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98 GM 1 ton ABS vibration/pulsing when almost stopped

blakes

Red Skull Member
Joined
May 19, 2020
Member Number
205
Messages
253
Loc
Storm Mountain CO
'98 Chevy 1 ton 4x4 C&C 454/NV4500 with hydro boost. I replaced the entire front brake system earlier this year including the wheel bearings with new ABS wire. Plenty of fluid.

The Brake light occasionally comes on. The ABS light never comes on (does come on when I turn on the key).

The brakes work great and have no vibration when stopping....until you get to less than ~5 MPH then I can feel the ABS pulsating the pedal. If I turn off the key while coming to a stop the ABS pulsating goes away.

How could I troubleshoot this? Does it have rear ABS as well?
 
Brake light comes on for low fluid level and for the set parking brake. So maybe a sensor going bad in the master, or the switch on the parking brake pedal is out of adjustment?

I believe the ABS system isn't even active under 15mph or so, but I could be wrong on that.

It does have ABS on all four wheels. It uses the VSS signal for the rear wheels though, so no speed sensors on the rear end.

You sure the front rotors aren't warped?
 
Sounds like an air-gap issue to me. The wheel speed sensor creates a/c voltage as the tone wheel inside the hub passes by it, if the sensor is too far away that voltage drops off and the ABS module thinks the wheel is locking. Usually that's from rust under the sensor picking it up but since you say you did the hubs recent-ish that's probably not your issue. Pull the sensor out and see if there's a thin spacer under them. I've had a couple aftermarket hubs where there should be one spacer but they have 2 or 3 and it causes issues.
 
Id hook the scanner to it and verify all 3 sensors are reading the same.

You did the fronts but you also need to check the speedo connetion on the tcase, it is the asb sensor for the rears.

You can also do pump testing with the scanner.
 
I had this problem in a 98 2500. Like mentioned above, I found that the wheel speed sensors had slightly moved due to rust build-up. I cleaned up each side and was good to go.
 
Another vote for wheel speed sensor.

The fact that it's low speed only is a huge tip off that it's a VR sensor signal quality issue. The signal is weakest at low speed (crack open a highschool physics textbook and flip to the chapter on electricity and magnetism if you don't understand why) so if something is right on the margin then the problem will be at low speed only.

ABS system is shitting bricks as the signal cuts in and out as the threshold drops below what it can pick up.
 
I replaced both fronts earlier this year.

What kind of scanner will read this data? Torque does not do this correct?
 
Loose wheel bearing will make a new sensor read bad.

I third or fourth this. I almost ran into a brick wall at Napa because the driver neglected to tell me this happens only at extremely low speed. Like when pulling into a parking spot at the auto parts store. 🤦
 
Loose wheel bearing will make a new sensor read bad.

I third or fourth this. I almost ran into a brick wall at Napa because the driver neglected to tell me this happens only at extremely low speed. Like when pulling into a parking spot at the auto parts store. 🤦
Ok

I will check the axle nuts and make sure they are still tight.

I'm still interested in "a scanner" that can read early GM ABS info like this.
 
My Autel does read every single sensor on my 02 silverados
 
I replaced both fronts earlier this year.
New does not necessarily mean good.

Years ago borrowed a friend's chevy to tow a trailer, he didn't mention it had this same issue. Pretty disconcerting when the pedal goes brrrt and the truck rolls another 5' into the intersection when it was already nearly stopped.

The issue was rust jacking pushing the front abs sensors up out of the hub. Doesn't take much of an airgap increase for the sensor to loose signal at low speed.
 
Just because you bought new bearings does not mean the air gap was set correct at the factory. Sometimes they come with shims under the ABS sensor. I have removed a shim or two over the years to correct this issue.

As far as scan tool goes..I have a Snap-on tool and believe it will be hard to catch the data to show what’s causing the activation. I’ve watched live data while this happens and can’t remember seeing a blip in the wheel speed data.
 
I dont look at the live data heart beat moniter.
I pull up the outputs in the abs section and watch it with the mph ## for each sensor. And watch for variation off one as i drive.

X2 on the shims for the cable heads. Ive got them mixed up before and had to restack them
 
I dont look at the live data heart beat moniter.
I pull up the outputs in the abs section and watch it with the mph ## for each sensor. And watch for variation off one as i drive.

X2 on the shims for the cable heads. Ive got them mixed up before and had to restack them
heck I didn't even know shims were even a thing on the cable heads for the ABS sensor.
 
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