replacements?

Thanks!
Dave

[Nicholas R]

Corrosion on a connection creates resistance and leads to an additional load on a circuit. That means that you're wires are going to heat up.

According to Ohm's law, this isn't quite accurate. V=IR ---> I = V/R, voltage is constant so as your resistance goes up, your current draw/load is going to go down. Corrosion usually leads to a lack of continuity in a circuit, not increased continuity. A higher resistance leads to lower current draw, not the opposite. Just like how bad, corroded grounds on our cars lead to circuits not working at all; because the resistance goes to infinity so the current goes to zero. If you're getting higher current draw and you're sure its a result of corrosion and nothing else, your corrosion must be causing circuits to bridge, creating new pathways for the current, causing shorts/high current/hot wires. In your case it sounds like your corrosion just bridged the bulb/socket terminals, creating a parallel path with the bulb with significantly lower resistance than the bulb, essentially creating a short circuit (significantly reducing the resistance in the circuit.) This reduction in resistance would significantly increase the current draw as you noted. As you said, this is why side marker gaskets and LEDs are important.

I've also had multiple incandescent bulbs simply overheat. I guess sometimes quality control at the bulb factory isn't that good and the resistance in the filament isn't quite what they say. Lower resistance in the filament leads to more current, higher temperatures, and melted housings. Another reason why LEDs are better. Better control and virtually no heat. :thumbup:

[Bitsyncmaster]

According to Ohm's law, this isn't quite accurate. V=IR ---> I = V/R, voltage is constant so as your resistance goes up, your current draw/load is going to go down. Corrosion usually leads to a lack of continuity in a circuit, not increased continuity. A higher resistance leads to lower current draw, not the opposite. Just like how bad, corroded grounds on our cars lead to circuits not working at all; because the resistance goes to infinity so the current goes to zero. If you're getting higher current draw and you're sure its a result of corrosion and nothing else, your corrosion must be causing circuits to bridge, creating new pathways for the current, causing shorts/high current/hot wires. In your case it sounds like your corrosion just bridged the bulb/socket terminals, creating a parallel path with the bulb with significantly lower resistance than the bulb, essentially creating a short circuit (significantly reducing the resistance in the circuit.) This reduction in resistance would significantly increase the current draw as you noted. As you said, this is why side marker gaskets and LEDs are important.

I've also had multiple incandescent bulbs simply overheat. I guess sometimes quality control at the bulb factory isn't that good and the resistance in the filament isn't quite what they say. Lower resistance in the filament leads to more current, higher temperatures, and melted housings. Another reason why LEDs are better. Better control and virtually no heat. :thumbup:

+1

The only exception (I can think of) is when your feeding a switching type regulator. With that circuit, lower voltage due to increased resistance will draw more current on the input to keep the regulator output the same. Some of Super brites LEDs use a switching regulator. Those LEDs will not dim correctly with PWM but otherwise may be more efficient.

[mr_maxime]

I replaced mine with the stainless screws. My only complaint is that because they aren't magnetic, it is easier to lose them.

It's a pretty easy swap. The only tip is that there are some plastic pegs that need to be filed in order for the new taillight board to slide in.

[bytes311]

I still have the old boards. They used to have grounding issues until I found screws that were small enough to fit through rivet holes on the tailboard. #2-56 screw/nut from Radio Shack or Fastenal.

[dtavres]

I replaced mine with the stainless screws. My only complaint is that because they aren't magnetic, it is easier to lose them.

It's a pretty easy swap. The only tip is that there are some plastic pegs that need to be filed in order for the new taillight board to slide in.
Thanks... I actually figured out the peg issue after several minutes... I ended up enlarging the holes on the boards, and they snapped right into place.

[Gregadeth]

I still have the old boards. They used to have grounding issues until I found screws that were small enough to fit through rivet holes on the tailboard. #2-56 screw/nut from Radio Shack or Fastenal.
I just did the tail light nut/screw fix over the weekend, after installing my LEDs. The #2-56 size is exactly what I used, but Radio Shack didn't have any so I got them from Home Depot. These were the smallest size Depot had but they're perfect for this. It's a pack of 10 screws and nuts (get one pack for each tail light since you need 20 total).

