LarryA

Brake lights on a 1937 Plymouth P4 Sedan

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I recently purchased a 1937 Plymouth.  The car is basically original and I have been trying to fix some of the 6 volt wiring issues.  Currently I have all of the exterior lights working properly with the exception of the stop/tail lights.  With the headlights and/or parking lights off, both brake lights work as they should.  With either the parking lights or the headlights on, both taillights work but if I step on the brake pedal, only the left light gets brighter.  I have replaced the bulbs and both work properly on the left side.  Anyone have any idea how to fix this?  Any help and advice is greatly appreciated.

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Check and clean every bulb base and socket with sand cloth. Same with every wire connection. 6 volt systems really need totally clean grounds to function correctly where a 12 volt system might overcome it.  Anything that is considered a ground clean it well and try again.

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Is the rear harness wiring to the rear of the car original? What about down in the engine bay? The original cloth wiring by now has become very weak and is beginning to break down and fall off.  There are a few points to mention here. Each bulb has 2 filaments and are wired independently. When you pull on your headlight switch, the wires from the switch run all the way back to tail lamp, carrying the 6V electricity to the 2 rear bulbs. There is also a wire to the license plate lamp that  is sliced into the same running lamps wire, as it too comes on at the same time as the tail lamps. Frayed cloth covering on any of these wires could be the problem, allowing for grounds to be created.

 

The brake wire is separate. There is a switch on the hydraulic brake line. When you press the brake pedal, it closes a switch and allows electron flow to the brighter filament in the bulb, the brake light. The entire wire harness should be closely inspected to look for  exposed wire and possible grounding.  This harness travels inside the right frame. Also the rear lamp assemblies should taken apart and inspected for wire trouble inside the housings. 

 

Being as only 1 light is acting up, I would start at the one problem tail lamp an inspect all connections there at the bulb. Follow the wires back toward the engine from there. Paying very close attention to any deteriorated cloth covering. 

I just built several new complete harnesses for my 1938 Plymouth. I drew this schematic detailing how the rear end is wired up. The colours only represent the wires that I chose to use on my new harness. Stock colors are different.

 

 

 

 

1938 Plymouth P6 Rear Frame Harness.jpg

Edited by keithb7 (see edit history)

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Here is the condition of the original wires that I found in my 1938 tail lamps. Very poor.

I started doing electrical repairs here and there, and the pile of replaced wires grew and grew.

 

 

 

 

Rear Lamp.jpeg

 

 

Replaced Wires.jpeg

Edited by keithb7 (see edit history)

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My basic trouble shooting with problems like this begins with checks for grounding.

Get a long wire with alligator clips on each end and find a good ground. (like at the battery)

Then test for voltage at the offending light supply using this ground source.

I have seen no grounding at sockets because there is no continuity thru the tail ight stand to a fender as well as a whole fender not grounded to the body, so this test ground eliminates all of those possibilities.

Often I will find light circuits searching for a ground thru another light circuit, This will really get a guy scratching his head.

90% of all electrical issues are groundless.

 

So, LarryA, if you do indeed come back to check out the advice here you will find that most in this group have probably had the same issues at one time or another.

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Thanks to everyone for the advice.  I will dig into this later this week and hopefully get it resolved.  

 

Keithb7:  Most of the wiring seems to be original but some has been replaced.  I was told the car was restored about 15 years ago.  The body, floors, paint, etc. are in very good condition but I’m not sure how far the previous owner(s) went with the restoration.  I guess I will find out as I continue to go through the car.

 

Again, thanks to everyone for their advice.  I’ll post an update once I spend some more time with my new toy.

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Just recently had my memory refreshed with an old hobby error.    As noted in diagram above, #1158 bulbs are required.   I had tail lamp issues.   Found that someone previous had jammed #1157 bulbs into the sockets getting marginal contact and ground.    1158s have parallel pins on the bulb.  1157s are staggered pins.  Just a thought.

 

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I found 1 rear lamp in my car that had the correct parallel pins. However it was a 12V lamp. 1156 if I recall. Watch out for those too.  Poor illumination is the result when a 12V lamp is used in a 6V system. 

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