Headlight ground wires

[Bitsyncmaster]

I measured the voltage drop from the negative battery terminal to the front frame ground (the radiator support bracket) with my cooling fans running. The drop was 0.0150 volts. Each of my fans were drawing 7.0 amps (engine not on just running battery voltage).

Frame resistance = 1.07 mohm. That's a "mill" not a "Meg" ohm.

So even with OEM fans drawing 40 amps and the headlights using the frame ground (20 amps) you still would only drop 0.064 volts on the frame ground to the fans and headlights.

So it was not voltage drop that made DMC decide to run ground wires for the headlights. Also to note, there is no reason to run a copper wire end to end of the frame unless your just using it as easy junction points.

[dmc6960]

Just had another thought, perhaps the wiring harness was primarily designed back when the car was going to have the ERM body/frame. In that situation there wouldn't have been a backbone frame to ground to up front.

Or we could accept that Lucas really didn't know what they were doing with much of it.

[Bitsyncmaster]

Just had another thought, perhaps the wiring harness was primarily designed back when the car was going to have the ERM body/frame. In that situation there wouldn't have been a backbone frame to ground to up front.

Or we could accept that Lucas really didn't know what they were doing with much of it.

That may be the case. They did use very large wire (I think 12 AWG, maybe 10 AWG) for each of the two grounds used for the headlights. That's not something Lucas would use for a couple 10 amp circuits (headlights).

[Bitsyncmaster]

Wish I never started this project.

I would only recommend it if you have the front fascia off. It's just to much work to get at the wiring. Anyway I have gone this far so the harness out of the car. I plan to make new side light wires very long and bring them up behind the grill. There is lots of room behind the grill.

[Ron]

Wish I never started this project.

Hang in there Bro!

[Bitsyncmaster]

Been making a little progress with what little free time I can find. I've got long wires on the side lights and a frame ground run up to the center grill area. I may make a little ground buss bar mounted on that stainless steel plate that runs across the front. I need to start my side blinker logic. I may just use my RPM relay boards.

[Bitsyncmaster]

Unwrapped the factory harness for the lights. All the grounds are junctioned to those two headlight ground wires going to the relay compartment ground. However the two pin connector for the trunk light (purple wire and black wire) is strange. That black wire went to the drivers side fender ground.

I had run a 10 AWG frame ground wire but I think I'm going to remove that and use a 6 AWG wire. The headlights with high beams on draw about 20 amps total and I don't want any voltage drops.

[Bitsyncmaster]

I was going to make my flasher logic with a micro (RPM relay boards) but after writing the simple logic in software, I drew up a simple truth table and all I need is an XOR gate, MOSFET and a resistor and maybe a few decoupling caps.

The quad XOR gate CD4030B is only 48 cents but my junk box had one. It can work up to 18 volts so this is the best way to do it.

I may hand draw up a schmatic if you electronics guys want to make the same.

[Spittybug]

I hate it when when someone leaves my XOR gate open and all the little MOSFETs escape into the neighborhood. Quad gates sound like they provide more protection.



I love Dave's zeal and his products are great for this community! I do not however understand half of what he says in some of these posts! Oh well...

[Bitsyncmaster]

XOR gate has two inputs and one output. When one input is one (above 6 volts) and the other input is zero (below 6 volts) then the output is one (12 volts). All other input logic will be an output of zero ( 0 volts). note: this is 4000 type logic running at 12 volts.

So with one input connected to the park light wire and the other connected to the turn signal wire you will get an output to drive the "FET" which drives the ground side of the side light. This will give you side light blinking with the turn signal of the park lights are off and blinking out of phase if the park lights are on.

So you need a +12 volt power wire going to the side lights and the ground side is switched by this circuit. That +12 volt power also powers this circuit. You would need two of these circuits but the "quad" gate has four of these XOR gates in one package.