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LMTV Abs troubleshooting

GeneralDisorder

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PGN is Parameter Group Number 65226. See the "(0)" at the end? That means the response to the DM1 PGN 65226 request was zero. In other words there are no active faults.

Check powers and grounds to the ABS module.
 

Lostchain

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Also, I have never been able to get wabco toolbox to connect on 1939, I always have to use 1708 …. For what it’s worth
 

GeneralDisorder

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Also, I have never been able to get wabco toolbox to connect on 1939, I always have to use 1708 …. For what it’s worth
Same. Odd since it's connected to the bus but yeah even on the new A1P2 trucks I haven't got it to work with J1939. Doesn't seem to be an issue though for troubleshooting so I haven't bothered to look into it.
 

aw113sgte

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Well can talk to abs ECU, but abs light stays on entire time of the drive. After I checked what the codes were....nothing active and nothing logged. Seems...strange. Places to look next?
 

GeneralDisorder

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Well can talk to abs ECU, but abs light stays on entire time of the drive. After I checked what the codes were....nothing active and nothing logged. Seems...strange. Places to look next?
Does the software say the light is on? Do the wheel speeds read correctly while driving?

I have noticed that you have to power cycle the unit after you clear codes to get the light to go out.
 

aw113sgte

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Does the software say the light is on? Do the wheel speeds read correctly while driving?

I have noticed that you have to power cycle the unit after you clear codes to get the light to go out.
It does say the light is on, it has been power cycled since since clearing codes. I haven't had the chance to check those readings while driving.
I tried turning off the abs light in software but it stayed on.

Looks like the lamp is controlled by a relay and for that light to turn off it needs to be pulled low by the ABS ECU. I'll check some voltages at that relay.
Does the ABS normally stay on till you're driving a few miles an hour or is it normally off after startup?

1699010498969.png
 
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GeneralDisorder

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It is normally off but it has weird behavior after clearing codes. I've experienced clearing codes and then the ABS light still not going out till you drive a few MPH then it goes out. I recall having to play with it after fixing a dirty wheel speed sensor and it wasn't straightforward how to get the light to turn off. But it did sort itself out.

I did recently do some ABS light troubleshooting on an A1P2 and that truck (2021 model built by Oshkosh - 06/21 🙄 ) turned out to have a poor ground from the chassis to the cab - they hadn't cleaned off the paint on the cab mount for the ground strap and it had about a 1.5v - 3v drop between the cab and the chassis. The ABS computer was running but was throwing a circuit code for the retarder relay control. That resolved on it's own once the ground was fixed.
 

Lostchain

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It is normally off but it has weird behavior after clearing codes. I've experienced clearing codes and then the ABS light still not going out till you drive a few MPH then it goes out. I recall having to play with it after fixing a dirty wheel speed sensor and it wasn't straightforward how to get the light to turn off. But it did sort itself out.

I did recently do some ABS light troubleshooting on an A1P2 and that truck (2021 model built by Oshkosh - 06/21 🙄 ) turned out to have a poor ground from the chassis to the cab - they hadn't cleaned off the paint on the cab mount for the ground strap and it had about a 1.5v - 3v drop between the cab and the chassis. The ABS computer was running but was throwing a circuit code for the retarder relay control. That resolved on it's own once the ground was fixed.
From the WABCO manual:

1699062334958.png

When I clear codes, light doesn't turn off until I drive the truck a little bit, usually by the time I reverse it out of the shop
 

aw113sgte

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Took it out and all speed sensors working fine. No codes except right after cranking the engine I got a fault code that then went away. I'm guessing just from the voltage drop, but maybe not?
I did try engaging the ABS on some gravel but I was only going about 10 mph or so so maybe it's below the ABS threshold. The rear tires just locked and skidded.
I tried disengaging the ABS light in the Wabco software, it claimed it did it but there was no tripping of the relay, and the line from the ABS ECU did not pull to ground as it supposedly should.20231104_101617.jpg20231104_104810.jpg20231104_101908.jpg
 
