brake lights

[lindbergh]

Need some help. While working on my brake light boards today, I shorted out one the boards and the brake lights went out as the fuse did it's job and blew. What I don't understand is why when I replaced the fuse the brake lights still don't work. The board I was working on is totally disconnected and I would expect the brake light I didn't touch to illuminated once again. I apparently don't have power as there is no power at the brake light switch. Any suggestions, I'm stumped. Thanks.

[Rich]

First, be sure that you really have "no power at the brake light switch". A 12V test lamp probe connected to a known-good ground point will verify it.

From there you'll have to work upstream until you get 12V+. The next point upstream from the switch is the output side of the brake light fuse at the fuse box.

Check:

The replacement fuse is good (continuity-check it).
There's 12V+ on the feed side to that fuse (again, use a test lamp).
You are dealing with the correct fuse position/receptacle in the fuse array.

[David T]

First, be sure that you really have "no power at the brake light switch". A 12V test lamp probe connected to a known-good ground point will verify it.

From there you'll have to work upstream until you get 12V+. The next point upstream from the switch is the output side of the brake light fuse at the fuse box.

Check:

The replacement fuse is good (continuity-check it).
There's 12V+ on the feed side to that fuse (again, use a test lamp).
You are dealing with the correct fuse position/receptacle in the fuse array.


It is a very simple circuit. Did you replace fuse #18? That is ONLY for the brake lights. From there the power goes to the brake switch and then on to the lights. If you do not have a good ground you may think you have no power when, in fact, you do have power but no way for it to get to ground. Time to get a meter or a test light and see what you really have. Maybe the bulb in the other board is NG?

[lindbergh]

First, be sure that you really have "no power at the brake light switch". A 12V test lamp probe connected to a known-good ground point will verify it.

From there you'll have to work upstream until you get 12V+. The next point upstream from the switch is the output side of the brake light fuse at the fuse box.

Check:

The replacement fuse is good (continuity-check it).
There's 12V+ on the feed side to that fuse (again, use a test lamp).
You are dealing with the correct fuse position/receptacle in the fuse array.



Ok, I have no power at the brake light switch and no power at the fuse box for brake lamps only, all other fuses have power. What is upstream of the fuse box?

[rdarlington]

Ok, I have no power at the brake light switch and no power at the fuse box for brake lamps only, all other fuses have power. What is upstream of the fuse box?

Can you test the brake switch itself?

[Jonathan]

Ok, I have no power at the brake light switch and no power at the fuse box for brake lamps only, all other fuses have power. What is upstream of the fuse box?

1) you might have possibly used a bad fuse as your replacement.

2) you might have coincidentally pushed out the connector on the underside of the fuse box when you did your fuse replacement. Or something else got bumped or loosened while in there.

Is everything in your fuse and relay area a stock set-up, or do you have some retro 80s creativity in there?

[lindbergh]

The fuse is good. The wiring is original OEM. The only wiring I added was for the cruise control. The connectors are visible in the fuse box so I am guessing that means they didn't push out. I tested the voltage with the two leads on the brake switch disconnected to eliminate the switch from the mix. I started to lean toward that when the 10 amp fuse blew it took the fuse assembly with it. All the lights on the other tail assembly work except the brake lights. I guess the next step is to jumper 12 volts to the #18 fuse and see if they work.

[Bitsyncmaster]

Your fuse may be up in the harness by the brake pedal. Mine is up there but your vin is a little later than mine. Did your brake lights work without the key on?

[Jonathan]

Your fuse may be up in the harness by the brake pedal.

I don't know much about aftermarket cruise controls, but would the brake lights power wire been routed to some other spot or control box prior to the fuse relay area? Cruise controls need to know when you hit the brakes, so maybe there are extra fuses? Is this what you meant, Dave, by the harness by the brake pedal, or are you mentioning something factory that isn't to do with the cruise control? There seem to be random inline glass fuses in our cars here and there discovered when doing troubleshooting work...

