Ground Fault/Loose Ground Wire on 1939 Cadillac LaSalle Coupe

[Payneangelfire]

I have been told by members of my local Cadillac LaSalle Club that I have a loose ground wire that is causing the following symptoms:  1) Headlights going dim three seconds after turning them on
2)  Headlights flickering at a very low light output  3) When headlights turned on the instrument lights and key light go dim, flicker, and go out  3)  Taillights also flicker when the headlights are turned on.
4)  The battery charge gauge gyrates quickly between D (discharging) and C (charging). 

When the headlights are not on, the instrument lights work and the taillights work fine.  The battery charge gauge also stays dead center in the gauge and does not gyrate.

I read in the owners manual that when there is a ground fault that the Thermostat Relay will cause the headlights to flicker.

I spent the last several hours and was able to eliminate several things.  The headlight grounds are not the problem.  I removed the bulbs from the headlights, turned on the lights, and the instrument lights and taillights still flickered.  I removed the floor switch, cleaned it, traced the ground wire and made sure it was well secured, then replaced the floor switch. The lights still flickered.  Finally, I pulled the battery and made sure the ground wire to the frame was clean and well secured.  Lights still flickered.

Finally, I crawled into the floor board and checked all the wiring that I could reach under the dash to make sure all connections were secure.

The headlights still flicker.

Does anyone have any suggestions?

John P.  #31819

[bcroe]

Welcome to the joys of restoring ancient car wiring.  After going
through the USUAL SUSPECTS, you may need to resort to using
your test instruments, to lead you to the fault. 

The most direct test might be, connect a volt meter (old analog
is best) between the battery hot terminal and the headlight hot
terminal.  It should be around 1/10 volt or less, and steady.  If
it is jumping around, or too high, move one lead toward the other
along the path till jumping or excess voltage stops, the fault is there. 

Hot lead good, connect that meter between the battery ground and
the headlight ground.  Readings should be similar. 

One method is with a (6 volt) test light, pretty long leads.   I would
start connecting it directly to the headlight terminals and see if it
flickers in unison with them.  If yes, move the hot test wire from
lights to the battery hot terminal (neg for yours).  If the test flicker
stops, the issue may be in the hot circuit between the battery and
the headlights.  In that case walk that wire down that path, till you
pass the point where the flicker comes back, the fault is there. 

The hot circuit is OK?  Put the hot test wire on the headlight, and
move the GROUND test wire from headlight to the battery ground. 
Look for that change from flicker to no flicker.  If you get flicker
even connected to the battery, you might have a bad battery or
charging system. 
Bruce Roe

[J. Gomez]

John,

The symptoms you are describing are a “tell-tell sign” the charging system is constantly flipping on and off either something with the gen/VR or a weak/low battery which can’t keep up with the extra power load.

Your step 4 with the ammeter fluctuating between C and D is such an indication of such possible issues with the charging and/or battery.

Also since you have indicated;

When the headlights are not on, the instrument lights work and the taillights work fine.  The battery charge gauge also stays dead center in the gauge and does not gyrate.

There should be some indication on the ammeter with the engine running of some slight charging or discharging reading somehow.   

A defective ground would have a different affect on either that side of the path or any other path up or down from the source.

If you have checked all your grounds straps are properly set metal to metal, no corrosion dirt or paint that could isolate them your next step is to check the charging system and verify the battery is in good shape.

Good luck..!

[bcroe]

Agree here, best to start by stabilizing the charge system.  Bruce Roe

[Classic]

I may be way off here, but my '37 has a (primitive) circuit breaker mounted on the back end of the headlight switch.  If yours has the same breaker, it could be tripping on and off rapidly.  If your stick your head under the dash while the headlights are on, you might be able to see it sparking.  If so, it will be very hot - Don't Touch.  Fix is the remove the breaker and clean it and readjust it (it's adjustable).  Alternatively, bypass it and install a modern 30A blade type breaker.  If it still trips, you have a short somewhere.

Just a thought.

Gene

[Payneangelfire]

Thanks Bruce, J. Gomez and Gene.  All with good info.  I'll apply the recommendations and see if I can find the problem.
John P. #31819