1986 M923 intermittent no-start condition

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.

These are the threads I have predominantly been reading to bone up on this problem:


Thanks to those, I got the wiring diagrams, learned a lot about how these boxes work and what they do, and even learned how to jump this thing if I really need to:



But I'd really like to properly resolve the issue entirely and just not have to worry about it anymore. Any assistance, advice, or guidance would be most appreciated.

Update:

The situation has been temporarily resolved to at least keep the truck operational. It's not ideal, but at least it works.

The solenoids in the PCB are not at fault. I have no idea if the components on the circuit board are faulty, I don't have enough information on how they work or what they do, to test them appropriately. The two big relay solenoids in the PCB tested fully functional and continue to work properly. As stated previously, the starter switch in the cab is not energizing the lead to the starter relay in the PCB like it should. I am still thinking it's one of the safety cutout switches somewhere in line between the starting switch and the PCB.

My solution at this point was to connect a heavy duty momentary starter button in the cab back down to the starting relay in the PCB, to jump the main power on the starting circuit relay, to the solenoid engage terminal. Which is the exact circuit the normal starting switch in the cab engages when it works.

I still don't know why the main switch won't close that circuit. Investigation and troubleshooting continues. But at least for now the truck starts and stops and drives normally once more. Will update again if/when I figure out what the actual problem is with the starter switch circuit.

You know that the control box in the picture you posted is made for a Non abs truck right?

Answer this.

When the truck will not start. When you flip the power switch up, do the gauges twitch or go back to their operating ranges like your coolant, trans temp or fuel? If No replace the pcb with the proper unit that does not have the circuit board. If Yes, check the neutral safety connection in the shifter.

That board is junk and you actually are using the wrong pcb box for a 939. There is one with 2 solenoids and diodes

Secondly there are zero cut out circuits between the pcb and the power switches in the dash.

The neutral safety switch completes the power circuit on the pcb to send power to the starter switch

simp5782 said:

You know that the control box in the picture you posted is made for a Non abs truck right?

Answer this.

When the truck will not start. When you flip the power switch up, do the gauges twitch or go back to their operating ranges like your coolant, trans temp or fuel? If No replace the pcb with the proper unit that does not have the circuit board. If Yes, check the neutral safety connection in the shifter.

That board is junk and you actually are using the wrong pcb box for a 939. There is one with 2 solenoids and diodes

Secondly there are zero cut out circuits between the pcb and the power switches in the dash.

The neutral safety switch completes the power circuit on the pcb to send power to the starter switch

Click to expand...

First off, thank you simp5782 for responding. I'm glad to have the knowledge of someone who knows far more than I do, to look at this thing. As I am sure you have already figured out, I have no idea what I'm doing with this PCB. I am being as forthcoming as I can with whatever information I have available for resolving this problem.

I need to mention that after the last time I took the box out, tested the solenoids, re-assembled it, and put it back in, now the truck will not start at all without using the bypass button. Previously, it would re-start if allowed to sit long enough. Now, it that is no longer true. Pushing UP on the start switch will not engage the starter. I'm not sure what changed, as the only thing I have been touching is the PCB itself, not anything in the rest of the wiring harness.

I'm not sure it's clear in the pictures, but there actually are two solenoid relays of different designs in the box, and the circuit board has diodes on it, though I do not understand at all anything that the circuit board does. It doesn't match up with my previous understanding of what the PCB should do and how it should be wired. Based on that, I believe you when you say it's not the right PCB for this truck.

I did NOT know that is for a non-ABS truck. It's what came in the truck when we got it seven years ago, and it has worked flawlessly the entire time, untill now. The ABS also works properly. If there is another box we should have on there, I would be delighted to know which one is correct and get the right box installed. The fact that the box we have does not match any of the diagrams I have seen, did raise a number of questions, but I didn't find any answers, which is what sparked this post in the first place.

When I turn on the master power switch, all the gauges come on, and show whatever their correct reading should be. Fuel, temp, oil pressure, air pressure, voltmeter, everything works. Flipping on the engine Run/Start switch, the ABS light comes on, I can hear the separate sets of clicks from the front and rear ABS modules cycling, then the light goes out; nothing changes on any of the gauges. They do not flicker, drop, or in any way change from showing a normal operating reading.

Additionally, when I push the start switch up to the START position, the gauges do not change, and it does not engage the starter. The voltmeter does not flicker or show any changes aside from the drop from the green down into the high yellow when the master switch is turned on, which it has always done since the first day we got the truck.

When I push the jumper button I installed, that will engage the starter, the voltmeter shows the appropriate minor drop in voltage as it sends power to the starter, the truck starts, and everything works. The gauges continue reading normally, and the Start/Run switch will shut the engine down like it's supposed to. As near as I can tell, the ONLY problem with the truck at the moment is that pressing up on the Start switch does not engage the starter. Everything else works.