[Timebender]

You can make the old boards work, but its still a good idea to change them out. I'm still running the old boards, but have large mounds of solder at every rivet, helping with continuity. Years ago I had continuity problems, but so far the solder has fixed it for now. One of these days I'm going to change out the boards, it just has not been a priority yet. Other projects keep getting in the way.

Like Shanghai Disney...

[DMCVegas]

According to Ohm's law, this isn't quite accurate. V=IR ---> I = V/R, voltage is constant so as your resistance goes up, your current draw/load is going to go down. Corrosion usually leads to a lack of continuity in a circuit, not increased continuity. A higher resistance leads to lower current draw, not the opposite. Just like how bad, corroded grounds on our cars lead to circuits not working at all; because the resistance goes to infinity so the current goes to zero. If you're getting higher current draw and you're sure its a result of corrosion and nothing else, your corrosion must be causing circuits to bridge, creating new pathways for the current, causing shorts/high current/hot wires. In your case it sounds like your corrosion just bridged the bulb/socket terminals, creating a parallel path with the bulb with significantly lower resistance than the bulb, essentially creating a short circuit (significantly reducing the resistance in the circuit.) This reduction in resistance would significantly increase the current draw as you noted. As you said, this is why side marker gaskets and LEDs are important.

I've also had multiple incandescent bulbs simply overheat. I guess sometimes quality control at the bulb factory isn't that good and the resistance in the filament isn't quite what they say. Lower resistance in the filament leads to more current, higher temperatures, and melted housings. Another reason why LEDs are better. Better control and virtually no heat. :thumbup:

Indeed the connection was bridged with corrosion. It was not a dead short that popped the fuse, hence my incorrect belief that it was a bridged connection with corrosion creating resistance, and was generating extra heat from that resistance (like a bulb or a heating coil). But a parallel path would indeed explain it better.

I was wrong.

[Nicholas R]

Like Shanghai Disney...

That has indeed gotten in the way of a lot of things...

[ccurzio]

The very first upgrade I did to my car was to replace the taillight boards with Grady's set (they are NICE) and threw in some LED bulbs. They've worked flawlessly for as long as they've been in there.

[Michael]

The very first upgrade I did to my car was to replace the taillight boards with Grady's set (they are NICE) and threw in some LED bulbs. They've worked flawlessly for as long as they've been in there.

My Grady boards (incadesant bulbs) are 7 years old next month. Have worked flawlessly.

[PJ Grady Inc.]

Thank you both Chris and Mike for the feedback. With our boards there really is no need to run LED's as the lights are bright enough.

A lot of people want LED's however so we're considering an integral LED board design which would eliminate the sockets entirely and be a more cost efficient LED design. If anyone is interested in that concept please PM or email me with your thoughts.

Rob

[ccurzio]

Thank you both Chris and Mike for the feedback. With our boards there really is no need to run LED's as the lights are bright enough.

Sure thing. Your boards seriously are top-notch.

I didn't upgrade to LEDs for the brightness, though that's a nice benefit. It was more for the response time and bulb longevity.

[Jonathan]

My existing circuit boards worked "most of the time", but I have been pulled over twice for not having operating tail lights. A simple walk to the back of the car and bang it with the palm of my hand got me out of tickets both times.

My driver's side brake lights are doing this. Sometimes they are on and other times they are not. They are either on as a pair (the driver's side pair that is) or not at all (as opposed to one or the other of the driver's side brake lights working or not working). A light tap with my palm on the fascia gets them to come back on for me too. All my other lights seem fine, passenger brake lights, reversing lights, running lights and turn signal lights, on both sides.

What's the most likely area for the problem to be (from the experience of any of you)?

Since it is happening as a pair, I'd imagine the bulbs themselves aren't the real problem. I have original style boards and regular incandescents as well. I also have the Radio Shack tiny nuts and bolts fix on them, from early on when I got my car, probably back in 2008 or so. My connectors look ok as does the wiring harness (just had it all out for rear fascia work). It was doing this I believe last season too, prior to disassembling the rear fascia. I recall a time or two someone said the lights weren't on, so I'm guessing this is not a brand new issue.

Should I focus on the connector, or the nuts and bolts tightening, or somewhere else on the circuit? I have not added any solder, but could do so if necessary. I have a solder gun and a little bit of experience using it, but not much. Thanks.

