Tracking Down An Instrument Panel Lighting Short-M38A1

[madmike]

Last night I finished trouble shooting why one of my front blackout lights was not working. I had painted the mounting bolt threads and it did not allow the ground to complete the circuit. Then I wanted to see if my panel lights worked. The high beam indicator was okay but with the headlights on, I turned the instrument lights on and immediately had sparks flying out from the panel. Then I heard the circuit breaker reset.

I have a new wiring harness and no frayed wires, and I am certain all the wires are hooked up as numbered. As I begin to trouble shoot this problem, I would like to know if I am looking at the light switch for a problem or if there is something else to check as well. The sparks came from the area of the temp gauge so I will look there first for evidence of a short. I want to make certain the gauge is not improperly connected.

Is the lighting switch serviceable? BTW, my panel is the later model type with the smaller, red shielded lights.

Mike

[Bretto]

Sounds like the wiring in the bulb socket is touching the metal housing and grounded out.

[jeeperjoe]

Check the end of the wires carefully. I found that the insulation on several of mine had frayed near it's end (and behind the contact) and allowed the bare wire to touch the metal shell of the Douglas connector. Then when it touched any ground, BANG!

[madmike]

Thanks to both of you. It's entirely possible there could be a frayed wire on the switch side because that part is old. I replaced insulators that were bad as I went along but that does not preclude the possibility that a wire is frayed.

I happened to think of one other possibility as well. The temp gauge wires could be on in reverse. That might cause a direct ground. I will be checking this tomorrow. I recall the manual was a little unclear on which gauges I have for wiring a couple of them. The panel is like for the M170 but I could have a gauge for an M38A1. Lots of possibilities I guess.

Took today off to go for a motorcycle ride.

[wesk]

Most of the light switces out there ar not serviceable. They were designed as throw aways. The very early (1950 thru 51) Bemdix switches were serviceable.

The way I read your post:

1-You have a brand new harness connected correctly.
2-When you turn power switch on no problem.
3-When you turn light switch on headlights and hi beam lite worked fine.
4-When you selected instrument lights the sparks occurred behind the instrument cluster. Then the CB popped inside the light switch.

If this is correct then you do not need to troubleshoot the light switch. You should unfasten the instrument cluster and support it away from the dash so you can observe it and inspect it. Look first for the burn marks left by the sparks. These will lead you to the issue fastest. If you cannot located any burn marks then thoroughly inspect the back of the cluster, all wires for cuts or any damage, the gauge connections for shorts. If you find nothing wrong then power up with the ignition switch. If no shorts then check each gauge for proper operation, if need be start the engine.Once you are satisfied the gauges work fine, turn the headlights on. If all is still fine select instrument lights. Remember you should still have the cluster separated from the dash and where you can see the wiring.

[madmike]

One thru four are correct. I pulled the panel and cannot find any evidence of burn marks and I have confirmed that the wiring is correct. It was the amp gauge that was different on the two models, not the temp. However, my temp gauge does not work yet. I put a new sending unit in and have a new gauge but I am reluctant to hook up the new one until I know if I have a problem there.

With just the panel lights on dim, I do not get lights on the dash. When I turn on the headlights, the high beam indicator works as it should. Still no panel lights though. When I switch to bright panel lights, the high beam light blinks briefly and then the CB trips. No sparks though with the panel disconnected. And no blinking of panel lights. It's as if they are not getting any voltage. I put new bulbs in the panel light sockets before this problem.

Should I check for voltage at one of the panel lights when it's on dim? If it has voltage I am not certain what that tells me since switching the lights to bright panel seems to be a problem. And I cannot check that voltage with the CB cutting out. Side Note: I still do not have a tail light because I now cannot get the screws out of the housing. I plan to remove the light housings and work on them on the bench after soaking the screws a bit. But I would not think any problem there would pop up just because I switch on bright panel lights.

[madmike]

A couple more points I left out. The same thing happens whether I am in the Service drive position or the Stop light position.

Also, with the ignition switch on, the Bat-Gen gauge goes to full discharge before the CB trips.

[wesk]

I have a new wiring harness and no frayed wires, and I am certain all the wires are hooked up as numbered.

If this is absolutely, 100% true then I would suspect the light switch. However this is not always as 100% as we would think or like it to be.

So start with a pinout check on the light switch plug to make sure the correct number wires are connected to the correct pins.



If the plug pins are correct then remove the light switch and do a circuit check on it.

http://www.willysmjeeps.com/v2/modules. ... _album.php

[madmike]

Okay. I found the short. There were two problems with the left panel light. The new wiring harness I got left a little bare wire that the Douglas connector insulator did not cover. I do not know for certain that it was grounding there but it was a potential problem. The larger problem was the insulator inside the socket. It is located right behind the bulb holder. After you unscrew the bulb holder it is inside the housing protecting the hot wire and the terminal connector from grounding. This is a rubber insulator, not the bakelite or hard shell one that slips into the housing.

That terminal connector must be totally insulated and the rubber insulator was not wrapped around the base as it should be. It was allowing a ground condition at the base. I would not have known it wrapped around and over the connector without checking the other panel light for comparison and I have no idea how it became unwrapped from the connector except maybe upon removal. It fits tightly in the housing and maybe that is how it wound up out of position.

I still do not know why it would only trip the CB on bright. Either way it was a ground but I ain't complainin' 'cause it's workin' now. I tracked the problem with an ohm meter and realized it was an unwanted ground condition.

I plan to keep a copy of the diagram Wes sent for future ref. since I still have other lighting issues and it looks like a helpful troubleshooting tool. It's good to know the switch is okay. I had no history of what worked or did not work since it was a non operable vehicle when I bought it. The help I got/get on this forum is invaluable and again I thank Wes and others for bailing me out. These ARE different, but interesting vehicles to work on.

Mike

[jeeperjoe]

Wes,

What's the story on the circuit breaker that's in the light switch? Does it resets after a fixed amount of time, or does it have to cool to a certain temperature or do you have to turn the switch off and then back on to reset it? My manuals contain NO mention of it that I can find.

[wesk]

Most of the manuals do not address the built in CB in the later light switches because the manuals were written when the Bendix light switches were in use and the M38 had the three CB's on the side of the battery box. They function by opening when they overheat and close once they cool off.

[madmike]

Mine would reset rather quickly. Within 10 seconds. You can hear an audible click both ways.