M939a2 Blinkers and Brake Lights Not Working

[DeMilitarized]

So I have a M931a2 and it currently doesn’t have any brake lights or turn signals. The brake lights weren’t working so we replaced the switch on the frame and everything worked. Then I took it for a cruise on some dirt roads which may have shaken something loose and hooked it up to our bolster trailer and now the brake lights and turn signals do not work. I pulled the switch harnesss and blinker harness off and cleaned it and put electric grease in it. Still nothing. The blinker indicator just lights up green instead of flashing. Brake lights don’t work on truck or trailer nor blinkers. Trailer wiring on the truck somehow got inverted so blackout running lights is regular running lights ext.

Where do I even start?

 

[kendelrio]

Check your blinker relay or the actual switch on the steering wheel. That could be the blinker issue. Next I would check the commonalities between the two circuits. Are they getting power? Is there a common ground they use that may be broken?

Good luck and please post steps and results to help build all of our knowledge!

 

[charlesmann]

Doesnt the brake light and turn signal run on the same circuit?

Are you getting power from light switch to brake light switch at the peddle, then out of the switch to the harness?

 

[simp5782]

Doesnt the brake light and turn signal run on the same circuit?

Are you getting power from light switch to brake light switch at the peddle, then out of the switch to the harness?

Brake light switch is on the drivers frame rail behind the transfer case.

 

[simp5782]

Turn main light switch to SERVICE.

Remove body harness plug behind intake tube. With a test light identify

Check for POWER at pin B wire #75 this is the brake light switch input down the frame. If present re connect the plug and check for power on pin A as the return for the brake light. If good report back.

If there is no power on the 75 wire then you have a bad light switch there are no relays or breakers between the light switch and that connector fitting.

Wire 37. Pin L in that connector at the air pipe is always hot 24v. So don't get confused with that.

Since the blinker light comes on there is power on 460/461G to the switch. In the air pipe up Check for turn signal pulse power at pin E or pin C. If No, Check for output on 467B pin F and 467A pin H with the turn signal on to see if there is power leaving with a pulse If there is, the flasher unit is good and you have a wire break.