[David T]

My driver's side brake lights are doing this. Sometimes they are on and other times they are not. They are either on as a pair (the driver's side pair that is) or not at all (as opposed to one or the other of the driver's side brake lights working or not working). A light tap with my palm on the fascia gets them to come back on for me too. All my other lights seem fine, passenger brake lights, reversing lights, running lights and turn signal lights, on both sides.

What's the most likely area for the problem to be (from the experience of any of you)?

Since it is happening as a pair, I'd imagine the bulbs themselves aren't the real problem. I have original style boards and regular incandescents as well. I also have the Radio Shack tiny nuts and bolts fix on them, from early on when I got my car, probably back in 2008 or so. My connectors look ok as does the wiring harness (just had it all out for rear fascia work). It was doing this I believe last season too, prior to disassembling the rear fascia. I recall a time or two someone said the lights weren't on, so I'm guessing this is not a brand new issue.

Should I focus on the connector, or the nuts and bolts tightening, or somewhere else on the circuit? I have not added any solder, but could do so if necessary. I have a solder gun and a little bit of experience using it, but not much. Thanks.

If both brake lights are not working I would suspect a failure point that both sides have in common. I would start with the brake light switch. It could be out of adjustment or bad or a bad connection. Check for power too. Once you are certain it is working reliably then move to the rear and see if you are getting power consistently when you step on the brake pedal.

[dn010]

I also upgraded to Rob Grady's boards long ago. After many, many years I was having issues with mine not wanting to work full time and it was mainly due to the connector on the board was corroded. It would intermittingly make contact with the plastic connector, especially made better after the "tap" on the housing. After using a small file on the plastic connector's flat pins, then 400 (or so) sand paper on the boards along with dielectric grease, my problem has never returned. You can also check if your bulbs/sockets have corrosion and clean any of that up while you're in there.

Other problems may be alleviated by cleaning the bulkhead connectors/pins, most of these connectors gave me issues with corrosion - one of the happiest days with my D was clipping those connectors off. :hihi2:



My driver's side brake lights are doing this. Sometimes they are on and other times they are not. They are either on as a pair (the driver's side pair that is) or not at all (as opposed to one or the other of the driver's side brake lights working or not working). A light tap with my palm on the fascia gets them to come back on for me too. All my other lights seem fine, passenger brake lights, reversing lights, running lights and turn signal lights, on both sides.

What's the most likely area for the problem to be (from the experience of any of you)?

Since it is happening as a pair, I'd imagine the bulbs themselves aren't the real problem. I have original style boards and regular incandescents as well. I also have the Radio Shack tiny nuts and bolts fix on them, from early on when I got my car, probably back in 2008 or so. My connectors look ok as does the wiring harness (just had it all out for rear fascia work). It was doing this I believe last season too, prior to disassembling the rear fascia. I recall a time or two someone said the lights weren't on, so I'm guessing this is not a brand new issue.

Should I focus on the connector, or the nuts and bolts tightening, or somewhere else on the circuit? I have not added any solder, but could do so if necessary. I have a solder gun and a little bit of experience using it, but not much. Thanks.

[Jonathan]

If both brake lights are not working I would suspect a failure point that both sides have in common. I would start with the brake light switch. It could be out of adjustment or bad or a bad connection. Check for power too. Once you are certain it is working reliably then move to the rear and see if you are getting power consistently when you step on the brake pedal.

Hey David, I thought that might get confusing. What I was trying to say by "pair" was that only the two bulbs on the driver's side are having problems, and not the two bulbs on the passenger side. You know what I mean? So the switch at the pedal should be ok, and everything back to where the two sides split somewhere in the harness. I'll have to look closer at the wiring diagram to see how far it goes independently.

[PJ Grady Inc.]

Hey David, I thought that might get confusing. What I was trying to say by "pair" was that only the two bulbs on the driver's side are having problems, and not the two bulbs on the passenger side. You know what I mean? So the switch at the pedal should be ok, and everything back to where the two sides split somewhere in the harness. I'll have to look closer at the wiring diagram to see how far it goes independently.

