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Brake lights on a 1937 Plymouth P4 Sedan


LarryA
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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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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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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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OK.  After disassembling the tail/brake light and cleaning everything thoroughly, it was functioning properly - at least for about 15 minutes.  That’s the point I created another problem. While mounting the headlight switch back into the dash, I bumped the hot wire, got a quick spark, and now no lights.  I checked the fuse behind the ammeter & it had continuity.  Continuity on each wire from the headlight switch rings out as it should, but when I connect the hot wire to say the tail lights, there is continuity on the taillight contact, the brake light contact, the brake light housing, etc.  To me, this is telling me I have a short circuit ahead of the headlight switch.  I am looking at the connections behind the ammeter as a starting point.  Is there an easy way to get.to this?  I used a mirror and a flashlight so I could see to get to the fuse.  As before any advice is appreciated.  

 

P.S.  I will be disconnecting the battery before re-installing any switches in the future. 

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Previous owners could have done almost anything to the car. There was not originally a fuse there in the tail light circuit but there could be one in there now.

 

Disconnect the battery. 

Hook up a continuity test lead to each end of the wire. Where it starts and where it ends. At the ammeter and the switch. Does it beep? Meaning continuity? Do similar with one test lead to ground. Check for grounds. 

 

Do you still have those small running lamps in the original headlight buckets? These are also shared with and wired into your rear running lamps. Could be grounding there. If Itecall, there is a bullet type connection down by the left frame in the harness beside the engine. Look for a black plastic type cylinder in the wire harness. Thats the old bullet connector in there. Trouble could be here too. Could unplug it there and eliminate that part of the circuit if headlamps have been updated to sealed beams. 

 

Best way to access wiring under the dash? Pull the front seat out. Lay some heavy blankets down over the seat mounting base. Lay on your back and enjoy the view. 

Ya gotta love old wiring. I do! 

Edited by keithb7 (see edit history)
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My ‘38 was nothing short of an electrical disaster. I’ve had a lot of fun fixing everything up. If you find the time and you can remove any distractions, electrical upgrades can be very rewarding. 

 

Keep up the good work. You’ll get it all sorted out. 

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  • 4 weeks later...

Just wanted to provide an update on my 37.  I ended up completely re-wiring my headlights, parking lights, taillights & brake lights.  Found all kinds of things which could have been contributing to my problems.  All my exterior lights are now working properly.  I just got the windshield wipers (vacuum motors) working today.  That  completes my inspection checklist so I can now get it titled & registered which I plan to do tomorrow.  I wanted to thank everyone for their advice, it was very helpful in resolving my electrical issues.  Thanks again.

 

Larry

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Good deal. Electrical has always been my weak point and most of that is caused by self induced frustration of not being able to see the problem easy. (I>E> leaky wheel cylinder)  But since I found the AACA and have been able to garner the help of so much experience from many many fine folks I find myself less frustrated and more confident in working on electrical and ignition issues. I am glad you worked the problem out. What a great feeling of success that must be !

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