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Head lights not working

weirdman

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Knoxville, Tennessee
The head lights on my m35a2 have not been working. Usually its the other way around with the head lights working but the signals arent working. Does anyone know if this is a wiring issue or just bad headlights. Thanks!
 
Last edited:

ender891

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He is definitely right, although a lot of people arent that skilled with those. Probably a
I greatly appreciate any responses to this thread. The young man who posted this is a 14 year old working on a deuce for the first time. He and I have been a great follower of this forum for years. His brothers have restored a few m35’s and he is working through this one with very little assistance, which is what he asked to do.

All of your experience is wonderful.

thank you all
 

NDT

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Thanks for the back story ender. Lots of times people say "how could BOTH headlights be burned out at the same time???" Well trust me the 30 year old bulbs seem to vent out and then they burn out immediately when you power them, I have seen it happen in front of my eyes more than once. Easy trick is to take the bulb to the battery and hold the wires against the terminals. 12 volts will be enough to cause the bulb to glow or not.
 

kendelrio

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It's also possible there's a short to ground which could cause the thermocouple to trip. Somerhing to check.

Best of luck and kudos to the young man for learning and tackling a project like this.

My deuce knowledge isn't recent, but being as I used to experiment on mine, I have a lot of "what not to do" knowledge. Feel free to ask away!
 

G744

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That schematic is for a very old wiring system. Chances are, your truck has a much later system with a much different turn signal arrangement.

However, the wiring numbers will be the same. Complete knowledge of the three-lever light switch is necessary to understand the entire lighting system as well.

The auto-reset circuit breakers are not "thermocouples", they simply pop open when the current thru them heats up an element, causing an open circuit.

Dennis
 

kendelrio

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The auto-reset circuit breakers are not "thermocouples", they simply pop open when the current thru them heats up an element, causing an open circuit.

Dennis

I bow to your knowledge oh wise one. How could I EVER mistakenly call a connector (or even a coupler) that is activated by heat (or, you know thermal energy) by the wrong name.

Nice pedantic mode you have there. Was what I said technically wrong? Yes. Did it at least point the OP in the right direction to what I was talking about? Possibly. Does it matter at the end of the day what it's called as long as the result is the same? No.

Wait til you hear me refer to an unknown item as a "Chu-chut" as in "Check the little chu-chut that sticks out from the rod to make sure it isn't broken".

Get over yourself.
 

silverstate55

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Perfect opportunity for that young man to learn how to use a multimeter & learn how to read a wiring schematic!

First things first, with a vehicle as old as these are getting, start with the basics: visually & physically inspect wires & connections...assume nothing, verify that each connector is actually connected, plugs are plugged in, wires aren’t broken off or have insulation worn through which causes the wires to short out against metal components, and so on. You’d be surprised how often these things are the causes of so many problems in lighting in these old trucks.

Check ALL grounds! I usually remove the grounds, remove all rust/paint/etc. in order to ensure that the ground wires or straps are making a definite metal-to-metal connection, then reinstall & paint over. This has been the second most frequent wiring issue in my experience.

With the multimeter, set it for DC Volts (if it’s digital, set it for 50V DC or whatever is well above the max of 24-volts you’re testing), and remember that you will NEVER get a perfect 24-volt reading unless you testing across battery terminals! The more voltage drop you get the farther from the batteries, demonstrates the inherent resistance in wiring & connections...but if it’s significant, like say more than 5 volts loss at a light fixture, you may have some corrosion or frayed/broken wires somewhere in between.

With these trucks’ wiring harnesses being as old as they are (most likely 40-60 years old!), there will be wiring issues to rectify.

Do these simple tasks before you tackle more difficult ones like the 3-lever light switch.

I’ve noticed on “newer” production turn signal switches, is that the wiring inside is much smaller than the older Mil-Spec units, and the newer units will fail much quicker due to smaller-diameter wiring....I have 3 or 4 I bought NEW several years ago, and all failed the first time I tried them. I kept them thinking that I’d replace the small wires with real-size wires someday to have as spares. So if these are an issue you keep having, try to source a take-off used unit that was known to be working.

If you haven’t done Clinto’s turn signal conversion yet, DO IT!

On the wiring diagram, it’s best to go to a print shop & have it printed out as large as you can (maybe even on poster board) for easy reading. Tell that young man that the ground symbol means that the wiring is using metal components (frame, body, etc...) as large wires to return the DC voltage back to the Negative terminal on the battery...that’s why it’s imperative to ensure good ground contacts.

Good luck, report your findings back here so you can be assisted through this.
 

NDT

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Hmm, are you aware the left lower lever on the 3 lever light switch has to be raised up to illuminate the panel lights? If it’s all the way down in “park” the headlights won’t come on.
 

fasttruck

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Power at dimmer switch ? if both high and low beams are out at the same time the problem likely lays elsewhere than bulbs. I gather from post 15 you understand the three lever light switch.
 

fasttruck

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Power up to wires 16 or 17 behind the bulbs ? There will only be power to one of these two circuits depending on the dimmer switch If no power, then work your way down the circuit to the 3 lever switch and see if you find any. If parking lights work, you have power as far as the switch ? Do tail lights come on when service drive circuit (headlights) is on ?
 

weirdman

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Knoxville, Tennessee
Yes, they do. When we tested the headlight connections, we went through low and high beams and nothing. We tested the ground on both headlights and grounds are fine. We were wondering if we should take apart the dimmer switch just incase it had corrosion but we decided not to because that's a whole different can of worms. We took off the covers on the rear lights and the front turn signals to see if we could get some new bulbs, while we were in there we cleaned them to make sure they worked and flipped some working bulbs around to make sure everything worked in them. We are stumped on where else the headlight problem could be coming from.
 

NDT

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The dimmer switch is not repairable. So to clarify, both high and low beam on both bulbs checks good when you checked them with a meter?
 
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