Turn Signal Don't Work with Headlights On

[CarZombie 25]

My turn signals work with the headlight switch off or in the parking light position, but if I turn the headlights on, the turn signals stop working (both left and right).  I actually saw this happen.  The headlights were on and I was testing the turn signals.  As I watched, the turn signal slowed down and then stopped.  They were working ok immediately before this.  

I did a Continuity Test on the headlight switch and it seems to be functioning correctly.  

 

Searching  the past electrical problem threads tells me someone is going to suggest checking the grounds, if so please tell me where to find the ground points, and which ground points are the most important, if there is such a thing. 

 

Thanks

[CarZombie 25]

Sorry, forgot to say the Car is a1970 convertible and both the front/back, left/right turn signals are affect (or is it effected) by this.     

[Mach1 Driver 560]

When you say they stop working do you mean they stop flashing but stay on? Put a voltmeter across the battery terminals when this is happening and tell us what you get- with both the engine running and not running. With the engine running you should get at 13-14v, ideally 13.8-14.2v. The battery should give you 12.6v before you put a load on it. It would also help to know if you have non-stock high amp headlights, if so you may need a higher amp alternator.

As far as the grounds go, I remember my first car had dim tail lights- all I had to do is take the entire cast housing out of the sheet metal, clean off the rust and reinstall them. Be sure to check the battery cables- at both ends. I've seen cables with broken wire strands under the insulation that would test ok with and ohmmeter but since their wire size was effectively smaller than normal they couldn't handle a load.

The flasher is current driven and some flashers work better than others in a low amperage condition (as the voltage goes down the amps go down with it). I'm sure someone on here could tell you which are best.

I'm sure others will have brighter ideas.

[LoneWolf2U 136]

First place to start is by installing a chassis ground wire at the battery, A simple connection that will cure most problems.Most chassis grounds are removed or of poor condition.

Next place to look is at or near the items that are dim on none lit, poor connections cause problems.

[CarZombie 25]

Thanks Mach1 D and R: The car has the stock electrical system.  No LEDs, no high power headlights.    I checked the battery and it was at 11.5 with the car off.   I know that is low, for fun I checked at the fuse box and it was only showing 8.5.   I hooked up my battery charger and switched it to the jump mod, 50 AMP, and I the turn signal worked with the headlights on.   So I have the battery charging (the car had been sitting for a while).  I also pulled the battery and cleaned the terminals and cables.  I found the ground from the battery.  I is grounded on the engine, with a cable running from the engine to the chassis.  I replaced the cable grounding the chassis and cleaned each ground contact point. (the engine and chassis).    I will see if this fixed anything tomorrow.  

 

Does anyone know where the ground points are for the headlights and the front running lights?  The wiring diagram shows ground points for these, but not there physical location on the car.  I don't know how to locate them with in the wiring harness.  I would like to check and clean these points also.  

 

Thanks again. 

[LoneWolf2U 136]

Check near the lamp to harness connection points, They are usually near each one or have one exiting from the loom nearby.

[CarZombie 25]

Again thanks to all who replied to this thread.  I am 95% sure the problem is fixed.   As I said in my 30 April post, I charged the battery and then cleaned the battery terminals and grounds associated with the battery. Using the wiring diagram and an old wiring harness I had from a parts car (1970 Mustang Fastback).  I found three under hood grounds and cleaned them.  The first is a ground by the passenger side horn.  My parts car wiring harness clearly showed this to be the original factory ground for the headlights, running lights, and turn signals.   I had added a second ground, by the driver side horn, years ago, and I cleaned this also.   The third under hood ground is located just to the right of the driver side engine compartment brace at the top of the firewall.   I am  not sure if this is an original ground , as it may have been added when I switch to electronic ignition.  I don't believe it is part of the lighting system, but what the heck I was there and it was there, so it got cleaned.  The final ground for the lighting system is located in the trunk and is attached to the trunk brace surrounding the  gas tank fill neck.  

 

Here is where the 95% part comes in.  While testing the lights, everything was working correctly, but the driver side front running light, the one under the headlight, stop working.  To make a long story short, after testing, complicated by having a bad bulb, I determined the running light had gone bad.  I could switch the running lights from side to side and the bad one would not work on the passenger side, but the good one would work on the driving side.  I have ordered  a new front running light.  When I install it and everything works, then I will call it done.  

 

So, it appears the problem was a low battery, possibly dirty grounds, and a possible  intermediate problem with the driver side running light which went totally bad at the end.   This is why I HATE electrical problems.  Once again I have learned to always check simple stuff first.   Now I know to check the battery charge before diagnosing a problem.   If anything changes when I install the new running light, I will let you'll know, but for now I again thank you for the help.  

[CarZombie 25]

Post scrip to this post:   I ordered a driver side front running light from Mustangs Unlimited.  They call it the driver side parking light.  Prior to attaching it to the car, I  tested it by plugging it in and grounding it.  It did not work correctly with the headlights on.  Without headlights, turn signal worked, but turn on the headlights or running lights and no turn signal.   Turns our the bulb shipped with the parking light was bad.  I  replaced it with another bulb and all lights are working correctly.   Luckily I remembered how the bad bulb screwed with me when I was determining the original unit was bad.   I HATE ELECTRICAL PROBLEMS, but it is soooo nice to be driving my Mustang again.