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Well I feel like a dumbo, one of the two ignition pins wasn't getting power due to a missing circuit breaker.
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.
Does the software say the light is on? Do the wheel speeds read correctly while driving?
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.
) 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: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
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?
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.
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.