Sometimes it makes more sense to replace your boards with something better as there are two options on the market. Ours carries a lifetime warranty which would suggest we have confidence in the product.
Rob

[David T]

Hey David, I thought that might get confusing. What I was trying to say by "pair" was that only the two bulbs on the driver's side are having problems, and not the two bulbs on the passenger side. You know what I mean? So the switch at the pedal should be ok, and everything back to where the two sides split somewhere in the harness. I'll have to look closer at the wiring diagram to see how far it goes independently.

If the problem is confined to one side (one taillight board) then it could be the connector to the board, the board itself, or the ground to the board. The boards themselves are notorious for bad connections, the rivets and the foil get a little corrosion and the shells for the sockets and then you have dim/intermittent connections. The bulbs also can get a little corrosion or the filaments can be bad and make intermittent connections. LED's and Rob's boards are the way to go if you can afford them. Any repairs to the OEM boards are like Band-Aids. It can work but it isn't the best solution.

[Tamir A.]

Ditto on the PJ Grady boards. Replaced mine with his probably 15 years ago, worked flawlessly ever since, just running regular bulbs.

T.



Thank you both Chris and Mike for the feedback. With our boards there really is no need to run LED's as the lights are bright enough.

A lot of people want LED's however so we're considering an integral LED board design which would eliminate the sockets entirely and be a more cost efficient LED design. If anyone is interested in that concept please PM or email me with your thoughts.

Rob

[PJ Grady Inc.]

Ditto on the PJ Grady boards. Replaced mine with his probably 15 years ago, worked flawlessly ever since, just running regular bulbs.

T.

A rare post from "the head honcho"! You flatter me Tamir. Thank you!
Rob

[DMCMW Dave]

Ditto on the PJ Grady boards. Replaced mine with his probably 15 years ago, worked flawlessly ever since, just running regular bulbs.

T.

Not casting aspersions on Rob's boards, they are fine, but as much as your car gets driven in the rain (i.e. never) anything would have held up, even the crummy originals!

[jwrayth]

Only problem I'm aware of with the Grady boards or the DMCH boards is if you are based in the UK and need to meet the rear fog-light requirement for MOT then the original boards are simpler to modify. More details over on the Eurotec forums (http://www.deloreaneurotec.uk/viewtopic.php?f=18&t=5071).

Still possible to have the rear-fog functionality as I modified my Grady boards, but just a bit more work with drilling, soldering then making sure it's weather proofed again. Otherwise the Grady boards are doing me good!

[PJ Grady Inc.]

Only problem I'm aware of with the Grady boards or the DMCH boards is if you are based in the UK and need to meet the rear fog-light requirement for MOT then the original boards are simpler to modify. More details over on the Eurotec forums (http://www.deloreaneurotec.uk/viewtopic.php?f=18&t=5071).

Still possible to have the rear-fog functionality as I modified my Grady boards, but just a bit more work with drilling, soldering then making sure it's weather proofed again. Otherwise the Grady boards are doing me good!

If there is sufficient interest I could do a batch of boards to Euro spec's provided it's standardized between countries.
Rob

[PJ Grady Inc.]

Not casting aspersions on Rob's boards, they are fine, but as much as your car gets driven in the rain (i.e. never) anything would have held up, even the crummy originals!
The originals started failing the year they were made so I can't get my head around that. I was there to see it. By the time we came out with the improved boards in 1990 there was a huge pent up demand. We have sold well over one thousand boards since then with only about a one percent failure rate overall. In the meantime we've made three upgraded revisions since then as improvements in materials and design permit. The newest version is almost indestructible.
Rob

[Nicholas R]

Does anyone using the LEDs (regardless of which boards) have cruise control? I've got cruise control and haven't switched the brake lights over yet because the LED brake lights do not draw enough current to trip the "cruise cancel." I'm thinking of taking something like a low resistance ceramic resistor, and soldering it to the traces of my circuit boards (still running stock boards) to create a parallel circuit for the brake lights, so that the brake circuit will draw enough amperage to trip the cruise control. Anyone ever tried something like this?

I know that the 2 tail light boards (left and right) are in parallel with one another, but does anyone know if the 2 brake lights on each board are parallel or are they in series? I want to calculate the size of my resistor for each board, but without knowing if those bulbs are in series or parallel, I cant calculate what the original circuit resistance would have been. The tail light wiring diagram unfortunately does not detail the boards themselves.

