M939 ABS ECU failure point

[RustyM923]

Thank you, Will.
I'll keep an eye out for one (or one to borrow)
-Tom

 

[trooper632]

Where are the fuses located for the ABS

 

[M35A2-AZ]

Where are the fuses located for the ABS

They are under the drivers side dash and are inline. 15a and 3a

 

[TexAndy]

Where is the ABS ECU located? I can't find it anywhere in the 4 volume TM pdf.

It's not the same thing as the Protective control box (PCB) is it?

I'm having the same symptom. 15A fuse under dash blowing as soon as I turn the switch, ABS light constant on.


eta: Well, they're definitely not the same thing. Went ahead and took the PCB out and took the cover off. It's just a couple of big fuses and little fuses.

 

[trooper632]

My ABS light stays on, but the fuses are good

 

[M35A2-AZ]

Where is the ABS ECU located? I can't find it anywhere in the 4 volume TM pdf.

It's not the same thing as the Protective control box (PCB) is it?

I'm having the same symptom. 15A fuse under dash blowing as soon as I turn the switch, ABS light constant on.


eta: Well, they're definitely not the same thing. Went ahead and took the PCB out and took the cover off. It's just a couple of big fuses and little fuses.

The ABS ECU is up above the center axle on the relay assy.

 

[TexAndy]

The ABS ECU is up above the center axle on the relay assy.

Thanks, I don't think I would have ever found it. I was going crazy trying to find it in that mess under the dash with flashlight.

Off to go look again...

 

[TexAndy]

Ok, there is something to the original post in this thread.

Before doing anything:

My ABS light was always on
Would blow the 15 amp fuse under dash every time
No self check clicks

After pulling the ECU and clipping the 5kp33ca diode in the picture:

ABS light still always on
No longer blows the 15 amp fuse under dash
It now does the two series of self check clicks when I turn it on.

I'm going to desolder the old diode from the board, then order a replacement diode from mouser and install it and see if that fixes the light.

 

[Suprman]

The diode is simply for protection. The abs ecu should have a voltage regulator before it this would better protect the ecu. The light can be on with a stored fault code you need a reader to clear it. But you cannot expect the abs to function properly with the light on. I have new abs ecu's coming in soon they are easy to swap.
Will

 

[TexAndy]

I would buy another ECU, but I just don't have the money.

Is there really no way to clear an error code on these things without the reader tool from haldex?

 

[Suprman]

Not that I know of. It may clear itself over time. I only know a little bit from playing around with the system on my trucks. The mil alternators are very unreliable and the abs ecu is very voltage sensitive it's a bad combination. A good voltage regulator would solve the issue there are cheap ones available that can take a wide range voltage input and put out a stable 12 or 24 v
Will

 

[TexAndy]

Thanks, I will look into doing exactly that.

I'm sure it was an alternator issue. I just replaced the voltage regulator on mine. Old one had VERY burnt out diodes on it.

 

[Suprman]

I have replaced regulators on all my trucks at one time or another. I had one alternator practically melt down.

 

[TexAndy]

Not surprising. When this one fails, I'll probably just go with a commercial AC delco alternator.

Slightly related, you can make a Prestolite regulator work with a Hellas alternator. Turn down the tongue on the Prestolite regulator so it'll fit into the Hellas body and just hook up pins 1,2, and 5. Ignore pin 4. It apparently works just fine.

 

[Suprman]

I personally don't like the Hellas alternators. I had one in my tractor I swapped it with another truck I had before I sold the other truck. After replacing the regulator that is.

 

[TexAndy]

There is a muni airport just down the road. I might see if they'll let me use the runway to test my ABS.

 

[Suprman]

Be careful. If the abs does not kick in, in a slide condition, with your foot on the brakes and the wheels not turning the drivetrain can stall the engine before the torque converter unlocks. If this happens you will loose power steering.

 

[TexAndy]

Will do, Thanks.

That's why I was thinking about doing this at the airport. No ditches or fences to run into if I lose power steering. Just clear grass field on either side.

 

[Suprman]

I would still be very careful trying it

 

[TexAndy]

The diagnostic tool for the ABS on these trucks is available in europe from a few vendors, but it's a bit expensive. In the neighborhood of 350 bucks.

Saw an old ebay listing of one for 40, tho.

I'm curious what trucks in europe use the same ECU or compatible ABS parts as are on the 939 series trucks.