Semi/Trailer ABS diagnosis

[dave]

There are an elite few in our shop with the ability to tackle ABS issues on tractors / trailers. Although I have never specifically been trained in the diesel world on ABS, I have taken what I have learned from automotive and have applied the same principles as far as diagnosis. I just have a couple of questions for the select few on this forum who might be trained specifically on ‘advanced’ tractor/trailer break systems:

Q: How do I pull ABS codes from a Mac?

Q: How do I pull ABS codes from a trailer?

Q: Does the tractor control ABS for the trailer?

Q: Can the tractor induce an ABS light for the trailer?

Up until this point, I have proceeded with ABS faults in a similar manner to how I would handle these issues in the automotive world; I would usually pull the harness from the ABS module and proceed to read AC volts from each tire as they were spun by hand. Not as easy to do in the big diesel world. I am open to suggestions.

[edit]: actually I just found a good link that answers a lot of my questions: http://www.fmcsa.dot.gov/documents/fhwa-abs.pdf

Topic

[college man]

I’m not a big rig guy. But great link. 😉

[Ryan]

for tractors there are a few options. To pull info out of the diagnostic port, i know bendix makes a plug in type light tester that will kind of lead you in the right direction. I dont think a tractor can induce a fault in the trailer, unless there is a bad connection in the 7-con plug going into the trailer. Most commonly in the trailer, ive found a broken corroded wire somewhere underneath in the factory wiring and factory connectors. As for the wheel sensors, you need to check the resistance with an ohmmeter. Its really impedence, but ohmmeter will read it. You dont have to spin the wheel to check. There should be a plug connection right by the brakes where the wheel sensor wire comes out, thats one place to check. Also to check from the abs computer to the sensor, just unplug it from the computer and check the ohms there. This goes for the tractors as well. I think that the impedence should be between like 900 and 2300 ohms for a good wheel sensor. If it needs to be replaced, then driving the trailer or tractor should reset the computer once it sees that the wheel sensors are working. Also, our shop has a tester called a “lite-check”. Thats the brand name anyways. That can read ABS faults both pending and stored, erase them, and tell you lots of info. Even tell you how many miles the trailer has gone with the abs powered up. Hope this helps.
ryan

[dave]

First, thank you for answering all my questions in depth, very cool i must say. Therefore, I have not only thanked you but also gave you mad karma points.

I just got one more question: How does the trailer ABS module communicate with the tractor to illuminate the ABS light? I know at least 1 pin on the trailer electrical connector is dedicated to ABS module power… but I am not sure how exactly the ABS light gets illuminated via a trailer ABS fault.

[college man]

Thank you for the help Ryan. 🙂

[Ryan]

I did forget get to mention a few things i guess. The tractor should have a power source for the trailer abs. We run freightliner cascadias and columbias, and they have a 30 amp fuse for trailer abs. If that fuse pops due to shorting,grounding,etc, that will cause the trailer abs to come on. How the tractor reads the abs computer, i think its done through pulses. the tractor and trailer computers “talk” by sending and receiving pulses to one another. Some of the plug in abs testers for trucks and trailers will pulse lights in the tester that you have to count and figure out what it means. Kinda like eric’s video on pulling those honda codes without a scanner. The trailer abs doesnt have its own power source, so thats why the lone plug in the cord is there, and why the trailer abs will cycle whenever its turned on, or whenever you brake, or when there is a fault. Like you said, there is kind of a mystery in figuring that abs stuff out, and like you, i got curious and just got to looking for answers.keep em comin and i’ll try to help if i can
ryan

[dave]

awesome. B)

[Author]

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