M35A2 Blinker and brake light failure and fix

[Barrman]

I used the Whistler to trailer my M715 55 miles to the Texas Rally the first weekend of November and back. No problems with the truck, just my civillian trailer throwing a tire. A week later I took the Whistler to a Boy Scout camp out. While going to gather my Scouts I noticed the left blinker indicator didn't light up. The right one did.

I got to town and did a walk around. The right side system worked and the left side didn't. I tried it on flasher and got the same result. I opened the engine side cover and shook all the wires at the blinker box, firewall and anything else that looked like it might be part of the blinker wiring. No help.

I didn't worry about it and did the camp out. I started messing with it a few days later and determined from looking through the wiring diagram that if one side flashed, the flasher was good. The next part of the system in line is the blinker switch. I tried a spare and got the same results except this time only the passenger front blinker worked. No rear blinkers on either side.

I had mostly oil in the tank, it got cold out, the truck was hard to start and I didn't need it again until February, so I just let it be. Then this week I volunteered to go pick up a M35 in San Antonio. I had to fix it.

While looking at it tonight, I figured out my brake lights didn't work either. I unplugged the wires at the brake light switch on the air pack and jumped them. No spark, no lights, no nothing. Hmm, bad 3 lever. I swapped in another and got the same result.

I put a test light on the wires at the air pack and my original 3 lever was putting power out. Back to the blinker switch or a break in the wires somewhere. I put a jumper on the wires at the brake switch. Then used the test light at terminal G on the blinker switch. Power there, wiring so far good. The brake lights and blinkers get routed through the blinker switch. I next tried jumping the 4 terminals to the 4 blinkers through the terminals. Only the passenger front worked.

I was thinking broken wires by now and pulled the cover off the drivers front housing. I found a black as coal bulb. New bulb in and the light worked. I pulled each rear off next. Passenger rear had a white as snow bulb. New bulb and it worked. Drivers side had a clear bulb but no fillament. New bulb and all was fixed.

I know, I should have checked the bulbs first. However, 2 failing at the same time on the same side is not normal. Then to have both rears gone within 1 hour of running time just isn't right. Add in the strange colors of the bulbs and I am stumped.

What I have come up with is this. I use a M105 trailer plug and front wire with a round 6 pin civillian female plug on the other end to adapt my MV's to the trailer. When my lugs broke and the trailer axle hit the ground with the M715 on it. Something must have shorted out. I had my flashers on as soon as I saw the tire bouncing down the road. Why the passenger front didn't go I don't know. More importantly, why didn't the flasher box go? I have had them burn up by looking at them wrong it seems before.

Time to check the trailer wiring before I hook it up again, too.

Just something to pass along in case somebody is stumped with a weird electrical problem. Check the obvious of course and it doesn't alway have to make sense.

 

[doghead]

Check your alternator output voltage, make sure it's not overcharging. Also check your battery cables connections, all of them(especially the alternator ground)

One truck I bought had every light blown(except 2) when I bought it(GL). I did the same thing you did, 2 hours of diagnostics, only to find almost every bulb was blown.

It had a bad ground connection at the frame. The alt was occasionally putting out max voltage (it was sensing no voltage). I think that is what blew all the lights.

I cleaned all the battery cable connections and have never had a problem with that truck since.

 

[Barrman]

Hmm, very interesting. I just so happened to get the dreaded "POP ssss" after cranking about 3 seconds on my first start attempt Thursday evening. The dog bone at one end was actually loose a little and had the white fuzzy growth on it. I could actually shake the end on the tie down post. I pulled all the terminals, wire brushed them and put it all back together tightly.

This is the first time in more than 2 years that I have had any issues with the battery connections. I also noticed when I fired it up last night that upon start up. The needle was all the way to the green/red line the first few minutes and then went on down to the hash where it is supposed to be. It has not done that in a while and I didn't pay enough attention to why it wasn't doing that I guess. All my other trucks do.

I didn't even consider that problem anything other than normal maintenance needing to be done. That is why I didn't mention it. However, I can now think this: Running lights off, the brake lights and left blinkers on. While stopping with a 3 legged trailer bouncing along empty over a rough road. Which is exactly what I was on the first turn after the tire troubles. I know I wasn't watching the gauges at that point so an alternator induced spike could have happened and I wouldn't have known.

Thanks Eric for the insight.

 

[rchalmers3]

Barrman, I have been caught in the same trap before: multiple failures masquerading as a single systemic failure. There is no way that I know of to anticipate multiple failures. Hindsight is sometimes all we got! Kinda like realizing later in life that school was a good thing.....

DH is correct for implementing 20/20 hindsight verification of the charging system. All I can add is that I hope your time spent figuring out the problem has given you greater understanding of your rig.

Rick

 

[vinnies10]

I have a rear passenger light out no power or nothing blinker stop light nothing bought all new bulbs cant figure it out. its a dumb question is there like a breaker or any kind of fuses? Its the only light that does not work

 

[Barrman]

There is a breaker in the 3 lever light switch. However, if it pops nothing will work for lights until it resets itself.

I would suggest you loosen both of the 3/4" mount bolts with a 9/16" wrench, clean the bolts, clean the ground contact on the back of the light bucket and clean the mount surface. You probably are just not getting any ground. If that doesn't work and you have the plastic bucket light. Pull the front cover off again and make sure the ground strap inside is still there and not broken.

 

[vinnies10]

I bought a brand new light housing and all I even jumped out the ground. I guess I may have a few broken wires or something stupid. I guess ill look into it more