Saturday my wife called. The ABS light was on in her '00 Blazer. It was braking hard and the ABS was kicking in at inappropriate times. I told her to drive it home and take the '93 Jimmy to finish her errands. I pulled the following codes from her rig:

C0221
C0222
C0223
C0281

The first three are all concerning the right front wheel speed sensor. The fourth is from the brake light circuit. This seemed like a fairly straightforward diagnostic procedure: check the voltage output of the RF wheel speed sensor. I cleared the codes and then drove it from the house over to my garage. The ABS light didn't come back on. I got curious and decided to drive it around the yard a while. No light. I did several quick stops to activate the ABS. Everything worked fine and the light didn't come back on. I drove it around town a while, making several fast stops on wet pavement. The ABS still worked perfectly. I scanned it again and the codes did not return.

Is this an actual ABS sensor problem or could something else have happened in the electrical system to cause those codes to set? If this is really a problem with the ABS sensor is there anyway to verify it since it seems to be intermittent at best?

 

I have experienced the WTF ABS function on another GM vehicle
It feels like the pedal is pushing back at you and your not actually going to stop , very unsettling
Codes leaned towards a front wheel sensor but replacement of that hub assembly did not fix the butt clenching ABS stops
I unplugged the ABS relay , disabling the ABS system but giving me standard braking function
Later on I replaced the other front hub , replaced the relay but still had issue
It was not until I replaced a rear hub assembly (front wheel drive) that I was able to regain ABS
At no time did it ever code for a rear sensor
I was OK driving for several years with the ABS light on because the relay was out

 

In the S-series trucks there is no rear axle ABS sensors; Only on the front hubs and the transmission. Check out this thread in the technical how to section which goes through the diagnosis and repair. I hope this helps you.

 

I believe the VSS serves as the "rear ABS sensor" on our trucks since they have a solid rear axle.

I've looked at the ABS bulletin thread in the past. That doesn't really apply in this situation. The ABS isn't kicking in at incorrect times. After I cleared the codes and tested the truck it worked perfectly which seems to indicate that, despite the stored codes, the wheel speed sensor is working just fine. What I'm really curious about is how to test for such an intermittent problem. If the codes aren't returning after clearing them then it seems to me that the sensor isn't a problem. Is ABS one of the things affected by a bad ignition switch? I'm deeply suspicious of the ignition switch on this truck but haven't replaced it yet.

 

That's what I meant when I said that the trucks use sensors on the front hubs and the transmission. I just was blanking on that it was the VSS at the transmission that it uses.

I don't have a lot of experience with a bad ignition switch, but I have read that it can cause all kinds of wierd problems. The other thing it could be is one of the wires from the front hub to the frame is broken inside the boot and intermittently looses contact. Because the wires are inside a boot it does still hold the wires together, but a bump or brakeing while turning might cause a momentary separation which would set the code. Broken hub wires is not uncommon, especially if you have older hubs.

 

That's some great information. A damaged wire seems to fit the circumstances. I've actually got this weekend off from gigs (first time since July!) so I'll get the wheel off and test out the sensor wire while moving it around to see if I can replicate the problem.

I know this truck has a suspicious ignition switch from my recent testing of the radio/overhead display circuit. I also know that many strange electrical problems can come from that darned ignition switch. I'm basically willing to blame the ignition switch for anything that goes wrong on the entire vehicle.