M931 blown electrical parts - I messed up and would like help

Hello all,

I have been a member for a little over a year here. I really appreciate all the help from forum topics that have been posted.

I own an M923, 929, and 931. I went a little crazy and bought three 5 tons within a few months! I also have a Unimog FLU419.

Let me start off with some background. I am an electrical engineer but I also like the mechanical side of wrenching. I have read all the TM's related to this problem and I have not been able to find what the issue could be by reading them and looking at the schematics.

I did something really stupid this weekend while warming up my M931. I started it, let it run for a bit, then turned off the battery disconnect but didn't shut the engine off for a bit. Within about 15 seconds, I saw smoke behind the amp gauge, and then POP. The gauge blew and all the dash indicator lights, buzzer, and any electric devices went out. I believe the ABS light still came on.

If I am not mistaken, by reading the TM's, the only circuit protection are two self-resetting circuit breakers in the cab, and some fuses in the ABS control circuit. Since the ABS light came on (and it was on before this event), I assume those fuses have nothing to do with the issue. Are the circuit breakers truly self-resetting?

One final piece of information - the engine shuts off now if the battery disconnect is turned off. It obviously didn't use to prior to this event. I assume that means a solenoid of some sort is closing off the fuel supply when battery power is cut.

I have learned my lesson and will not do this again. I did not know it would cause an issue, but it makes sense to my why it does now - removing the batteries from the alternator charging circuit.

Any help in getting this rectified would be appreciated because I am not seeing what circuit protection would cause this.. unless I melted some wires...

Thanks,

Dan Gilfillen

Ouch. What happened is, when you disconnected the batteries with the switch, the alternator no longer had reference voltage and it ran away. Likely the protective control box has burned up components inside. There is a thread here about repairing it.

NDT said:

Ouch. What happened is, when you disconnected the batteries with the switch, the alternator no longer had reference voltage and it ran away. Likely the protective control box has burned up components inside. There is a thread here about repairing it.

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Here you go ;

https://www.steelsoldiers.com/showt...-Box/page3&highlight=power+control+box+repair

Post #21

This can and does happen a lot, and there is no good reason for it to be this way.

Above posts are correct. Strongly suggest to switch to a modern civilian alternator, as it eliminates this from being able to happen again.

This is in my top 3 design flaws of these trucks:
1. Self-destructing alternators
2. Torque rod end links that fall off
3. Transfer case destruction if lo-reverse used

Update...

Hello again all. I’ve been super busy at work as well as getting some vacation cabins ready to rent.

I finally was able to replace the PCB with a known-working unit. The results are the same. Also, I tested both circuit breakers on the engine side firewall and both are OK.

I just double-checked the TM and it appears there are also 2 circuit breakers under the steering column area. I’ll verify both of those are OK when I am back near the truck.

It appears the failsafe control module is what is causing me to have no indicator lights or warning buzzer when there is insufficient air brake pressure. I have 3 trucks so I can swap the module and verify.

Does anyone have other ideas as to what the issue could be? Please note I have lost all indicating lights except the FRONT AXLE IN green light. The lower temperature gauge does work but the upper one does not and neither does the voltmeter.

Please also note that if I turn off the battery disconnect even for a split second (and turn it back on) - the engine stalls and will not keep running even if the disconnect is turned back on. The engine will, however, start right back up.

I hope these troubleshooting steps help.

Thanks,

dgilfillen

dgilfillen said:

Please also note that if I turn off the battery disconnect even for a split second (and turn it back on) - the engine stalls and will not keep running even if the disconnect is turned back on...

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First, you need to stop doing this!

Second, did you check the bulbs and make sure your not getting power, AND ground in the sockets?

Hummmm, not trying to be a smart butt, BUT i assume the trucks owner has and is proficient with a PSM 6 ( multi meter) start at the batteries test everything from there on, USE the t.m. T.O. to me, its a pretty simple system Did i read that he had changed the PCB ? wondering if he melted a ground off when the short occurred ?

Jbulach said:

First, you need to stop doing this!

Second, did you check the bulbs and make sure your not getting power, AND ground in the sockets?

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I really wish this guy was my brother. Holiday dinners would be amazing.

No offense taken. I did something dumb in the first place to cause the problem so I’ll put in the sweat to fix it.

I am proficient with a multimeter. I have looked over the schematic for this area. It has to be either one main ground problem or the failsafe module - because some devices have a working wire 27 and others don’t.

I’ll keep folks posted as to what I find. Thanks again for the help!

Jericho said:

... Did i read that he had changed the PCB ?...

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Yes, but then he did an improper shut down again, as some sort of trouble shooting, with his know good PCB.