ABS light on, No ABS codes?

[DieselAmateur]

As the title states, My ABS light is on, but when using GMTDScan Tech, the only DTCs I get are 46 and 88

I know DTC 88 is related to TDCO, that's another story for another thread. Thought a local mechanic with the correct scanner set the timing, but according to Scan Tech actual and desired timing are both at 11, and TDCO is zero. He said he'd look at it this weekend.

DTC 46 is malfunction circuit indicator lamp (MIL) fault. Despite much googling, I have yet to actually find how to address this issue. Where is the circuit located, or is it a general grounding issue?

I've checked all grounds, just put a brand new ground strap from engine to firewall.

No ABS- related fuses are blown

If the truck sits overnight, the ABS light will sometimes go away the next day, or randomly turn on halfway through the morning commute, or stay off until the drive home.

This is the first time I've had the issue since owning the truck, which has been since 2016

Front brakes were done 2 years ago, rear brakes completely redone (cylinder, backing plate, hardware, etc) two months ago. There's no brake fluid leaks. The front left wheel hub was also replaced about 2 months ago, but there's no code for any WSS/ABS sensor, so I don't think it's a sensor issue.

Interestingly enough, the light has yet to come on when towing my equipment trailer. Which leads me to think it's something of an electrical issue, as power is going to the trailer brakes and whatnot. But it's been going on for a few weeks now, and I'm stumped.

Any suggestions on where else to look/ troubleshoot are much appreciated!

 

[JMJNet]

Check the FAQ carefully, there is a way to read ABS code using paper clip by shorting a different set of pins on the ALDL connector.
I used to know how to do it but I forgot.
It is usually point to bad Wheel Speed Sensor but there are more codes.

 

[ak diesel driver]

It can also be from a wheel bearing starting to go bad and putting metal fuzz on the sensor

 

[DieselAmateur]

If the sensor goes bad, wouldn't it throw a code?

I had an alignment done when the snow tires came off, which was when the driver's side wheel bearing was found to be bad. I'll check the passenger side, but I imagine it would have been caught when doing tires and alignment, but still a possibility, thanks AK

I know about the paper clip trick, but will that give codes that the scantech software wont?

 

[DieselAmateur]

Found on the Place forum what you were referencing @JMJNet , jump pins A and H, H second from left on bottom, A top right. I'll try that and see if anything different comes up.

Any idea if its related to the MIL code? Anyone know how to resolve that one?

 

[ak diesel driver]

pull the sensor to check for fuzz, the bearing can still "feel" good

 

[JMJNet]

Found on the Place forum what you were referencing @JMJNet , jump pins A and H, H second from left on bottom, A top right. I'll try that and see if anything different comes up.

Any idea if its related to the MIL code? Anyone know how to resolve that one?

Then you have to count the blinking ABS light.
There is a code list somewhere on the manual which is probably stored as PDF.
Otherwise, go to DieselPlace, they have all the PDF of manual.

The code will indicate which side is bad.

 

[DieselAmateur]

I did the paper clip trick, and am only now more confused. I got codes 25, 26, and 27, all of which apply to the accelerator pedal position, and seem to contradict each other. DTC 25 is APP circuit high, DTC 26 is APP circuit low, and DTC 27 is APP circuit out of range. I would think that these codes would cause the service throttle soon light to go on, not the ABS?

Also peturbed that the $160 I spent on GMTDScan Tech isn't showing these codes, but the paper clip is. What good is the software if it can't do the work of a darn paper clip?

 

[JMJNet]

Sorry for linking a thread from another forum:

ABS Codes /Clearing Codes on 95 6.5L

www.dieselplace.com

Your code for ABS is in Post #1.
Sounds like you are using the regular DTC code for interpretation instead of ABS code.
Those set of codes indicates the Left Front Wheel Speed sensor is bad.

There is no code reader (I know of) that can read ABS code for this truck.
Well may be the GM stealership code reader.
GMTDScan Tech can do other things like setting TDC for IP, etc.

For code reading in OBD-I, we just need the greatest engineering invention called "paper clip".

 

[DieselAmateur]

Thanks JMJNet, I realized this after I posted. Didn't know until now that there were separate DTC codes for ABS and everything else in ECM.

Apparently the Innova 3123 GM OBD 1 code reader can pull ABS codes. Found the PDF for it here, it has a complete list of ABS codes

Innova 3123 GM OBD1 Code Reader User Manual

Innova 3123 GM OBD1 Code Reader User Manual - Download as a PDF or view online for free

www.slideshare.net

Perhaps the ABS codes that pertain to the GMT400 trucks should be in the library/ sticky? A lot of the links in the technical reference library don't work anymore

The driver side (front left) wheel bearing was replaced ~2 months ago during alignment, so I'll have to call the shop and see if the sensor is under warranty. Gotta love the short life span of new parts!

If it helps anyone in the future, here's a list of all ABS codes


21 - right front speed sensor or circuit open.
22 - missing right front speed signal.
23 - erratic right front speed signal.
25 - left front speed sensor or circuit open.
26 - missing left front speed signal.
27 - erratic left front speed signal.
29 - simultaneous drop-out of front speed sensors.
35 - vehicle speed sensor or open circuit.
36 - missing vehicle speed sensor signal.
37 - erratic vehicle speed sensor signal.
38 - wheel speed error.
41 - 54 control valves
61 - 63 reset switches
65 - 66 open or shorted pump motor relay.
67 - open motor circuit or shorted BPMV output.
68 - locked motor or shorted motor circuit.
71 - 74 memory error.
81 - brake switch circuit shorted or open.
86 - shorted anti-lock indicator lamp.
87 - shorted brake warning lamp.

Thanks @JMJNet and @ak diesel driver for your timely help!

 

[Husker6.5]

Check for the obvious and see if the pigtail for the WSS is plugged into the body harness and that there is good electrical contact between the two. Then if that checks out ok, unplug the WSS and check its impedance.

 

[DieselAmateur]

I checked the connections and wiring, it all checked out

I have a multimeter, any idea what the resistance value should be?

 

[Husker6.5]

It should be a few Ohms. Zero would indicate a dead short, infinite resistance an open in the coil/lead. Iirc a reading of 3-4 Ohms is normal. If I get a chance I'll grab my Chassis manual and see if they have the resistance values in it.

 

[FellowTraveler]

I pull those ABS sensors to inject synthetic bearing grease since I had split the unit bearing housings to rebuild them many years ago used synthetic bearing grease to pack them.

 

[EWC]

Check for rust under the sensor mount . Enough rust will pull the sensor away from the ring .

 

[DieselAmateur]

The shop that did the wheel bearing warrantied the whole assembly as it was only 2 months old, so problem solved for the moment Chalk it up to cheap China parts/ electronics?

Brakes were still spongy when I picked the truck up after hours at the shop, pulled over 20 minutes later and the brake hose is leaking at the hard line! Torqued it down with a pair of vice grips and leak is gone. Don't people test drive a vehicle after it's repaired anymore? Sheesh.

Thanks to all who contributed to this thread, much appreciated!

 

[Husker6.5]

So I take it that they replaced the whole unit bearing assembly?

 

[DieselAmateur]

Yup, whole assembly was replaced. So far so good Now I'm just dealing with my radio going out when I hit a pothole which forces me to find the next pothole to try and bump it back on again But I'll take it over faulty brakes!