M939 ABS ECU failure point

[jarhead1086]

My ABS light stays on, but the fuses are good

I think I am going to have at least one extra ABS ECU when I am done getting mine working. You could try it to troubleshoot and buy it and keep it if it works. My alternator works fine, but it had electrical wire splices when I got it, so I am keeping eye on it. My voltage is steady for now. I have my safety due this month and want all this important stuff in order. Thanks again to everyone's input on this.

 

[Suprman]

For 350 it would be cheaper to swap out the ecu for a new one.

 

[TexAndy]

Not to dig this grave up too much, but I went ahead and soldered in a new diode on my ECU and put it back on the truck. I actually desoldered the old diode and pulled it out of the board completely and soldered a new one in.

I'm guessing I got the polarity on the diode right. I get the the ABS self-check clicks, the 15a fuse doesn't blow (like it did when the original diode was installed on the ECU) but the light does not go out. I'm guessing if I had got the diode backwards, it would act just like the dead diode would?

Just for grins, I looked up the diagnostic scan tool on ebay and managed to snag one reasonably cheaply. I'm guessing there's not a ton of demand for these scan tools. So next step, I'll try hooking that up and seeing if I can clear the light that way.

 

[TexAndy]

Well, I got my info center.

Plugged it in and first fault is an S2B 04. Which translates as "2B Sensor/wiring open or short circuit." The manual lists possible causes as:

Sensor failures, sensor wiring open or short circuit. Sensor resistance should be 980 to 2350 ohms. Verify wire ties on sensor cables connected to air hoses are not too tight. Excessive grease may prevent secure sensor connections.

Other stored faults were
ECU 99 = "Internal ECU fault" Most likely the dead diode inside the ECU, which I replaced.

B+HI 92 = "Supply voltage at the ECU greater than 33V" .What most likely killed the ECU voltage protection diode. My truck had a dead alternator with a burned up voltage regulator when I got it

ISO 91 = "Faulty supply or 15amp fuse blown (Incorrect power sequence)" This one is self-explanatory.

SLW 43 = "Slow recovery, yellow channel" The possible causes listed in the manual are "Slow brake release, foundation brake mechanical faults, dry bearings, broken spring, restricted piping. Modulator fault. Check for kinks and blockages, etc. Incorrect piping or sensor wiring crossed. Inspect exciter ring for damage. Failed primary reservoir."

SLW 42 see above

After clearing the stored faults, the only one that shows up is S2B 04.

So the good news, I think, is that replacing the voltage protection diode "fixed" the ECU. The bad news, is that there are faults in the system unrelated to that. And I have no idea what the wheel sensors even look like or what the wiring is supposed to look like or where they may be found. But at least I know what terms to use in researching the problem, thanks to the scan tool.

All in all, I think it's a good idea to have access to one of these scanner tools if you run into problems with your ABS. Yes, you may have faults completely related to the ECU. Then again, you may have other faults.

 

[74M35A2]

ABS sensors are just very simple VR (Variable Reluctance) sensors, which, are just coils of wire. You should be able to measure that channel’s resistance at the ECU connector, and know if the problem lies on in the ECU circuitry or the harness/sensor.

 

[TexAndy]

ABS sensors are just very simple VR (Variable Reluctance) sensors, which, are just coils of wire. You should be able to measure that channel’s resistance at the ECU connector, and know if the problem lies on in the ECU circuitry or the harness/sensor.

Awesome. Pretend (not difficult) that I'm a wee slow. Where do I stick the pointy metal probes?

Reading a little further, I saw here https://www.steelsoldiers.com/showthread.php?79389-M923A1-Abs-Light/page3 where 2B is reported to be passenger side, rearmost axle. Tracing where the sensor cable goes into the wheel, I noticed where the cable was rubbing up against metal. The insulation was worn all the way through, all the way around.





eta: on closer inspection, the wire underneath the plastic conduit is completely severed. I'm guessing that's my problem. Will try stripping the insulation back and splicing it back together and see if that fixes it.

 

[TechnoWeenie]

Well, I got my info center.

Plugged it in and first fault is an S2B 04. Which translates as "2B Sensor/wiring open or short circuit." The manual lists possible causes as:

Sensor failures, sensor wiring open or short circuit. Sensor resistance should be 980 to 2350 ohms. Verify wire ties on sensor cables connected to air hoses are not too tight. Excessive grease may prevent secure sensor connections.

Other stored faults were
ECU 99 = "Internal ECU fault" Most likely the dead diode inside the ECU, which I replaced.

B+HI 92 = "Supply voltage at the ECU greater than 33V" .What most likely killed the ECU voltage protection diode. My truck had a dead alternator with a burned up voltage regulator when I got it

ISO 91 = "Faulty supply or 15amp fuse blown (Incorrect power sequence)" This one is self-explanatory.

SLW 43 = "Slow recovery, yellow channel" The possible causes listed in the manual are "Slow brake release, foundation brake mechanical faults, dry bearings, broken spring, restricted piping. Modulator fault. Check for kinks and blockages, etc. Incorrect piping or sensor wiring crossed. Inspect exciter ring for damage. Failed primary reservoir."

SLW 42 see above

After clearing the stored faults, the only one that shows up is S2B 04.

So the good news, I think, is that replacing the voltage protection diode "fixed" the ECU. The bad news, is that there are faults in the system unrelated to that. And I have no idea what the wheel sensors even look like or what the wiring is supposed to look like or where they may be found. But at least I know what terms to use in researching the problem, thanks to the scan tool.

All in all, I think it's a good idea to have access to one of these scanner tools if you run into problems with your ABS. Yes, you may have faults completely related to the ECU. Then again, you may have other faults.

I'm necroing this post with a slight correction.

A=curb side (passenger)
B=Road side (driver)

S2b 04 would indicate drivers side sensor fault, not passenger.


Just got the Haldex reader.... exact same issue here.... all the stored codes, cleared them all and S2B 04 is the remaining code.

I'll do what you did and search for cut wires.

 

[TexAndy]

I'm necroing this post with a slight correction.

A=curb side (passenger)
B=Road side (driver)

S2b 04 would indicate drivers side sensor fault, not passenger.


Just got the Haldex reader.... exact same issue here.... all the stored codes, cleared them all and S2B 04 is the remaining code.

I'll do what you did and search for cut wires.

Its been so long since I messed with this, I'll have to go back and look.