M37B1 Headlight / electrical problem.

[qualityhardware]

Over the past few weeks, my 1961 M37B1 has been eating up headlights on the passenger side. At first I thought it was just bad 24v headlights, but after the third one died in a two week period, I checked the wires feeding the headlight. I am not getting a current through the bright light wire, but the regular light wire is pumping 48 volts into the lamp. Each dead lamp looks like a blown flashbulb. I've done nothing to the harness since I bought the truck four years ago.

I am wondering if this is a blown circuit breaker or if there's anything wrong with the main light switch.

Any ideas on the best way to diagnose this?

Thanks guys.

 

[Tuko]

I wonder if current is jumping due to bad insulation sending 48v downstream?

 

[Ferroequinologist]

Is the generator putting out the correct voltage?

 

[nattieleather]

The truck is 24V so if your getting 48V at the head lamp then somewhere in the wire harness the low beam wire (16 or 17 I forget which) and the high beam (17 or 18 I forget) have to be hooking up in series in order to get 48V. I would start tracing the harness wire to see where your getting the problem.

 

[qualityhardware]

Is the generator putting out the correct voltage?

Yes. Checked the generator and many other wires and everything else checks out at 24v.

 

[qualityhardware]

The truck is 24V so if your getting 48V at the head lamp then somewhere in the wire harness the low beam wire (16 or 17 I forget which) and the high beam (17 or 18 I forget) have to be hooking up in series in order to get 48V. I would start tracing the harness wire to see where your getting the problem.

Will recheck. Followed the wires from the headlight into the main light switch switch and didn't see anything obvious, but will try that again and look harder.

 

[RangerBob]

The truck is 24V so if your getting 48V at the head lamp then somewhere in the wire harness the low beam wire (16 or 17 I forget which) and the high beam (17 or 18 I forget) have to be hooking up in series in order to get 48V. I would start tracing the harness wire to see where your getting the problem.

You can't put two 24V lines from the same source in series to get 48V.

 

[RangerBob]

Yes. Checked the generator and many other wires and everything else checks out at 24v.

If you are getting 48V anywhere in the vehicle and have only two 12V batteries in series, the problem must be with the generator and/or regulator. Have you measured the voltages from the generator and regulator at idle and with some RPMs to test the regulation?

The only other thing I could even remotely conceive of with the symptoms as described is the batteries in series with the gen/reg rather than in parallel!! Then to see 24V and 48V at the same time, one line would have to be tapped off either the gen/reg-only side or batteries-only side for 24V and the other line across the combined series circuit of gen/reg and batteries for 48V. I think this configuration is highly unlikely, just the only thing I could think of to meet your description.

 

[foxtrk2]

is ther high voltage coming when running or just battery power if running it has to be spike from generator either from regulator going bad or gen itself spikeing past regulator no other source for that much spike in power if not running

 

[Tuko]

Why would it only blow out one bulb if the voltage is spiking?

 

[foxtrk2]

not sure on why only one possibly weakest link in system ergo bad ground there or some simple thing like that ive seen weirder things happen

 

[M543A2]

Check the grounds; headlight wire loom to body, body to frame. Grounds do really weird things when they are dicey. I had headlight trouble with an M37; checked and renewed grounds and the problem went away.
Regards Marti

 

[vtdeucedriver]

Check the ground for that side. On your fender shield panel you should have one douglas connector that is either rivited or screwed on to serve as a dead end ground. If you were getting a voltage spike, you would also see it on the Gen gage. Dont forget there is a headlight harness that connects to the main harness that clips on the fender shields, You can disconnect them there to check your voltage to the headlight. Also check your rubber gromets that shield your wires coming from the headlight bucket and going into the fender channel. You might have a chafing issue going and this is a comon locations when those rubbers are deteriorated.

 

[qualityhardware]

Check the ground for that side. On your fender shield panel you should have one douglas connector that is either rivited or screwed on to serve as a dead end ground. If you were getting a voltage spike, you would also see it on the Gen gage. Dont forget there is a headlight harness that connects to the main harness that clips on the fender shields, You can disconnect them there to check your voltage to the headlight. Also check your rubber gromets that shield your wires coming from the headlight bucket and going into the fender channel. You might have a chafing issue going and this is a comon locations when those rubbers are deteriorated.

I checked everything tonight and used a different ground than the official ground at wire 91. Wires 17 and 18 came in at 22.9 and slightly above for voltage when grounded directly to the frame. Am going to disassemble the ground wire and see what I can do to clean it up, as I think that is where the problem is.

I also checked voltage with just the batteries and also with the engine running at low and high RPM. Using the frame as the ground, everything still stayed at 22.9 volts.

Will advise results after checking the ground wire in detail with the engine and generator running.

Thanks for the suggestions fellas. They are helping very much.

 

[qualityhardware]

Closing the loop on this. I pulled the headlamp assembly and the bracket under the fender that sheltered the headlight harness. Yanked the headlight harness for the passenger side and wires 17 and 91 had completely rotted rubber in multiple places exposing bare wire that touched fenders. Getting info on new Douglas connectors, crimping tool, and Prestolite wiring to restore the headlight harness, but the rough cost/benefit analysis is leading to just buying a new overall harness.

In any case, all advice has helped, and as usual with SS, the more I learned about what I was dealing with, the more I refined my searches and found the answers I needed on the site. Thanks to all with your help.

 

[qualityhardware]

One more update. I cheated and bought some regular 14ga wire and 14ga waterproof connectors from Autozone. Rigged up a new headlight harness and the waterproof connectors worked perfectly with the Douglas metal shells and bakelite connectors. Some propane torch work made everything shrink wrap very nicely and the new harness is delivering a proper ground. Popped in a headlamp and ran the truck at different speeds. No power spikes and the lamp is now working perfectly.