no running or brake lights, completely baffled

no running or brake lights, completely baffled - 07/31/11 04:33 AM

I am completely baffled by the latest electrical problem on our 70 RR. Went to the local track but rained out (it is the wet time of year here). Went to leave, turn on lights. Nothing but headlights. Drive home anyway, what else to do. Decided to do some trouble shooting. There are two hot leads to the headlight switch. One is 16 ga, the other is 18 ga (both seem light to me but that is a different discussion). There is 12 volts at each. The 16 ga I think drives the headlights, the 18 ga drives everything else. When I jump from the 16 ga terminal in the connector to the taillight terminal in the connector, the running lights all work. When I jump from the 18 ga one that is supposed to drive the running lights (it comes directly from the fuse block terminal marked stop/tail) nothing. Even the 12 volts go away when it is jumped together. When I use the 16 ga wire to jump I still get 12 volts at the 16 ga wire and the lights work.

To make it more frustrating, the brake lights also do not work. Even if I pull the wires off the brake light switch and jumper them together, no brake lights. There is 12 volts at the wire that is supposed to have it. The turn signals work fine, so I do not think it is the turn signal switch but ???.

I have removed and reinstalled the fuses in the fuse block, so I do not think it is corrosion there but maybe?? I am getting 12 volts but perhaps under load the resistance is too high due to corrosion? We are ground zero for corrosion here.

We have three British cars, all older than the 70 RR. Their electrical systems are 1/8 as complicated (the entire car fits on one page)and work better. This is not the first time I have been tempted to gut the entire electrical system and streamline it with Lucas fittings I can trust.

Posted By: A12

Re: no running or brake lights, completely baffled - 07/31/11 04:56 AM

Do the 4-way flashers work? Have you checked the flasher relays (turn and 4-way) to see if you have power through them?

Oh and you're sure that the taillight bulbs are not blown.....had that just happen to me as I was doing some rewiring and must have shorted out the bulbs working on something else

Posted By: David_in_St_Croi

Re: no running or brake lights, completely baffled - 07/31/11 05:07 AM

Both flasher relays were replaced recently. The turn signals work fine, I turned them on and walked to the back of the car to confirm. When I jump from the headlight power source to the terminal for the running lights all the lights work front and rear. It was actually easy for awhile as the power went out so I could see the glow from inside the car when I hooked up the headlight power source. Still walked to the back of the car to confirm.

Thanks for the help and reminder to check the simple stuff. It is something more than bulbs as nothing works, no dash, no front lights, no rear lights when I jumper from the source from the fuse box.

Posted By: macmic87

Re: no running or brake lights, completely baffled - 07/31/11 11:57 AM

just my guess, but it sounds like there is high resistance that has developed in the circuit with the 18 gauge wire. with the high resistance with no load (just checking for voltage) you will show 12 volts. as soon as the load is applied (connecting to running lights) the voltage wil drop and lights won't work. using the wiring diagram start checking the connections from the batttery back. maybe even the simple act of disconnecting and reconnectiong a connection will clean it enough to make it work better. even remove fuse and check condition of connectors.

Posted By: dOoC

Re: no running or brake lights, completely baffled - 07/31/11 12:03 PM

How about the license plate lite ?

Posted By: HYPER8oSoNic

Re: no running or brake lights, completely baffled - 07/31/11 01:53 PM

You say you are at ground zero for corrosion, check your grounds on both the brake and running lights circuits. It's also a possibility that the
18 gauge wire has gone brittle.

Posted By: David_in_St_Croi

Re: no running or brake lights, completely baffled - 07/31/11 02:43 PM

Thanks for all the help so far.

Grounds are good I believe, I have a separate ground wire for the taillight assemblies that fixed a problem a couple years ago.

When I hook up to the wire that feeds the headlight source the lights all work, which is why I think the grounds are okay in this case.

I agree with a couple of posters, I will have to start cleaning contacts, probably start with the fuses, although I have removed and reinstalled them which will frequently improve the contact enough for a circuit to work.

This portion of the harness is all original so as mentioned maybe the feed wire itself is bad and will show 12 volts with a multimeter but as soon as a load is on it the resistance is too high (this could also apply to a connection of course).

It really is amazing the gauge of wire the factories used when these were new.

I agree grounds (or the lack of) can cause some baffling problems so I will explore that but I think in this case it must be on the feed (hot) side but ....

Posted By: mopar_man

Re: no running or brake lights, completely baffled - 07/31/11 05:47 PM

If you have signal lights then a lot of the wiring checks out ok ,its not your brake light bulbs nor your wiring going back there.
you say you have 12v but when you add load it goes away ? Thats very possible , amperage and voltage is inverseally proportional to each other. I suspect you have a loose connection somewhere in the lighting harness. There may be 12v there until you turn on the lights or apply the brakes and then it opens. Check the back of the fuse box for a loose or corroded wire .

