M939 ABS ECU failure point

My alternator/generator went bad and was making 30+ volts and blew my abs ecu. I replaced the alternator and then the ecu and all is good. I opened the old ecu to see what was going on in there. There is a protection diode in the module it is a 5KP33CA. This type of diode is designed to protect electronic circuity from over voltage. This diode in my ecu is shorted. I have not tested the ecu with the shorted diode removed yet but I believe if your abs ecu is blowing fuses this diode is probably the culprit and in a pinch you may be able to open the ecu and clip out the diode and get the ecu working again. I make a red line going to the diode, it is clipped and upright, normally both ends would be soldered to the board.

awesome!!!!!!!

Another fine example of the knowledge and wisdom on this forum!! I kneel before the amount of info. that's here for the taking. Thank you to everyone that shares these facts. Regards--Allen(Old dog trying to learn)

Can it be replaced?

Possibly if you were very careful desoldering the old one and soldering in a new one. The board is double sided with surface mounted components on each side. The best bet for replacement would be to try and cut the old one out closest to the diodes body as possible and solder the new one onto the old ones leads coming off the board. In theory you could just cut out the bad one and put a similar protective component connecting it to the positive side right after the fuse under the steering column and finding a body ground.

That is good info. thanks!

Thanks for sharing the diode info. I just replaced a cut line to my velocity sensor while I had my hub and drum off. New sensor and cable to the connector. I put in a new fuse, since it was missing one when I got it, and now the ABS light stays on. They must have fixed it by removing the fuse to the annoying light? I will pull this out and look it over and smell for BBQ.

I had to replace ABS ECU on my M927. I will look into the old one to see if it failed the same way. If I fix it, I will keep it as a spare. Will post an update.

If the light is on and the fuse does not blow then the diode is not shorted. It will cause the fuse to blow every time you put a new fuse in and turn the power switch on. If you had a error like a disconnected sensor the light could be on due to a stored error code. This does not mean the abs is not operational.
Will

My problem was exactly that: light on and fuse blows every time.

Then the ecu diode is probably shorted. Open the box snip the diode out and then see if it works.
Will

Suprman said:

If the light is on and the fuse does not blow then the diode is not shorted. It will cause the fuse to blow every time you put a new fuse in and turn the power switch on. If you had a error like a disconnected sensor the light could be on due to a stored error code. This does not mean the abs is not operational.
Will

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I did not drive it yet, just ran it to build pressure. I hope it just needs to clear the error code like you said. I will cross my fingers and drive it to see if it goes off before I pull it out. I have had the battery cables off since then to trickle charge one at a time but that didn't clear it. I will post my fix to this tread when I get it. I want to learn any failure mode with these things before they happen. I may buy that diode to put in my spares. Thanks again.

Is there an easy way to clear error codes? Or read them for that matter?

Baradium said:

Is there an easy way to clear error codes? Or read them for that matter?

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You will need a Midland / haldex infocentre scan tool. They pop up on epay from time to time
http://www.haldex.com/en/North-Amer...ock-Braking-System-ABS1/Handheld-Info-Center/

juanprado said:

You will need a Midland / haldex infocentre scan tool. They pop up on epay from time to time
http://www.haldex.com/en/North-Amer...ock-Braking-System-ABS1/Handheld-Info-Center/

Click to expand...

Is the infocenter that you're referring to the AQ15855? Or is there a specific 24volt one that is needed for these trucks?

It is a specific tool

Suprman said:

It is a specific tool

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Is it more specific than the one they currently offer?

http://epi.hbsna.com/products/dept....4B7EB0158D62777735F4&dept_id=3916&parent_id=0

It has a different connector. If you look under the passenger side of the dash there is a large connector this is the ste ice diagnostic connector and to the left of that is a 4 pin connector thats the abs diag connector. Did you check both of the fuses?
Will

Suprman said:

It has a different connector. If you look under the passenger side of the dash there is a large connector this is the ste ice diagnostic connector and to the left of that is a 4 pin connector thats the abs diag connector. Did you check both of the fuses?
Will

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Hi Will, Thanks for walking me through this.

I did see that 4 pin connector under the passenger side. I think that newer Halden Info center (vs. Info "centre" that I see works on these M939's) has a 2 pin plug (no good for me).

I did check both fuses and they are both good. I replaced them anyway.

What I'm noticing is that my voltage from my alternator is bouncing around. It's going from 26.0v to 28.2v rather regularly (bouncing around).

When I turn on the battery, then put the start switch to "run", I do hear the 2 set of three clicks from the relays or solenoids under the truck (front, then rear, or maybe the other way...hard to tell from the cab). The light is solid and used to go off after about 3 seconds.

I've had it come on after one run (100 miles) before, but it went off before I parked it. This time, it's still on after coming on about 30 miles into a 65 mile trip. I've started and warmed it up several time, but it's still on. Nothing appears loose (connections that I've checked). I had pulled the rear hub about 150 miles ago for an unrelated problem and saw the exciter ring and sensor. They both looked clean on that corner.

I'd like to find one of those info "centre" models to rent or buy to get this light off. Any pointers on the common failure points? If I had another ECU, I'd swap it in to see if that's it, however the sets of clicks are still happening. I'm not sure if that's a positive for the ECU being alive.

Then there's the alternator....perhaps I should sort that out first....could be related...?

Usually high voltage will kill them. 26-28 volts should be ok. This is the correct model Haldex tool.
Will