HEMRWL
New member
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- Location
- Cincinnati, OH
Hello all,
I have an M1078A1 LMTV in our shop that is stumping me. When it came in, the driver stated that while driving it had a loss of power and when they went to shut it off, the switch on the dash would not work, so they used the power shut off switch below the steering wheel next to the diagnostic plug. I've been told using that itself could cause some issues. When the work order came to me, I went out to turn on the rotary battery switch and the truck immediately had power (dash switch was in off position and remote switches under cab were normal). I tried to start it, but it would just crank and never fire up. My theory is, because of that constant power, the fuel injectors and computers aren't engaging correctly. Here is a list of things I've looked at:
Batteries - Hooked up correctly, all have good charge
Rotary Battery switch - Functions correctly
Bus bars and relay in battery switch box - Working as they should
LBCD - Did the functions check from the Neihaus website, all good
ECM - Hooked up diagnostics computer, a few codes about "over voltage" but its reading the ECM so I assume its not burnt up
PDM in dash - All relays and CB are good.
The PDM in the dash is where it gets weird. I checked the two posts TL41 and TL42 for proper voltage and they're all reading 22v. One should be 12v and the other should be 24v. When I unhook the hot cables from the dash and read them individually they read what is expected. But as soon as they are hooked up, the 24v seems to be back feeding in to the 12v side (which is probably where I am getting the "over voltage" fault). The K1 and K2 relays are both working as they should and I have swapped out a known good PDM with the one in the truck. I am losing my patience with this truck, every time I think I have an "A-HA" moment, it quickly shuts me down.
Anyone have any ideas as to where else I should check or recheck? Much appreciated.
I have an M1078A1 LMTV in our shop that is stumping me. When it came in, the driver stated that while driving it had a loss of power and when they went to shut it off, the switch on the dash would not work, so they used the power shut off switch below the steering wheel next to the diagnostic plug. I've been told using that itself could cause some issues. When the work order came to me, I went out to turn on the rotary battery switch and the truck immediately had power (dash switch was in off position and remote switches under cab were normal). I tried to start it, but it would just crank and never fire up. My theory is, because of that constant power, the fuel injectors and computers aren't engaging correctly. Here is a list of things I've looked at:
Batteries - Hooked up correctly, all have good charge
Rotary Battery switch - Functions correctly
Bus bars and relay in battery switch box - Working as they should
LBCD - Did the functions check from the Neihaus website, all good
ECM - Hooked up diagnostics computer, a few codes about "over voltage" but its reading the ECM so I assume its not burnt up
PDM in dash - All relays and CB are good.
The PDM in the dash is where it gets weird. I checked the two posts TL41 and TL42 for proper voltage and they're all reading 22v. One should be 12v and the other should be 24v. When I unhook the hot cables from the dash and read them individually they read what is expected. But as soon as they are hooked up, the 24v seems to be back feeding in to the 12v side (which is probably where I am getting the "over voltage" fault). The K1 and K2 relays are both working as they should and I have swapped out a known good PDM with the one in the truck. I am losing my patience with this truck, every time I think I have an "A-HA" moment, it quickly shuts me down.
Anyone have any ideas as to where else I should check or recheck? Much appreciated.