Posted By: HYPER8oSoNic

Re: no running or brake lights, completely baffled - 07/31/11 11:59 PM

Quote:

Thanks for all the help so far.

Grounds are good I believe, I have a separate ground wire for the taillight assemblies that fixed a problem a couple years ago.

When I hook up to the wire that feeds the headlight source the lights all work, which is why I think the grounds are okay in this case.

I agree with a couple of posters, I will have to start cleaning contacts, probably start with the fuses, although I have removed and reinstalled them which will frequently improve the contact enough for a circuit to work.

This portion of the harness is all original so as mentioned maybe the feed wire itself is bad and will show 12 volts with a multimeter but as soon as a load is on it the resistance is too high (this could also apply to a connection of course).

It really is amazing the gauge of wire the factories used when these were new.

I agree grounds (or the lack of) can cause some baffling problems so I will explore that but I think in this case it must be on the feed (hot) side but ....

Could be a combination of both, electrical circuits ARE tricky. However the feed wire may have an open spot/bad connection or "fracture" due to age/corrosion.

Posted By: David_in_St_Croi

Re: no running or brake lights, completely baffled - 08/01/11 01:06 AM

Thanks everyone,

Got it sorted. The terminal for the feed wire for the circuits was bad. I went to pull the terminal off on the hot side of the fuse block and the terminal came apart. It was a female spade terminal with the wire coming in at 90 degrees. I was able to trim enough wire back to use a standard female spade connector to replace the bad one.

Regardless, I think it is time to go through the electrical system and give it a thorough inspection/renew. I have already rebuilt the harness to the rear and to the engine. Really need to redo the engine one as the car has a MSD 6AL so there are a bunch of redundant wires I can eliminate. I have to believe the under-dash harness can be greatly simplified as I run no accessories on this car. Going up on wire gauge is not a bad thing either.

Anyway, the problem seems to have been high resistance in the supply wire for those circuits to the fuse box due to corrosion/deterioration in the removable female spade terminal.

Best regards, Dave

Posted By: MoparforLife

Re: no running or brake lights, completely baffled - 08/01/11 01:18 AM

You have got to remember that back in those days the front 'park' lights did not light in front on any other light switch position than park. They did not light with the headlamps when the headlamps were turned on. So don't expect them to. They turn signals are on a separate element than the park lights in front or in the rear the brake/turn signals operate on the same element but the tail lights use a separate element.

Posted By: David_in_St_Croi

Re: no running or brake lights, completely baffled - 08/01/11 01:40 AM

In 1970 the front lights work when the headlights are on, as far as I can tell. Just checked.

Correct about the turn/brake elements layout.

Now, whether the headlights work or not seems to be at the whim of current replacement dip switches quality......

Posted By: macmic87

Re: no running or brake lights, completely baffled - 08/01/11 02:03 AM

Quote:

just my guess, but it sounds like there is high resistance that has developed in the circuit with the 18 gauge wire. with the high resistance with no load (just checking for voltage) you will show 12 volts. as soon as the load is applied (connecting to running lights) the voltage wil drop and lights won't work. using the wiring diagram start checking the connections from the batttery back. maybe even the simple act of disconnecting and reconnectiong a connection will clean it enough to make it work better. even remove fuse and check condition of connectors.

Quote:

Anyway, the problem seems to have been high resistance in the supply wire for those circuits to the fuse box due to corrosion/deterioration in the removable female spade terminal.

holy cow i got one right!!

Posted By: MoparforLife

Re: no running or brake lights, completely baffled - 08/01/11 02:59 AM

I sure don't remember the park lights being on with the headlights as early as 1970. Could be but i sure didn't think so. I was thinking that it was quite a bit later than that.

Posted By: A12

Re: no running or brake lights, completely baffled - 08/01/11 05:14 AM

Quote:

I sure don't remember the park lights being on with the headlights as early as 1970. Could be but i sure didn't think so. I was thinking that it was quite a bit later than that.

'68 B-body or at least a '68 road runner did not run the front parking/running lights when the headlights were turned on but the '69's did. The only thing that I found when I was trying to figure out if I had a '68 or '69 underdash wiring harness was one yellow wire on the '68 that shut off the front parking lights and I think that's because the '68's had four (4) side marker lights that may have (by engineering standards???) been determined to be too much draw with the headlights or high beams on?????? "69 models had four (4) reflectors and did away with the four side marker lights.

Posted By: A12

Re: no running or brake lights, completely baffled - 08/01/11 05:19 AM

https://board.moparts.org/ubbthreads/show...rue#Post6727449

Did the '70 models have side marker lights and reflectors?

Posted By: buildanother

Re: no running or brake lights, completely baffled - 08/01/11 12:38 PM

68-side lights only
69-side reflectors only
70-side lights and reflectors