M923 No low beams

[44CCKW]

I just got a M923 cargo truck.Everything works great,But I have no low beams.I checked the headlights and they are both fine.The high beam switch works well,But I am thinking the low beam side of the switch may be bad.Wiring all looks good.Is their any solenoids or fuses I do not know about??

 

[KaiserM109]

I'd be supicious of the floor switch. I'll bet it is a straight cross with civilian switches.

 

[juanprado]

Most likely culprit is the floor mounted dimmer switch. It looks like and mounts like a civilian very common switch but it is 24v not 12v and has the military wiring connectors not a 3 wire slip on pigtail.

Possible other cause is 2 burnt out headlights. Bulbs have 2 filaments. One can work and the other can be burnt out on old school incandescent lights. Redneck way to test is reverse #17 (high) & #18 (low) wires at light and turn on. #91 is ground.

I actually found one of my headlights wired wrong because one lead wire was short on the harness so they plugged it into the closest connector instead of the correct connector on the bucket. Real easy to tell on the leds when they light up.

 

[44CCKW]

Well,I replaced the dimmer switch with a brand new one and still only have high beams.I also switched the 18 and 17 wire around,But when I do this I have no head lights at all.Could there be a inline fuse somewhere???Im thinking of buying two new headlights,But I'm not really sure that is the issue.When I switch the wires around,the bulbs should still light.Am I right to think this???

 

[juanprado]

Yes, When you reversed the wires it would let you know if the bulbs were ok as you were switching high beam to low beam. They should have lit up either way if good whether you were in low beam or high beam. Sounds like the bulbs are bad. Verify with a test light you have power to 17 and 18 one or the other should have power depending upon in which position the dimmer switch is in and then the reverse.

Check voltage first to those wires. If you have voltage and bulb does not light I would say the bulb. If you have no voltage possible culprit the wiring. If the bulb lite in one position and not the other, I would rule out the ground as it works or it does not.

I captured the screenshot from the P2P program. It is an excellent resource. Link here:
http://www.steelsoldiers.com/showthread.php?82818-M939-P2P-Program-On-SS

 

[44CCKW]

Thanks for the schematic,Very helpfull.Ok,I,ve done some testing.I disconnected the dimmer switch by pulling all 3 wires off the switch.With a jumper wire I jumped from the 16 wire(hot wire) at the dimmer to 17 terminal on the back of the headlight.The light went on.Then I switched to the 18 terminal and got nothing.I am thinking bad bulbs???But then I also tested the wires at the headlights with a test light.17 had power.18 had none.This remained the same even when I switched the dimmer.Also have no power on 18 at the dimmer.

 

[o1951]

Before you spend $ for bulbs, remove wires 17 and 18 from bulbs. Leave wire 91 connected. Jumper wire directly from 24 volt hot side of batteries to 17 - light? now try 18 - light? if it lights on only 17 or only 18, bulb is bad. If lights on both 17 and 18, bulb is good. If it lights on neither, both bulb fillaments are blown ( unlikely) you have a bad ground at 91, or you are not on 24 volt hot side of batteries.
Let us know, will tell what to look for next.

 

[44CCKW]

I will try that this weekend.Wire 16 at the dimmer is a hot wire.It feeds the dimmer power.So that is why I jumped from there.I would think by using 16 wire as my power source,The bulb would light if I touched the jumper wire to the terminals on the back of the bulb.Like I said one terminal did light 17 and one did not 18.I am confused by the 18 wire not having power at the dimmer.I would think that the power would switch from 17 to 18 when you step on the dimmer.Would the headlight main switch have anything to do with this??

 

[o1951]

.Would the headlight main switch have anything to do with this??

NO!
Suggest you get power from known source - the battery and test the bulbs. Don't buy anymore parts you don't need. We can go from there.

 

[Danl]

I had no high Beam on my 915, after much checking and testing , The bad thing was in the black out switch for some reason the black out switch was shutting off the high beam power, relacated power lead and all is working well, if this truck is new to you, you might fine that some one in the motor pool might of put the wire on the wrong side of the black out power, try useing black out , and I'm sure that in black out moad the low beams will work

 

[44CCKW]

I think you might have solved my problem.When I turn on the driving lights and switch the floor dimmer to low beamI have no low beams.But if I slowly switch the dim lever on the main headlight switch,The low beams come on for a few seconds.I have another switch ,So I will try the new one and see if this cures the problem.

 

[44CCKW]

Well,Still no low beams.I tested both headlights with a jumper wire directly from the positive side of the batteries.High beam worked,Low beam did not.So I replaced both headlights.BUT still no low beams.So far I have replaced the dimmer switch,Both headlights and the main light switch.Its got to be a wire or fuse issue.I just have to find it.But,I do not under stand why the low beams do not work even when I run a direct wire to the batteries.I did not remove the headlights to test them.I ran a jumper from the battery to the terminals on the back of the headlight bucket behind the headlight.Could the terminals be bad?

 

[o1951]

Not sure what you mean - the actual terminals on the lamp with the wiring connector unplugged, or the back of the wiring connector.
Suggest you not buy anymore parts until this is diagnosed.

 

[KE7FVB]

Can anyone help with this one? My driver side low beam keeps burning out, I have replaced it 4 times in the last year and it's only the driver side low beam, any thoughts?

 

[steelandcanvas]

Can anyone help with this one? My driver side low beam keeps burning out, I have replaced it 4 times in the last year and it's only the driver side low beam, any thoughts?

Lamp nice and tight in the bucket? All power and ground connections clean and tight?

 

[KE7FVB]

Yes, I replaced the bucket assembly with a new one and added a ground wire for good measure but it still burned out

 

[o1951]

Like Steel & C said in previous post, light must be tight, and bucket must be secure. If anything is loose, will bounce and stress out filament.
Are you using good bulbs, not the ones made in Chinaroo?

Have to ask - kinda dumb, but you are using correct voltage bulbs, right.

If you answer is yes to all, then the only rather unlikely thing left is voltage spikes. That can be caused by a poor or loose connection that is arcing and causing spikes as truck is driven. Now seeing as only left one blows, you only need check wires going to that one.

 

[KE7FVB]

They are the correct voltage, everything seems tight, secure and the brand is Wagner so I guess it's time to start checking the wires. What's funny is the right hand side vibrates like crazy and it has never burned out. I'm kind of at a loss with this one. Is it possible that the voltage spikes when I start it after it has sat for a few weeks to charge the batterys?

 

[infidel got me]

Chinaroo -- ain't that up the road from hancho en mexico

 

[o1951]

They are the correct voltage, everything seems tight, secure and the brand is Wagner so I guess it's time to start checking the wires. What's funny is the right hand side vibrates like crazy and it has never burned out. I'm kind of at a loss with this one. Is it possible that the voltage spikes when I start it after it has sat for a few weeks to charge the batterys?

As they both are on at same time, then both should have issues. If you have a voltmeter, and want to, you could check battery voltages. over 15 volts per battery is excessive.

Check where Wagner is made. I bought some GE bulbs for my house, and they were short lived. Made in Chinaroo! I called GE to complain, and they sent me replacements made someplace else. The replacements are very good. Course, I do not know -maybe that case of short life bulbs was dropped or had a rough trip from overseas.