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GeneralDisorder

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Yeah I looked at that, but since it only popped up during cranking (low voltage condition) I wasn't giving it much trust. There was no memorized or active flag either. Considering I don't have any other things to test though, it's next on my list
The ABS computer seems to be especially sensitive to low voltage conditions. And as @Lostchain pointed out the manual does indicate that many fault codes require an ignition power cycle to clear - so you may not be able to clear the code since it will return after the ignition cycle. Sounds to me like you need to address the low voltage during cranking. Insufficient battery, connection, or too much starter draw?
 

aw113sgte

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The ABS computer seems to be especially sensitive to low voltage conditions. And as @Lostchain pointed out the manual does indicate that many fault codes require an ignition power cycle to clear - so you may not be able to clear the code since it will return after the ignition cycle. Sounds to me like you need to address the low voltage during cranking. Insufficient battery, connection, or too much starter draw?
Not sure there actually is a "condition" just that the most voltage sag will be during cranking. It has 4 fresh batteries, all measuring well voltage wise. Truck cranks quickly. Obviously that's not an all-inclusive test of battery, grounds, etc. I'll look into this as well though.
 

GeneralDisorder

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Not sure there actually is a "condition" just that the most voltage sag will be during cranking. It has 4 fresh batteries, all measuring well voltage wise. Truck cranks quickly. Obviously that's not an all-inclusive test of battery, grounds, etc. I'll look into this as well though.
The starter is cabled directly to the batteries. The ABS computer gets it's power from the PDP which is on the other side of the LBCD, the power distribution studs under the PDP, through CB's and then gets it's ground through the cab which is cabled to the chassis. It was also the case with the A1P2 that I was recently troubleshooting the ABS light on that it cranked great and had new AGM batteries. The issue was the cab to chassis ground that was dropping 1.5v to 3v due to high resistance through the paint they didn't grind off. You can't judge the voltage at the ABS computer by the state of battery charge or how great the starter sounds. You do have a "condition" since there should not be sufficient voltage sag during cranking to cause the ABS computer any issues. My guess is that it's seeing less than 10v - possibly a lot less. But it's not that difficult to test and know exactly what the voltage is during cranking.
 

aw113sgte

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Checked resistance to all 4 modulators from the connectors at ABS, all right at 5.1-5.3 ohm so all in spec (4-9 ohm)

Checked resistance to ground from both pins, both at 0.2, but need to check voltage under load still.
 

GeneralDisorder

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Resistance tests rarely tell you there's a bad connection because the meter is using it's own voltage and tiny current to test the resistance. At those small voltages and currents the poor connection will probably seem fine. 0.2 Ohms could just as easily be the zero on your meter. You must check under load - non loaded testing is pretty much meaningless for diagnostics of circuits that actually have to carry a load capable of doing actual physical work.
 

aw113sgte

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Resistance tests rarely tell you there's a bad connection because the meter is using it's own voltage and tiny current to test the resistance. At those small voltages and currents the poor connection will probably seem fine. 0.2 Ohms could just as easily be the zero on your meter. You must check under load - non loaded testing is pretty much meaningless for diagnostics of circuits that actually have to carry a load capable of doing actual physical work.
Yep, I've got to test the V=IR, but under load. This truck has me buying more and more tools. I'll have to see if my old oscilloscope can record with a starter trigger.
 

GeneralDisorder

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If you can back-probe the ABS computer while cranking you can watch the voltage on your meter. I mean there's lots of things I would consider pulling out one of my half dozen oscilloscopes for but simple voltage drop tests I wouldn't bother. If you can't back probe the ABS controller you could carefully front probe it with an automotive light bulb and socket for a simple visual test. If it dims significantly when cranking you likely have an issue. I would start by watching the voltage at the distribution studs under the PDP while cranking using both a cab ground and then double checking using the ground bolt on the alternator.
 
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