[bdmiko]

I know that the 2 tail light boards (left and right) are in parallel with one another, but does anyone know if the 2 brake lights on each board are parallel or are they in series?

My boards are out right now so I took a quick look and the 2 brake lights on each board are in parallel. This makes sense because if one bulb burns out the other bulb will still work.

[Jonathan]

My boards are out right now so I took a quick look and the 2 brake lights on each board are in parallel. This makes sense because if one bulb burns out the other bulb will still work.

That's another good piece of troubleshooting info. If both bulbs intermittently go on and off, you can focus on portions of the circuit either before they split, or after they combine again (assuming they do, i.e. into the ground connection).

[Chris4099]

Does anyone using the LEDs (regardless of which boards) have cruise control? I've got cruise control and haven't switched the brake lights over yet because the LED brake lights do not draw enough current to trip the "cruise cancel." I'm thinking of taking something like a low resistance ceramic resistor, and soldering it to the traces of my circuit boards (still running stock boards) to create a parallel circuit for the brake lights, so that the brake circuit will draw enough amperage to trip the cruise control. Anyone ever tried something like this?

I ran into this when I installed cruise control (I already had LED brake lights). The diagnostic was not passing for the brake test at first. The fix I did was add a simple relay to the brake circuit (by the cruise control wiring). The relay wasn't wired to switch anything, just turn the coil on/off with the brake lights. The cruise controller sensed the relay coil and it then passed the diagnostic. It also made a small clicking sound (was mounted behind me) which was good to confirm the brake lights were working as well. After having driven for an unknown amount of time with a bad brake light switch, I liked the confirmation.

[refugeefromcalif]

Does anyone using the LEDs (regardless of which boards) have cruise control?

My D has cruise control and switching over to LED brake light bulbs has rendered it inoperable. With the cruise on, The brake light LEDs are also on Dimly. (Like the middle Running lights). But they do brighten fine when I use the brakes.
Switching cruise off everything works like stock. I do want to find an easy fix for this without going back to incandescent bulbs for the brake lights. The LEDs are bright and quick.

George

[Nicholas R]

Thanks for the help! So four 27 watt (max) incandescent bulbs in parallel on 13v circuit.

VI=W 27w/13v=2.07 amps per bulb.

V=IR so 13v/2.07a=6.28 ohms per bulb.

(4/R)^-1=Circuit resistance so (4/6.28Ω)^-1=1.57Ω of resistance total (with incandescent bulbs). This makes sense because at max draw, 13v/1.57Ω=8.2amps, and that circuit is fused at 10 amps.

---------------------------------------
With four 3 watt LED brake lights, 3w/13v=.23 amps per LED, 13v/.23a=56.5 ohms of resistance per LED.

(4/56.5Ω)^-1= 14.12Ω of resistance total (with LED bulbs) 13v/14.12Ω=.92amps total current draw (this is why we love LED's right)
---------------------------------------

Adding something like a 25Ω, 10 watt ceramic resistor parallel to each bulb circuit:

http://www.daytonaudio.com/media/catalog/product/cache/1/image/800x800/9df78eab33525d08d6e5fb8d27136e95/0/0/004-25_hr_0.jpg

((4/56.5Ω)+(4/25Ω))^-1 = 4.333Ω total circuit resistance. Still a little higher resistance than stock but not too much more, and no where near as high as with the LEDs, but also less current draw than the incandescent. (13v/4.333Ω=3 amps) I'm sure it would still provide plenty of current to trip the cruise control circuit, while still drawing less than half the amps that the stock circuit does. If it's not, you could work your way down to either a 20Ω, 15Ω, or even 10Ω, before you would reach the resistance and amperage of the stock circuit.


I am not an electrical engineer, so if anyone can check my math, or knows a reason why this wouldn't work, it would definitely be appreciated.

[Nicholas R]

I suppose if you only wanted to make one modification, you could go upstream before the circuit splits and install one 5 ohm resistor, parallel to the rest of the circuit (grounding one side). (4/56.5ohms+1/5ohms)^-1=3.7ohms total. Still in the ballpark. Only thing is that it would have to be something like a 40 watt resistor, so it would probably get pretty hot.

I still like the idea of 4 smaller resistors soldered to the circuit boards. Plus since I'm still using my stock ones, I don't mind defacing them, haha.

