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JFN1
JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 4413
Experience:  GM Master Technician
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GM 3500 Duramax: C0055 (rear wheel speed sensor circuit)abs

Customer Question

C0055 (rear wheel speed sensor circuit)abs and brake light on, speedo drops off every few minutes for split second then goes back into position although it consistently reads 7 mph off correct mph.
Submitted: 1 year ago.
Category: GMC
Expert:  JFN1 replied 1 year ago.
My first question: is this a new issue that started happening for no reason, or did you purchase the truck used and inherit this issue or something like that?
Customer: replied 1 year ago.
purchased the truck with the lights, no history of how long they've been on.
Expert:  JFN1 replied 1 year ago.
My first concern is that the truck has been "tuned" which means someone got in there with a computer and started tweaking the calibration around. It's very easy to make the speedo act up when you do this, as well as cause all sorts of other issues. Let's assume, for the moment, that this isn't the case, and that there is something more normal wrong with the truck. Do you have any engine or transmission codes relative to the speed sensor circuit?
Customer: replied 1 year ago.
no other dash lights, the only issue is the one mentioned truck runs and drives perfect
Expert:  JFN1 replied 1 year ago.
OK. The reason I ask is that the VSS signal is handled by the ECM, then ECM feeds this signal to the ABS module over a discrete circuit. However, if the speedo is also dropping out, the ECM also generates this signal and feeds it to the speedo. If the circuit from the VSS is touching ground anywhere, the signal drops. I'd want to verify that this signal also drops when watching the scan tool under engine data, I'd just want to see it happen in real time to make sure I was after the right component. Then, if wiring and all that is OK, this will apparently be an ECM internal issue.
Customer: replied 1 year ago.
So if the vss signal drops when the speedo needle drops during the drive with the scan tool this will tell me the the problem most definitely lies in the ecm? is there a way to test if the reluctor wheel inside the transfer case is sending intermittent poor signals to the VSS?
Expert:  JFN1 replied 1 year ago.
A reluctor wheel doesn't have the ability to send an intermittent poor signal, it's just a piece of metal with teeth machined into it. There's nothing to go wrong, unless that wheel is loose or something obvious like that, but that doesn't sound like what's going on here. If you have a higher end scan tool, there is a suffix that goes with this. It will look like C0055 5F, or something like that. Do you have the suffix for your trouble code, by any chance? It will tell us more precisely what the computer is seeing wrong with that circuit.
Customer: replied 1 year ago.
just scanned the truck this is the exact message Rear wheel speed sensor circuit fault signal amplitude less than min. (Symptom 18) - current dtc, test not passed since dtc cleared, test failed since dtc cleared, history dtc, test not passed since power up.
Expert:  JFN1 replied 1 year ago.
Very good, that's what I was looking for. This suffix tells us that there's a problem with signal quality, not a missing signal necessarily, but that the amplitude isn't sufficient or something like that. Here's a snippet: DTC C0055 18This DTC may indicate a missing pulse signal or mechanical fault, refer to the Diagnostic Fault Information table for possible mechanical faults or conditions. Check for DTCs in the ECM or internal fault in the ECM or EBCM. If the air gap between the sensor and the reluctor was excessive, you might get this. Just make sure the sensor is installed all the way, and that there's not dirt or debris holding it away from the reluctor. If there isn't a mechanical issue, it's telling us to go looking at the ECM. If this were in my shop, I'd have a lab scope on that signal, to see what's happening at the moment it sets the code. If there's nothing wrong with the input signal, I would suspect faulty ECM.
Customer: replied 1 year ago.
What I do have is the same exact truck that I can swap components off of to determine the weakest link. With this being said can you give me the order that you would try swapping from best chances of fixing the problem to the least.
Expert:  JFN1 replied 1 year ago.
You've already swapped the VSS and the wiring. You cannot directly swap ECM's from one vehicle to another, due to a variety of programming issues, unless you are equipped to deal with these programming issues. I would be at this with a scan tool, like I said before, monitoring the speed signal very carefully, even graphing it on the scan tool, so I could capture the moment that the code sets, and see what the computer is seeing. It's not a simple matter of swapping parts, this really needs to be pinpointed to a particular system (probably the ECM), then checked to make sure there isn't a grounded circuit or some other cause, followed by replacement of that component and the requisite programming that comes with it (ABS, ECM, TCM, etc. all requires programming when replaced). If you've done everything you say you've done, and you're convinced that it's not the VSS circuit, it seems like an ECM is what your problem is. But I would do a lot more diagnostic work with the scan tool and lab scope before condemning it, unless you just want to take an educated guess at it and try an ECM.

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