Trouble-shooting the Protective Control Box in a 5-ton

[SSchwake]

My 1991 M923A2 had the same type issue - turn on batt switch which energizes gauges, but nothing at all from the start switch. I've purchased a new PCB from Boyce, but discovered a previous owner placed a jumper between wires 14/49 at the PCB box connector. I've combed my Resources for a schematic or pin-out identifying these wires but have come up blank.

Does anyone one know what has been bypassed with this jumper, or more importantly, have access to a diagram detailing the PCB connections?

 

[Andyrv6av8r]

 

[tobyS]

View attachment 686175

The link doesn't work (at least for me).

My 931A2 has a problem with battery going down and when low, the fuel cutoff solenoid will not pull in to allow it to start. If I manually pull it in, it usually starts and stays. When well charged (from my charger), all seems to work fine. Are those symptoms of a control box problem or a voltage regulator?

 

[Andyrv6av8r]

 

[Andyrv6av8r]

https://www.steelsoldiers.com/showthread.php?113634-Ever-see-inside-the-Protective-Control-Box

Check out this thread

 

[Jbulach]

The link doesn't work (at least for me).

My 931A2 has a problem with battery going down and when low, the fuel cutoff solenoid will not pull in to allow it to start. If I manually pull it in, it usually starts and stays. When well charged (from my charger), all seems to work fine. Are those symptoms of a control box problem or a voltage regulator?

Thats a common problem. It takes full voltage to pull it in. If your getting good charge voltage while it's running, your batteries are probably on their way out.

 

[74M35A2]

A2 should fire almost instantly if in a good state of tune.

Batteries over 5 years old will likely be well below their rated CCA value, and will quickly drop voltage when cranking, even though you can not hear this in starter speed until voltage is way down.

The other aspect of the a2 shut down solenoid is that the longer the truck is cranked, the hotter this solenoid will become, and the resistance goes up, current drops, and so does its holding power.

A fast starting a2 with batteries less than 5 years old (even civilian ones) and a modern alternator is the best weapon for combating a2 fuel shut off solenoid issues.

 

[CARMAN]

There are several of these threads going so not sure which one to add to?? When I picked up my truck it would not start. We swapped the PCB and no problem. I took the bad one with me and it looks very basic like the one SUPRMAN posted. This may have been done at the 2010 re-build.? I want to rebuild this one but have not seen actual part numbers listed. M923A2. Is Prestolite even a source anymore.

 

[CARMAN]

 

[74M35A2]

24v continuous duty solenoids. Cole-Hersee is one mfg of them.

 

[CARMAN]

Was just looking at my PCB. One of the solenoids says 12V. The other 24V. The newer on is the 12. As a reference my rusty one says....SA8 or SAB 4418. 24V. 1A

 

[74M35A2]

Should both be 24v. One controls dash power, the other controls signal to starter solenoid.

 

[CARMAN]

 

[74M35A2]

I'm not sure what else you want. Get 2 new 24V continuous duty solenoids, and go forward. There are posted part numbers in other threads if you need/want.

 

[CARMAN]

Just showing you the way mine was with a 12v.

 

[MKD73]

Looks like I have fallen into the same situation with my 923
I do not know if this is the most recent thread but my question is when I turn the starter switch to the start position I can hear a relay click. Is it possible the relay(s) have still failed or could it possibly be something else?

Thanks for the help!

 

[simp5782]

Looks like I have fallen into the same situation with my 923
I do not know if this is the most recent thread but my question is when I turn the starter switch to the start position I can hear a relay click. Is it possible the relay(s) have still failed or could it possibly be something else?

Thanks for the help!

Usually they just fail. crack it open and see if you have power going across them when you hit the power on and hit the starter switch.

 

[74M35A2]

The starter is built much more durable than the solenoids within the PCB. You could test for crank signal being sent to the starter, but with the age of these trucks, it really just makes sense to rebuild the PCB with 2 new solenoids if you want it to be reliable.

 

[MKD73]

Cut my PCB open today. I just want to confirm the top solenoid is more than likely the power solenoid and the bottom (black, 4 post) solenoid is the starter solenoid? I get power across the top solenoid and when I have someone move the starter switch to the start position I get power across the two large lower solenoid posts.

I have moved the shift lever to confirm the neutral safety switch is working.

Am I missing something with the PCB? The batteries all test in the middle 12 volts and seem to load test fine, (with my harbor freight battery load tester). Could one 'weak' battery be causing all this trouble? Like I said they all test ok but they are older batteries. I didn't want to start replacing batteries that still have life left in them if there is something else I am missing.

Thanks again for your help!

 

[Suprman]

I had one of them new fancy southern automotive pcbs. Didn’t last very long. Only drove the truck a few times with it installed. Opened it up. Burnt traces on the board. Has a non military, low quality look inside. I will stick with rebuilding the older ones from now on.