[Mario]

To be honest, I'd check your math but it's a bit much to follow. :P Why not just keep everything in terms of watts?

The four stock bulbs draw 27W total. The four LED bulbs draw 12W total. So you're looking to add 15 watts to this circuit.
We want to spread that out over 4 resistors, so 15W / 4 resistors = 3.75W drawn by one resistor.
You can combine Ohm's law and the wattage calculation to get R = V2 / P:
132 / 3.75 = 45.1 ohms ideal resistor value. The next highest standard value is 47 ohms.

P = V2 / R, so a 47 ohm resistor with 13V across it dissipates 3.6W. Four of them in parallel brings us up to 14.4W.

So, four 47 ohm, 10W resistors would bring you to just about the stock draw.

If you used 25 ohm resistors, the four resistors alone would draw 27W, bringing your total draw up to 39W.

[Nicholas R]

To be honest, I'd check your math but it's a bit much to follow. :P Why not just keep everything in terms of watts?

The four stock bulbs draw 27W total. The four LED bulbs draw 12W total. So you're looking to add 15 watts to this circuit.
We want to spread that out over 4 resistors, so 15W / 4 resistors = 3.75W drawn by one resistor.
You can combine Ohm's law and the wattage calculation to get R = V2 / P:
132 / 3.75 = 45.1 ohms ideal resistor value. The next highest standard value is 47 ohms.

P = V2 / R, so a 47 ohm resistor with 13V across it dissipates 3.6W. Four of them in parallel brings us up to 14.4W.

So, four 47 ohm, 10W resistors would bring you to just about the stock draw.

If you used 25 ohm resistors, the four resistors alone would draw 27W, bringing your total draw up to 39W.

The 4 stock bulbs draw 27W each, not total. Total wattage stock is 4 times that (around 100-108 watts). Thats based on a typical brake light bulb. My reading showed most are 27watts on the brake filament.

I think we did the same, I just calculated the total circuit resistance where you calculated the total circuit wattage. I agree though, staying in watts is cleaner. I didn't want to shoot for the full stock amperage; only about half. I'm assuming this would still be enough to trip the Cruise control. If not, I would just swap for lower resistance resistors.

Also one of the electrical engineers I work with recommended this type of aluminum housed resistor since its easier to mount, and better at dissipating heat. I think that's a good call.

http://www.arcolresistors.com/wp-content/uploads/2011/07/5034-19-arcol-330x220.jpg

[Nicholas R]

Still, dividing your resistance by 4 to account for 4 bulbs instead of 1, but then multiplying it by 2 to get half of the stock draw instead of the full draw, gives 47ohms/4x2=23.5ohms, which is roughly what I figured (25ohms). Which is good! :D

[Mario]

The 4 stock bulbs draw 27W each, not total. Total wattage stock is 4 times that (around 100-108 watts). Thats based on a typical brake light bulb. My reading showed most are 27watts on the brake filament.

Ah, I see. Looks like I slightly misread your post.

Yes, the aluminum-cased resistors are better, but ONLY if they're mounted to a heatsink! If they're left in open air, they usually perform worse than similarly-rated ceramic resistors.

If we're using 25 ohms, we're going to need to dissipate 6.8W in each resistor.
Here's a 25 ohm, 30W resistor in an aluminum housing (Vishay is a good company): Link (http://www.mouser.com/ProductDetail/Vishay-Dale/RE75G25R0C02/?qs=sGAEpiMZZMtbXrIkmrvidJsfrzGjJJtW9d6gnz%2fsSn8% 3d) (Mouser part # 71-RE75G25R0)
Here's a 25 ohm, 25W ceramic-cased resistor (the tolerance over temperature is poor, but it's fine for this case): Link (http://www.mouser.com/ProductDetail/Xicon/280-CR25-25-RC/?qs=sGAEpiMZZMtbXrIkmrvidGsGvnANYh8pjerVHUjO9LQ%3d ) (Mouser part # 280-CR25-25-RC)

If we look in the datasheet for the Vishay resistor, we can see that without mounting it to a heatsink, the rated power dissipation goes from 30W to 10W! And that's at 25C ambient temperature. At 70C ambient (could easily happen on a hot day in the enclosed taillight area), they are further derated to only 65% of 10W (6.5W), which is right at the limit of how much we need to dissipate.

