12 Circuit Wiring Harness Problems

[Jim Coleman]

Newly installed wiring harness is giving me problems.

Head lights work on high and low beam. Electric fan works on the radiator, electric water pump works, ventilation fans work.

Starter turns over; turn signal indicators and high beam indicator work. Instrument lights work when headlight switch is in park position.

Turn signals: all 4 lights flash on left and right from the turn signal lever.

Tail lights go out when headlights are turned on. Brake lights don't work.

I have a new CPP GM style tilt column without the key. Have a keyed ignition switch in the dash panel.

Thought it might be the turn signal switch, replaced it and it didn't make any difference.

Was told it was a ground problem, so I spent most of the day re-running the grounds, they now are all connected to the ground post on the battery. Each light has a ground that is attached to the main ground wire that goes to the ground post on the battery.

Replaced the flasher - it's flashing at a normal rate, but all 4 lights (including the license plate light) flash when the turn signal lever is put into position. I checked the wires that go to the column plug and they appear to be OK.

Any suggestions would be appreciated.

Thanks,

Jim

[Cruisin428]

Do you have the wiring harness schematic?

If it was me, i would look at the hazard flasher switch and circuit. It has to play nice with the turn signals. (Having no idea of the wiring of your harness... this is just very general ideas)
Do you have diodes between the hazards and turn signals? You need to know how the hazard and turn siganls wire together?
Disconect the turn signal switch, put 12v to one side of the switch going to the lights, do all 4 corners light up? Try the other side of the switch going to the lights, do all 4 corners light.
Disconect the hazard switch, do the same test ? When you get to the point that you can energize one side and the other independently, then you will need to work backward reataching swithches and the flasher to find where you are getting the issue.
Brake lights - pull the break switch, put 12v to the light side, do they light up? Do you have 12v at the break switch? You will know if its wireing from the switch to the lights or power going to the switch!
Licence light would need to attach "upstream" of the flashers but after the headlight switch? Or you are feeding 12v back somewhere on to that wire? Try to see if it lights up when doing the above tests, when the turn and hazards are disconected, i would think you should be able to control it with the headlight switch!
Tail lights work with the headlight swith on "running light" or the first setting? Then go out when it is pulled to turn on the head lights? Check the wireing at the headlight switch and the switch itself, they should stay energized on both settings but arnt! In fact, I might start at the headlight switch, also, with the above test you may need to disconect parts of the headlight switch also to isolate the issue.
This would be more precise with a wiring diagram, but in general disconect switches and the flasher untill you can control the lights with a 12v jumper wire. Then work backwards reattching untill you come back to the error you have or discover an error causing your problems!
Good luck!

[Jim Coleman]

https://static.speedwaymotors.com/pd...structions.pdf





This is the harness I used.

Thanks,

Jim

[HighPlainsDrifter]

Hi,
I had their 21 ckt harness, worked but the headlight switch was shorted, bought another GM one at the parts store, much better quality also.
Perry

[Jim Coleman]

Perry,

I first tried using a double circuit GM switch, but went to the Speedway single circuit switch because the instructions showed it in use.


Cruisin:

Checking the harness wires that go to the turn signal switch with 12V (both flashers pulled):

Blue RF Turn signal & indicator - light & indicator work

Lt Blue LF Turn signal & indicator - light and indicator work


Yellow Left rear Turn signal - all 4 and the license plate light lights up


Green Right rear Turn signal - all 4 and the license plate light lights up


Note: both of these will flash if the turn signal flasher is installed.


Not sure where to go from here...…


Thanks,

Jim

[spdbrake]

Jim,

Did you run a ground to each light or are are they daisy-chained (Series circuit)?

If they are you may have Christmas lights.

If so, if you remove 1-2 bulbs the remaining bulbs will be brighter, but most likely one bulb removed will break the circuit.

Also ensure you have the correct PN turn signal bulb and Brake/running lights bulbs and they are not swapped.

It almost seems like your rear lights, turn signals and license plate already have power to them and the turn signal switch is supplying a ground.

The only other thing I can think of right now is your definition of green,pink red, etc... may be off. You may want to start over or get input from another set of eyes. I find that is the most helpful as most of the time the answer or mistake on my part is staring me in the face. I just can't see it.

[RockBit]

Most wiring harnesses are set up for separate right and left turn signal indicator dash lights. Cobras usually only have one indicator dash light that serves for both right and left turn signals.

If you have both right and left hooked up to a single indicator light on the dash, all four corners will flash if either right or left is on. You have to put diodes in the lines to the single indicator light so that one side won’t power the other.

I have seen this before on Cobras.

Good luck.

[Jim Coleman]

Spdbrake,

Grounds are run individually to each light assembly. The wire colors listed are as per the wiring harness instructions. The bulbs are the correct ones.

Rockbit,

This harness has a left, right and high beam indicator lights which are working correctly.

