M934A2 Brake light and signal light issues

[Sknttu]

I have a 92 934A2 that has me at a loss. I have scoured the active threads and can’t find anything that has helped so if you have an idea, I appreciate your input.
The issue:
I recently went to drive the truck. Started it up turned lights on and noticed I had no brake lights when I pushed the pedal. I next tried the turn signals and no front or rear lights for them either. In retrospect, I remembered smelling something “electrical”. I have a second truck. 923A2 and I tried changing the turn signal activator and no changes. I then went through the TM’s on trouble shooting this and they pointed to bad flasher. Order one in, plug it in and...nothing. Try that flasher on the working 923 and it doesn’t work. Thinking is was bad, I return it and they send me another. Plug the new one in and this time I hear a pop and the potting on the back looks grey on the inside, like it’s burned. I try it on the working truck and it did not work. I started trying to discover a short. I made a small jumper to go from the input wire on the flasher plug to the output wire. I moved the turn signal to the left and the front and rear signal lights work, to the right and they work as well. I never had sparks and never had a hot jumper wire either and no continuity with a ground. Next, one thread recommended the Grote flasher, I ordered 2 to be safe. Before I plugged it in, I went back through the TM for the flasher and compared to working truck and all is well. I noticed that I have 24v coming to flasher from turn signal when it is activated but then I found when it is turned to the right I also have 12 volts to the output wire side. This is 12 with no flasher connected to the wire that runs back to the turn signal unit to go to the lights. The 12v was at front and rear lights as well. I disconnect the harness and test all the leads to the turn signal and none have 12v. I try with the working truck’s turn signal on the 934 and I have the same issue: 12v coming from output wire with no flasher attached. The 934 turn signal unit works fine on the 923. I install the Grote flasher to try it and nothing happens. Put the same Grote on the 923 and it works perfectly.
I’ve checked my grounds that I can find. The only thing I haven’t tried is the brake light switch and the 3 way service light switch. It works for headlights and tail lights and dash lights in appropriate positions.
PLEASE HELP BEFORE I FIX IT WITH A MATCH AND A CAN OF GAS!!!

Steve

 

[simp5782]

Stop ordering stuff and throwing parts at it.
First thing is, Have you downloaded the P2P program? To show you wish wires to check?

Are the bulbs known to be good?

Check to make sure #57 off the turn signal flasher is good. You said you have cleaned grounds but re position that one away from the steering shaft to a new location.

From the switch check to see your voltage output on wire 460-461 out of the turn signal switch going to the 3 way switch. This can be done right there on your column with it attached. I had the same problem and ended up having to jump wires right there to make my right turn signal work.

Another way to check this is go to your trailer plug. With the turn signals ON, check the connectors and see if you have a trailer signal turn signal. This will be 22-460/22-461 from the turn signal switch to the rear plug. This is where I ended up jumping the wires as mine had a broke 460 inside the connector at the turn signal switch. If you have a signal at the trailer plug then you know your flasher and switch are good.

You simply check the brake light switch with brakes applied see if you have power on both sides of the switch. Its on the frame rail generally 6 inches behind the transfer case


Oh and you should have 24v on signal wires and not 12v.

 

[Sknttu]

Thanks for the reply. I'll be sure and check the voltage at the trailer connector. I wasn't just buying parts to throw at it though I did confirm via my other working truck that the flasher units were bad. The 12 volts I am finding should not be there. I have no 12 volt accessories on it. This stray voltage is seemingly back feeding to the output side of the connector from the flasher back to the turn signal unit.


By by trying the brake switch and the 3-way light switch, I wasn't implying that I replace them, just making sure I trouble shoot them according to the TM's. Plan on doing the brake switch as soon as I get a chance. Didn't do it recently because I didn't think about it before I stopped working on it this weekend.

Im not familiar with the P2P program you mentioned...

 

[Jbulach]

P2P is in a ZIP file at the bottom of the TM’s for M939 series.
My money’s on a bad ground or two, back feeding from a bad ground will do some crazy things.


Sent from my iPhone using Tapatalk

 

[simp5782]

Well the turn signal unit gets its power from the light switch which in turn gets it's power from the switch. 24v from main switch to light switch then it is disbursed

The p2p program provides wiring schematics and air line diagrams for the M939 series trucks. Some small things may differ from truck to truck but it is a good place to show you what you are looking for.