The ceramic resistor is rated to 100% capacity (25W) when not mounted on a heatsink and at an ambient temperature of up to 70C.
Another graph in that datasheet tells us the case temperature will then be 90C plus the ambient temperature. On a hot day, that could be as high as 160C (320F)!
I don't know what the case temperature would be for the aluminum one, but I suspect it would be something similar, if not a bit higher.

Anyway, point is, 6.8W is a lot of power and needs to be handled properly if you don't want to melt anything. I'd recommend the aluminum-cased resistors properly mounted to an adequate heatsink, or the ceramic resistors mounted held in free air and away from anything they could melt.

According to Chris4099, adding just a single relay in his brake circuit was enough to trigger his cruise control. So maybe adding 27 additional watts is a bit overkill! I'd see if you can get it to trigger with a smaller load.

[SS Spoiler]

love it when these guys talk dirty.....

[Nicholas R]

Ah, I see. Looks like I slightly misread your post.

Yes, the aluminum-cased resistors are better, but ONLY if they're mounted to a heatsink! If they're left in open air, they usually perform worse than similarly-rated ceramic resistors.

If we're using 25 ohms, we're going to need to dissipate 6.8W in each resistor.
Here's a 25 ohm, 30W resistor in an aluminum housing (Vishay is a good company): Link (http://www.mouser.com/ProductDetail/Vishay-Dale/RE75G25R0C02/?qs=sGAEpiMZZMtbXrIkmrvidJsfrzGjJJtW9d6gnz%2fsSn8% 3d) (Mouser part # 71-RE75G25R0)
Here's a 25 ohm, 25W ceramic-cased resistor (the tolerance over temperature is poor, but it's fine for this case): Link (http://www.mouser.com/ProductDetail/Xicon/280-CR25-25-RC/?qs=sGAEpiMZZMtbXrIkmrvidGsGvnANYh8pjerVHUjO9LQ%3d ) (Mouser part # 280-CR25-25-RC)

If we look in the datasheet for the Vishay resistor, we can see that without mounting it to a heatsink, the rated power dissipation goes from 30W to 10W! And that's at 25C ambient temperature. At 70C ambient (could easily happen on a hot day in the enclosed taillight area), they are further derated to only 65% of 10W (6.5W), which is right at the limit of how much we need to dissipate.

The ceramic resistor is rated to 100% capacity (25W) when not mounted on a heatsink and at an ambient temperature of up to 70C.
Another graph in that datasheet tells us the case temperature will then be 90C plus the ambient temperature. On a hot day, that could be as high as 160C (320F)!
I don't know what the case temperature would be for the aluminum one, but I suspect it would be something similar, if not a bit higher.

Anyway, point is, 6.8W is a lot of power and needs to be handled properly if you don't want to melt anything. I'd recommend the aluminum-cased resistors properly mounted to an adequate heatsink, or the ceramic resistors mounted held in free air and away from anything they could melt.

According to Chris4099, adding just a single relay in his brake circuit was enough to trigger his cruise control. So maybe adding 27 additional watts is a bit overkill! I'd see if you can get it to trigger with a smaller load.

Awesome! This is great information!! I didn't realize the aluminum ones needed mounted to a heat sync. I think you're right about adding 27 extra watts. I wish there was a way to know the minimum current required to trigger the system. Still, ceramic resistors are so cheap that It may be worth it to just start with higher resistance resistors (starting maybe around 50 ohms) and work my way down until I find a resistance that works. I think I'll get some of these resistors on order so they're waiting for me when I get home.

[DL4567]

http://www.arcolresistors.com/wp-content/uploads/2011/07/5034-19-arcol-330x220.jpg

Eyyyy!! I sure know where I've seen a very similar gold resistor before.....

http://i14.photobucket.com/albums/a337/nick-a-tron/trap5.jpg

For some reason it always surprises me to find pieces of props used in real life, haha.
Alright, you can now carry on with your serious electrical discussion. :)

[ccurzio]

I looked at the trap, Ray.

[Seabee]

I'm surprised no one motioned electric grease for the connections. I cleaned the connections and added the electric grease to the bulb connections over 6 years ago and still no problems to date, that's more than I can say for my 2012 Buick enclave. I use the red spray on my battery terminals with great success also.