The LF and RF turn signals are working individually without flashing all 4.


My plan is to take the harness apart and replace the yellow and green wires which I think are the problems.

Thanks,

Jim

[HighPlainsDrifter]

Hi,
also check and see if the column has a button on the right side opposite the turn signal lever, you push in or pull out that button to make the 4 way flashers work so they may be joined that way.
Perry

[Jim Coleman]

Spent the day on the harness, took the rear section completely out of the loom and checked for possible shorts. Found a trouble area between 2 wires and fixed it.

Now, all the turn signals work correctly, except when the headlights are on.

They work correctly with the park lights on. Anyone have any ideas as to what's causing this?

Brake lights still are now working. I haven't tried them with the headlights on or with turn signals.

The only thing left is the turn signals working with the headlights on. The turn signal indicators will work for awhile with the headlights on, then they quit working. The turn signals will work for awhile (very slowly).


Edit: I'm told that this is a classic ground problem -- I thought I had the ground problems solved. Will recheck and redo them tomorrow.


Thanks,

Jim

[Blas]

What type of bulbs are you using for turn signals?
Blas

[Jim Coleman]

Dual filament bulbs. Turn signals are on brightest side.

Thanks,

Jim

[Cruisin428]

Unplug the headlights and make sure you have the same issue with the turn signals, with the head light switch on running lights and then with head lights on (but no bulbs). See if they will work longer than before!

The install sheet helps a little, unfortunatly, it leaves out what the switches are actually doing, it makes sence, you would need to look up the actual wiring for the switches to start putting the whole "picture" together! Hopefully its simple and you find your ground problem!

[Jim Coleman]

I will pull the headlights (halogen) and see...

Here's what Autowire says:

My turn signals work, and my lights work, but when my lights are on, my turn signals will not work.

This is a classic bad ground problem.The problem is that the light sockets themselves are not grounded properly or at all. For anything electrical to work properly, you must have two (2) things, a 12-volt feed and a ground. The bulb usually picks up a ground through the case of the bulb, but in this instance it’s picking up an improper feedback ground through the opposite wire in the light socket itself. The reason the turn signals quit working when the lights are on, is that a 12-volt signal is now being sent down the opposite wire, which was acting as a ground before. Once the lights are turned off, everything will work fine again.

Solution: Be sure that whenever your lamp is grounded, you have a good known chassis ground. This usually involved scraping paint or rust from the mounting surface and, in extreme cases, running a separate ground wire to the lamp assembly itself.


I thought I had the ground problem solved. I ran wires to the negative battery post, and then ran wires to each light. Will have to recheck and see what happened.

[CHANMADD]

Originally Posted by RockBit

Most wiring harnesses are set up for separate right and left turn signal indicator dash lights. Cobras usually only have one indicator dash light that serves for both right and left turn signals.

If you have both right and left hooked up to a single indicator light on the dash, all four corners will flash if either right or left is on. You have to put diodes in the lines to the single indicator light so that one side won’t power the other.

I have seen this before on Cobras.

Good luck.

Not true...put the flasher in the power wire before the switch.

[CHANMADD]

You have to run a ground cable from the battery to the frame. Then you need to ground each circuit (light) as close to the light as possible ,to the frame. Ground the frame and then ground the circuits to the frame....cheeeeeez.

[Jim Coleman]

Today I ran even more grounds hoping to solve the problem that Autowire talked about.

I'm pretty sure the car is now completely grounded -- ran new wires from the ground bus wires to each headlight.

Didn't work.

Disconnected the headlight bulbs, and the turn signals worked with the headlight switch in the headlights position. Thought it might be the bulbs, swapped them for some old ones I had, the old bulbs worked on high and low beam but the turn signals still won't work with the headlights on using the other bulbs.

I am at a complete loss as what to do next, but will try replacing the headlight plugs, I have a new unused pair -- will try them tomorrow and will test run each ground wire on the plugs directly to the negative post on the battery which should eliminate any ground problems.


Thanks,

Jim

[Jim Coleman]

Replaced headlight plugs, made no difference.

Took the front end of the harness that has the headlight and turn signal wires out of the loom looking for cuts or abrasions that could cause a short --nothing found - all wires are OK.

The only thing left that I haven't done is the headlight switch. I have a GM switch and will remove the Speedway switch tomorrow and replace it with the GM switch.

Thanks,

Jim

[Jim Coleman]

Tried the GM headlight switch -- no difference. Went back to the Speedway switch.

Took the turn signal switch connectors apart, and the wires run to them -- didn't see anything wrong.

Had a idea this morning, went out and bought another turn signal switch and hooked it up without installing it in the column. The turn signals started working with the headlights on and the indicators plus everything else started working correctly.

Even though I had already replaced the turn signal switch once, I either ruined it or the switch was bad. In any case, the new replacement turn signal switch cured the problems.


Thanks,

Jim