So... if the only cutout between the Start switch and the PCB is in the neutral safety switch... then it sounds likely that this is the culprit responsible for my no-start? I have tried moving the gear selector back and forth, into and out of neutral, and trying the Start switch again, but this has not changed anything. I wonder if the neutral safety switch is just broken entirely now.

Further question... What potential harm or problems may have been caused by having the wrong PCB in the truck all these years?

Obviously I need to obtain the correct PCB for this truck, and I will get right on that immediately. Unless it's better to rebuild this one in accordance with the instructions provided in the four links I posted earlier in this thread. It seems pretty straightforward to re-wire the box with new diodes and no circuit board... I think?

But in the meantime, I still have to trace out and resolve this no-start condition. I will look at the neutral safety switch as soon as I have an opportunity. This may not be untill the weekend, depending on other factors. Fortunately the manual shows where it is. I will report back my results when I have investigated this.

Also, here is a better view of the circuit board inside the PCB.

simp5782 said:

You know that the control box in the picture you posted is made for a Non abs truck right?

Answer this.

When the truck will not start. When you flip the power switch up, do the gauges twitch or go back to their operating ranges like your coolant, trans temp or fuel? If No replace the pcb with the proper unit that does not have the circuit board. If Yes, check the neutral safety connection in the shifter.

That board is junk and you actually are using the wrong pcb box for a 939. There is one with 2 solenoids and diodes

Secondly there are zero cut out circuits between the pcb and the power switches in the dash.

The neutral safety switch completes the power circuit on the pcb to send power to the starter switch

Click to expand...

One further question...

My understanding is that in addition to the neutral safety switch, there is at least one safety switch to prevent engaging the starter on the engine if it is already running (alternator voltage and/or engine oil pressure). Is that not correct?

My initial assumption was that an engine running no-start safety might be the component that is causing the no-start condition, since it only happened once the engine had been running for a while, and would subside over time allowing the truck to be restarted again. Is there no such engine running safety switch, or is it simply not possible for that to be a problem with that in this system?

Wire #5 is that. Usually If the alternator is working fine then that is not the issue. Exciter wire tends to freak out and blow the pcb. Stick your test light in wire #5 plug with the pcb hooked up and see if you have power there when it is causing an issue. Not likely but possible .

Bypass the Neutral safety switch (nss) by hooking the 2 connectors together and trying it for awhile and see if it acts up.


Take the pcb you have. Gut it. Buy 2 solenoids. Cole harsee #24063. And bolt or spot weld them into the panel you have. And I can provide you with the proper wiring information. You will need some of the diodes if you want to protect it properly in case you hook the battery up backwards

simp5782 said:

Wire #5 is that. Usually If the alternator is working fine then that is not the issue. Exciter wire tends to freak out and blow the pcb. Stick your test light in wire #5 plug with the pcb hooked up and see if you have power there when it is causing an issue. Not likely but possible .

Bypass the Neutral safety switch (nss) by hooking the 2 connectors together and trying it for awhile and see if it acts up.

Take the pcb you have. Gut it. Buy 2 solenoids. Cole harsee #24063. And bolt or spot weld them into the panel you have. And I can provide you with the proper wiring information. You will need some of the diodes if you want to protect it properly in case you hook the battery up backwards

Click to expand...

Thank you for that advice, I actually anticipated that and I'm one step ahead for that one. Already ordered the solenoids, they arrived last Saturday. I got the big three-terminal ground-through-bracket units, Cole Hersee 24124 SPST 24vdc 85amp continuous duty, same model as the top one that's already on the panel now. Not super cheap, but not super expensive either. I also bought a new rotary cut off tool to get into the tight spaces to cut the old solenoids and their brackets off the panel.

I didn't know which diodes to get, so I haven't ordered those yet. Looking at the diagrams, installing a diode with these may be weird but not impossible. Your recommendations for wiring it up are most welcome, I want to make sure I do this right.

Since I don't have a spot welder, I figured I'd just drill out bolt holes and hard mount the solenoids to the panel. Your instructions say that's okay, so that's what I'm going to do.

Probably should have mentioned before, the alternator is one of those aftermarket replacement units from Those Military Guys, I bought the conversion kit a couple years ago after the original alternator cooked itself and the magic smoke got out. I actually forgot I'd done that till I started looking at the wiring harness a few days ago. Does that aftermarket alternator make a difference?

Will check on the #5 wire at first opportunity. Probably the same time I start digging around for the neutral safety switch. This weekend, may be soonest available time. Thank you again for taking the time to explain all this. I really appreciate it.

IsaLandr said:

Thank you for that advice, I actually anticipated that and I'm one step ahead for that one. Already ordered the solenoids, they arrived last Saturday. I got the big three-terminal ground-through-bracket units, Cole Hersee 24124 SPST 24vdc 85amp continuous duty, same model as the top one that's already on the panel now. Not super cheap, but not super expensive either. I also bought a new rotary cut off tool to get into the tight spaces to cut the old solenoids and their brackets off the panel.

I didn't know which diodes to get, so I haven't ordered those yet. Looking at the diagrams, installing a diode with these may be weird but not impossible. Your recommendations for wiring it up are most welcome, I want to make sure I do this right.