[Rich]

Ok, I have no power at the brake light switch and no power at the fuse box for brake lamps only, all other fuses have power. What is upstream of the fuse box?

In case you still need this answer go to this link and scroll down to Tail Light Circuit, where the brake light wiring/colors/fuse are all shown:

http://dmctalk.org/showthread.php?40-All-Wiring-Schematics

Upstream of the hot side of Fuse 18 is main power feed. For lots of things besides brake lights, things that apparently are all OK in your car.

[Bitsyncmaster]

I don't know much about aftermarket cruise controls, but would the brake lights power wire been routed to some other spot or control box prior to the fuse relay area? Cruise controls need to know when you hit the brakes, so maybe there are extra fuses? Is this what you meant, Dave, by the harness by the brake pedal, or are you mentioning something factory that isn't to do with the cruise control? There seem to be random inline glass fuses in our cars here and there discovered when doing troubleshooting work...

The glass fuse for the brake lights in some cars is a factory difference. Don't know what VINs have this but I know many other cars are also wired that way.

[lindbergh]

Your fuse may be up in the harness by the brake pedal. Mine is up there but your vin is a little later than mine. Did your brake lights work without the key on?

I don't see another fuse up by the brake switch area. Are there 2 fuses for the brakes? Yes, the brake lights worked without the key on. The cruise control should be no factor. I had the brake lights on for over an hour, is it possible the wires melted? The 10 amp fuse blew immediately on the short. Fuse #18 was the one blown and is the only fuse I know about for the brake lights. How does the fuse box come out?

[Bitsyncmaster]

I don't see another fuse up by the brake switch area. Are there 2 fuses for the brakes? Yes, the brake lights worked without the key on. The cruise control should be no factor. I had the brake lights on for over an hour, is it possible the wires melted? The 10 amp fuse blew immediately on the short. Fuse #18 was the one blown and is the only fuse I know about for the brake lights. How does the fuse box come out?

I did the same thing you did and blew the fuse. But the fuse that blew was the glass one up at the brake pedal. I don't think I had any fuse in #18 of the fuse block.

[lindbergh]

I did the same thing you did and blew the fuse. But the fuse that blew was the glass one up at the brake pedal. I don't think I had any fuse in #18 of the fuse block.

I'll look again. It sounds reasonable since there is evidently an open circuit somewhere, but fuse #18 was blown, but maybe it was always blown, it was the first time I looked at it.

[David T]

Pull fuse #18 and check both sides for power. If you do not have power to one side of the # 18 fuse location in the fuseblock (with the fuse removed) the connector may have backed out or some other problem preventing you from getting power to #18. On some cars there is an inline fuseholder also ahead of the brake switch. Even if there is and you blew that fuse you should still have 12 volts on the #18 position. You could temporarily jumper power into the brake light circuit to prove out the rest of it. Use a fuse.

[DeLorean03]

Check the black bulkhead connector in the engine bay - back right of the engine bay underneath the coil cover. A pin on the black bulkhead connector is responsible for supplying power to your brake lights. If it backed out, your symptoms could happen - ask me how I know.

[Ron]

Check the black bulkhead connector in the engine bay - back right of the engine bay underneath the coil cover. A pin on the black bulkhead connector is responsible for supplying power to your brake lights. If it backed out, your symptoms could happen - ask me how I know.This wouldn't cause fuse #18 to be dead...



I guess the next step is to jumper 12 volts to the #18 fuse and see if they work.That's what I would try. (Assuming no glass fuse.)
If that doesn't make them work, see if you then have 12v at the brake switch.

FYI- Fuse 18 should have 12v at all times. (Brown wire going to a multiple branch.)

[DeLorean03]

Reading through this thread, I am just asking, OP: what known grounds are you utilizing to know that you have no power at Fuse #18's connectors?

I'm not trying to belittle you, but we aren't there to know that the procedure you're doing is accurate. Casey used to tell me to do things all the time when troubleshooting, and I thought I was doing things correctly and would come up with a wrong diagnosis. Then he'd come over and show me what I was doing wrong, and suddenly the real problem showed its ugly head; and we were off and running to fix the problem.

