M939 PCB-ABS Relationship?

[74M35A2]

My ABS ECU is dead from improper shutdown. I was habitually dropping both switches to off, and would notice the volt gauge swing to red as the engine was coasting down, and then the gauge would bottom out to off. ABS 15A fuse blows as soon as replaced and battery switch is flipped on. Have ordered a new replacement ABS ECU from SUPRMAN after following those threads.

Not wanting to blow the replacement ABS ECU, I am considering to add some type of voltage limiting device. But, perhaps the PCB is supposed to be doing this, or controlling connectivity if improperly shut down? I say this because I suspect my PCB to be partially faulty somehow as I can re-engage the starter with the truck running. The truck does start, run, and all gauges work correctly. ABS did also until recently.

So I am suspecting my PCB to be somehow not functioning correct. Also it is a very high probability of improper shut down in the field, so I am assuming the PCB would normally protect against this, but mine may not be working correctly?

In summation, I am wondering if there is a known relationship between proper PCB function and protection of the ABS ECU during an improper shutdown sequence?

 

[wheelspinner]

The systems are grounded together at the mounting stud of the PCB. Faulty shutdown can trace back to the ABS ECU.

 

[The HUlk]

I can re-engage the starter with the truck running. The truck does start, run, and all gauges work correctly. ABS did also until recently.

So I am suspecting my PCB to be somehow not functioning correct. Also it is a very high probability of improper shut down in the field, so I am assuming the PCB would normally protect against this, but mine may not be working correctly

From what I undetstand the PCB disables the starter solenoid based on a signal from the alternator. A missing alternator signal or disconnection may cause this. Check for voltage on wire 566 PIN F on the PCB when running. Not sure if it's AC or DC.

No experience but folks have told me to shut down the engine, then the battery switch or damage to the PCB is likely.

 

[Jason O]

I don't see what the PCB does to protect the ABS unit or any other device during an improper shutdown. The way the truck is wired, its the PCB that allows the operator to create this condition. (Alternator keeps supplying "garbage" power as the engine coasts to a stop, w/o the batteries in the circuit acting as a buffer) I wonder why the truck was not designed with the alternator connected to the "C" side of the PCB (with the batteries). That way, when the "power" switch was shut off C-D contact would open, resulting in a clean break in voltage at D.

 

[74M35A2]

So, it seems even if I were to install a brand new PCB and ABS ECU, the PCB does not protect the ABS ECU from improper shut down spikes, and this improper procedure is said to be able to harm the PCB as well.

I'm not installing a brand new ABS ECU without adding protection, this is not happening again.

 

[74M35A2]

Jason, exactly. Is the alternator main charging output connected through the PCB, or just the energize signal for the alternator, as well as an AC feedback line to tell the PCB that the engine is running?

Also supposedly so has a timer and oil pressure circuit in it? I replaced my electric oil pressure gauge with a mechanical one, and nothing changed with the running of the engine, so either that part of the PCB is shot or it uses a pressure switch, not sensor, to detect oil pressure.

The entire PCB concept seems corrupt anyway. As long as batteries are installed correctly, you have oil in the truck, and you don't hit the starter once the truck is running, what would be the reason to have this failure prone part?

Why does it have a 12v lead into it? I thought that was only used for the heater blower low speed?

I am assuming one relay is for main power, and the other one is for the starter solenoid.

Sorry for repeat questions, there does not seem to be an all in one posted on this silly box.

Sounds like maybe the alternator output should be moved to the other side of the relay as you suggest Jason, or just screw it to the back of the starter B+ terminal like today's big rigs are wired.

 

[The HUlk]

I don't see what the PCB does to protect the ABS unit or any other device during an improper shutdown. The way the truck is wired, its the PCB that allows the operator to create this condition. (Alternator keeps supplying "garbage" power as the engine coasts to a stop, w/o the batteries in the circuit acting as a buffer) I wonder why the truck was not designed with the alternator connected to the "C" side of the PCB (with the batteries). That way, when the "power" switch was shut off C-D contact would open, resulting in a clean break in voltage at D.

My guess is that when the battery switch is turned off while the engine is rotating the PCB circuit draws high voltage from the alternator because the ballast effect of the batteries is taken away and perhaps deactivation of the voltage regulator. That is my guess as to why the voltage guage goes red then falls out as the alternator slows.

 

[The HUlk]

Jason, exactly. Is the alternator main charging output connected through the PCB, or just the energize signal for the alternator, as well as an AC feedback line to tell the PCB that the engine is running?

Also supposedly so has a timer and oil pressure circuit in it? I replaced my electric oil pressure gauge with a mechanical one, and nothing changed with the running of the engine, so either that part of the PCB is shot or it uses a pressure switch, not sensor, to detect oil pressure.

Is this an A1? Which alternator?

 

[74M35A2]

It may be because if this alternator needs a remote source line, and this line drops to 0V during an incorrect shut down (drop batt switch first) so that drives the alternator to increase its output.

I'm a starter and alternator guy, so if that is the case, we are changing that in a hurry.

 

[74M35A2]

M925A2, stock alt. I have not studied the diagrams enough yet (at all, really).

 

[The HUlk]

M925A2, stock alt. I have not studied the diagrams enough yet (at all, really).

Wire 568 to the alternator is a voltage reference which tells it to excite based on voltage. I'm guessing you're correct that turning off the battery drops this reference and the alternator goes full bore while winding down.

 

[Jason O]

I've been trying to post a screen shot of the diagram from the P2P program with no sucess...

I'm thinking #568 is the volt reference (spliced to #5 and "D" on PCB)

#566 goes to "F" on PCB. I think this is the starter "overrun" circ that some PCB boxes have. That feature makes starting a cold engine that needs cranking as it begins to "catch" and sputter more difficult. It cuts the starter out too quickly.

Could one of you guys post the schematic?

 

[The HUlk]

Could one of you guys post the schematic?

I meant 568, sorry for any confusion