IsaLandr
Tartaned Goði
- 186
- 18
- 18
- Location
- Centralia, WA
Looking for troubleshooting guidance and suggestions on resolving this intermittent no-start condition. The issue only started on July 3rd, when we were prepping the truck to participate in some Independence Day operations.
The truck is a 1986 AM General M923 with the ABS system. It has the standard Cummins NHC250 and Allison automatic, no major modifications or changes to either of these.
So, here's the issue: If the truck sits for a while, it will start up with no difficulty. Runs well, builds air pressure, transmission operates, everything works. Shut it down, it will re-start. Can do that several times with no problem. Engine starts easily, starter spins at the correct speed, no drag, everything seems normal. Once it is running, the truck operates normally and can be driven with no issues whatsoever.
The issue starts if the truck runs for 20 minutes or more. Once it has been running for a while, if we shut it down, it will not restart untill it has sat for a while again. Anywhere from half an hour to several hours or even overnight, though it is not consistent. Whacking on the Protective Control Box was thought to have temporarily resolved the issue to start it a couple times over the July 4th weekend, but that seems to have been coincidental, as that method has not worked since and in retrospect, we whacked on the box after the truck had been sitting for a while, and it restarted. Thus making us think that's what worked... but in reality, I'm pretty sure now that it was the time left sitting that actually did the trick.
I have removed the PCB and opened it up, tested the solenoids. both in the truck and out of the truck. The solenoids test good, but I can't speak for the circuit board, as I don't fully understand all of its functions or how it works. I can verify that both solenoids work, however. Both activate immediately and create positive connections when they activate. Resistances are within acceptable tolerances, and when the truck will start, they work properly as they should.
In my testing, what I have discovered is that when the truck actually starts, pushing the START switch up to the start position energizes the starter solenoid, and when the truck won't start, the solenoid engage circuit isn't getting any voltage. In the image below, it is the pole labeled "ENGINE START". Under normal starting conditions, that terminal has no voltage untill the starter switch is pushed up to START, which then energizes that pole with 25vdc, which activates the solenoid, and starts the engine. When it won't start, pushing the starter switch up to START does not send any voltage to that terminal. Thus I believe the issue is somewhere upstream from the Protective Control Box, and not the PCB itself.
The part I have labeled "ENGINE RUNNING SAFETY" is essentially a guess, as that appears to me to be a large diode to prevent damage from connecting batteries backwards and according to the wiring diagrams provided in the various PCB repair threads I have been reading, that is Pin A, which has all the stuff that the primary battery switch controls listed on it. I should probably have labeled it "REVERSE POLARITY PROTECTION" instead.
So, what this sounds to me like is that one of the engine safety no-start circuits isn't disengaging once the truck has been running a while, and that once it cools down or loses pressure or whatever it is that is triggering the no-start disengages, it will start normally again. Almost like there is a pressure sensor that doesn't depressurize immediately, and takes time to drop down to a point where the starting circuit will energize again. But I could be way off in that assessment, I just don't understand enough about how the various circuits work in these trucks.
So I'm looking for advice and suggestions on how to identify and locate what the offending circuit might be. I do have the pertinent manuals, it has not proven as helpful in this endeavour as I might have preferred. I have ordered two new replacement solenoids with which to rebuild the PCB since I have it open, but I do not believe that will resolve this issue.
Below are the wiring diagrams I have been referencing:
Hopefully in all this confusion somebody can point me in the right direction to figure out what the issue actually is so I can fix it.
The truck is a 1986 AM General M923 with the ABS system. It has the standard Cummins NHC250 and Allison automatic, no major modifications or changes to either of these.
So, here's the issue: If the truck sits for a while, it will start up with no difficulty. Runs well, builds air pressure, transmission operates, everything works. Shut it down, it will re-start. Can do that several times with no problem. Engine starts easily, starter spins at the correct speed, no drag, everything seems normal. Once it is running, the truck operates normally and can be driven with no issues whatsoever.
The issue starts if the truck runs for 20 minutes or more. Once it has been running for a while, if we shut it down, it will not restart untill it has sat for a while again. Anywhere from half an hour to several hours or even overnight, though it is not consistent. Whacking on the Protective Control Box was thought to have temporarily resolved the issue to start it a couple times over the July 4th weekend, but that seems to have been coincidental, as that method has not worked since and in retrospect, we whacked on the box after the truck had been sitting for a while, and it restarted. Thus making us think that's what worked... but in reality, I'm pretty sure now that it was the time left sitting that actually did the trick.
I have removed the PCB and opened it up, tested the solenoids. both in the truck and out of the truck. The solenoids test good, but I can't speak for the circuit board, as I don't fully understand all of its functions or how it works. I can verify that both solenoids work, however. Both activate immediately and create positive connections when they activate. Resistances are within acceptable tolerances, and when the truck will start, they work properly as they should.
In my testing, what I have discovered is that when the truck actually starts, pushing the START switch up to the start position energizes the starter solenoid, and when the truck won't start, the solenoid engage circuit isn't getting any voltage. In the image below, it is the pole labeled "ENGINE START". Under normal starting conditions, that terminal has no voltage untill the starter switch is pushed up to START, which then energizes that pole with 25vdc, which activates the solenoid, and starts the engine. When it won't start, pushing the starter switch up to START does not send any voltage to that terminal. Thus I believe the issue is somewhere upstream from the Protective Control Box, and not the PCB itself.
The part I have labeled "ENGINE RUNNING SAFETY" is essentially a guess, as that appears to me to be a large diode to prevent damage from connecting batteries backwards and according to the wiring diagrams provided in the various PCB repair threads I have been reading, that is Pin A, which has all the stuff that the primary battery switch controls listed on it. I should probably have labeled it "REVERSE POLARITY PROTECTION" instead.
So, what this sounds to me like is that one of the engine safety no-start circuits isn't disengaging once the truck has been running a while, and that once it cools down or loses pressure or whatever it is that is triggering the no-start disengages, it will start normally again. Almost like there is a pressure sensor that doesn't depressurize immediately, and takes time to drop down to a point where the starting circuit will energize again. But I could be way off in that assessment, I just don't understand enough about how the various circuits work in these trucks.
So I'm looking for advice and suggestions on how to identify and locate what the offending circuit might be. I do have the pertinent manuals, it has not proven as helpful in this endeavour as I might have preferred. I have ordered two new replacement solenoids with which to rebuild the PCB since I have it open, but I do not believe that will resolve this issue.
Below are the wiring diagrams I have been referencing:
Hopefully in all this confusion somebody can point me in the right direction to figure out what the issue actually is so I can fix it.