Like I said, not trying to question you, just making sure we know what you're doing to come up with things like "no power to Fuse #18" or "the brake switch is not the problem." I point this out because upstream of Fuse #18 is a massive index of brown wiring where pretty much everything in the car gets solid power - the other end of Fuse #18 goes to the brake lights.

Then again, you stated you had the brake lights on for over an hour. With the wiring in this car known for being sub-standard, maybe that was too much for the brake light power wire, and the wiring to Fuse #18 has lost its integrity and cannot function as it should.

[lindbergh]

Found it. Fuse 18 is a spare (apparently) in my DeLorean (I realize there are very few DeLoreans exactly the same). The fuse was as bitsyncmaster suggested an inline glass tube fuse under the dash. A little hard to find since it was up high and slightly back on left side of steering column (up and to the right of the inertia switch). If you are looking for it specifically it is easy to find but otherwise might be overlooked. Replaced the British Lucas fuse with a Buss 10 amp and all is well. Thanks everyone for your help.

Just FYI the all leads and power was checked using an ohm meter. Fuse #18 has no power to it and is not used , all other fuses have power as described in the shop manual although in my belief the amperage ratings are in most cases higher then what is necessary.

[DMC5180]

I suspected as much. Btw: there are no wires (terminals) at #18 location. My car has the inline configuration also.

[Bitsyncmaster]

I think if your car does use that fuse #18 it only works with the key on. So the schmatic is not correct in that it shows always powered.

[uhhair]

Okay so I have a question about this glass fuse (currently having trouble after installing a 3rd brake light, not getting any brake lights at all now.

The old glass fuse indicates it's a Lucas 12v, 25 amp fuse. What is the exact modern day glass replacement fuse I need? At the parts store, they had SFE, AGC, etc glass fuses. Not sure what I need at all to replace this. Thanks.

[Patrick C]

Okay so I have a question about this glass fuse (currently having trouble after installing a 3rd brake light, not getting any brake lights at all now.

The old glass fuse indicates it's a Lucas 12v, 25 amp fuse. What is the exact modern day glass replacement fuse I need? At the parts store, they had SFE, AGC, etc glass fuses. Not sure what I need at all to replace this. Thanks.

I spliced in a new blade fuse holder and use a 20 amp fuse. No problems with it in 7 years since I did this.

[Nicholas R]

Okay so I have a question about this glass fuse (currently having trouble after installing a 3rd brake light, not getting any brake lights at all now.

The old glass fuse indicates it's a Lucas 12v, 25 amp fuse. What is the exact modern day glass replacement fuse I need? At the parts store, they had SFE, AGC, etc glass fuses. Not sure what I need at all to replace this. Thanks.

The exact modern day glass fuse you need is a 12v 25amp fuse.

Do yourself a favor though and just do what Patrick did.

[Bitsyncmaster]

I think my "glass" fuse was 13 amps or some weird value like that. Most any 15 or 20 amp fuse that length would work. But I do like the idea of replacing with the blade type so I would have spares with me.

[dtavres]

I know this is an old thread, but wanted to add a bit...

Last week I noticed I had no brake lights. I just got the car and have driven it less then 6 hours, so I'm not sure if the brake lights ever worked since I got the car.

A few weeks ago I replaced the old circuit boards with the new ones from DMC (plus the LEDs), thinking the 'no brake lights' problem was due to the old boards - but that still didn't fix it. (Although, all the other tail lights work great now - no more smacking the lens to get it to come on!)

I read this thread and a couple others, and finally pulled the green leads off the switch near the brake pedal, and connected them with the 10 amp fuse I had pulled out of #12 (my door switch wasn't engaging when closed, so I killed the battery once.)

WITHOUT the key in the ignition, I got out of the car, and there they were... nice bright brake lights!

I pulled the 10 amp fuse off and now assume that the culprit is the switch at the brake pedal. (ARG!)

I hope this helps someone else when they read this thread years from now :)