Since I don't have a spot welder, I figured I'd just drill out bolt holes and hard mount the solenoids to the panel. Your instructions say that's okay, so that's what I'm going to do.

Probably should have mentioned before, the alternator is one of those aftermarket replacement units from Those Military Guys, I bought the conversion kit a couple years ago after the original alternator cooked itself and the magic smoke got out. I actually forgot I'd done that till I started looking at the wiring harness a few days ago. Does that aftermarket alternator make a difference?

Will check on the #5 wire at first opportunity. Probably the same time I start digging around for the neutral safety switch. This weekend, may be soonest available time. Thank you again for taking the time to explain all this. I really appreciate it.

Click to expand...

Please post a picture of your alternator. If it is a brushless unit then you should direct wire it to the battery and the exciter wire should not even be hooked up to cause any issue for you.

This is the label on the PCB that is in the truck now:



Again, it is the PCB that came in the truck. I can only assume the Oregon National Guard put that in there before the truck was excessed.

The NSN and ORD numbers match what is shown in this thread:

and the part numbers listed here:

So I really don't know what's going on with that box or why it's wrong.

simp5782 said:

Please post a picture of your alternator. If it is a brushless unit then you should direct wire it to the battery and the exciter wire should not even be hooked up to cause any issue for you.

Click to expand...

I do not believe it is a brushless alternator. If I remember correctly, it's a 24vdc 12SI ACDelco unit.





And this is the wire that was left unconnected when I put the new alternator in. As I recall, the instructions that came with the new alternator said to leave that wire disconnected.




I have no idea if that makes any difference or not.

IsaLandr said:

I do not believe it is a brushless alternator. If I remember correctly, it's a 24vdc 12SI ACDelco unit.

View attachment 806056
View attachment 806057
View attachment 806058

And this is the wire that was left unconnected when I put the new alternator in. As I recall, the instructions that came with the new alternator said to leave that wire disconnected.

View attachment 806059
View attachment 806060

I have no idea if that makes any difference or not.

Click to expand...

You have a brushless alternator. Run a wire from the alternator directly to the battery. Eliminating it from the pcb system. The exciter wire that inhibits starting while the truck is running is not hooked up so that has nothing to do with your issues of not starting.

I am going with the chipboard being bad. Over a neutral safety


For giggles. Make sure the screw connector for the rear wire harness is tight. This is behind the air tube under the floorboard

If you do not have one. Invest in a power probe circuit tester.

They have power probe 4s on ebay for $209 shipped for new ones.

simp5782 said:

You have a brushless alternator. Run a wire from the alternator directly to the battery. Eliminating it from the pcb system. The exciter wire that inhibits starting while the truck is running is not hooked up so that has nothing to do with your issues of not starting.

I am going with the chipboard being bad. Over a neutral safety

For giggles. Make sure the screw connector for the rear wire harness is tight. This is behind the air tube under the floorboard

Click to expand...

I will endeavour to check the screw connector tomorrow after work.

How did you determine that it is a brushless alternator? I could not find any information from my original order or on the website which indicates which kind it is and I haven't seen any markings on the alternator itself identifying it.

simp5782 said:

If you do not have one. Invest in a power probe circuit tester.

They have power probe 4s on ebay for $209 shipped for new ones.

Click to expand...

Power Probe IV ordered and on its way. Thank you for the heads-up.

IsaLandr said:

I will endeavour to check the screw connector tomorrow after work.

How did you determine that it is a brushless alternator? I could not find any information from my original order or on the website which indicates which kind it is and I haven't seen any markings on the alternator itself identifying it.

Click to expand...

Only requires the single power wire. And your exciter wire is not being used. Which means the alternator does not require a signal to operate. It produces power anytime it spins.

simp5782 said:

For giggles. Make sure the screw connector for the rear wire harness is tight. This is behind the air tube under the floorboard

Click to expand...

This is the correct connector, yes?




I had a few minutes before work this morning, so I went out and checked the screw connector. It's put together pretty tight. So tight, I couldn't get it loose with a pair of channel locks. I don't think it's coming off there anytime soon.

IsaLandr said:

This is the correct connector, yes?

View attachment 806096
View attachment 806097

I had a few minutes before work this morning, so I went out and checked the screw connector. It's put together pretty tight. So tight, I couldn't get it loose with a pair of channel locks. I don't think it's coming off there anytime soon.

Click to expand...

Yes. That is it. Use a chisel and a hammer to loosen it. Best to check it for a corroded pin. The neutral safety switch runs thru that connector. Add stabilant 22 or dielectric grease to all pins

simp5782 said:

Yes. That is it. Use a chisel and a hammer to loosen it. Best to check it for a corroded pin. The neutral safety switch runs thru that connector. Add stabilant 22 or dielectric grease to all pins

Click to expand...

Duly noted, thank you. That goes into my maintenance list for this weekend. Along with checking the neutral safety switch itself.