99 Tahoe 4x4 2dr Base, off road w/ABS problem RESOLVED by pulling fuse

[K7KY]

99 Tahoe 4x4 2dr Base back, our only car, back in shop with ABS issue. Shop was tasked with flush and refill brake system and find problem. .When stopping speed declines to abt 9mph, pedal drops and front wheels shudder hard until stopped. Pedal near floor and barely effective. No codes. This happened once a week ago. Then, there was a code for intermittent rt/ft speed sensor. We replaced both front sensors. No effect on problem and still no codes or dash lights.

Road test. Parked,, there's a small reading from all 3 sensors. I don't recall exact reading, but very small and all 3 the same. Mechanic says there should be zero reading when stopped. Beginning to move the rear sensor increments up normally; front sensor outputs don't move until 9mph, then both increment up and both read exactly the same, but a bit below rear sensor (fronts are new, rear probably old). With no fault codes in 2 shop grade analyzers, we think something electronic is amiss in ABS module, but doesn't seem right with no fault codes or dash lights.

So far, we've been unable to locate a rebuilt ABS module and we're not certain this one is faulty. I bought a shop grade analyzer specifically to read air-bags and ABS. I would appreciate any insight or experience you have with OBS ABS issues. TIA

 

[Eman85]

From my experience front sensors build up rust under them. removing the sensors and cleaning the mounts does the trick sometimes. Front hubs are the other problem as the bearings wear there is play causing erratic readings between the wheels causing the ABS to react. Never replaced a module for a problem.

 

[K7KY]

TU Joseph.... Last year the rt/ft wheel speed sensor set a code (I forget the #), reading low or high. That was resolved by cleaning the mount surface. This time code said rt/ft erratic and ABS was dangerously malfunctioning. We replace both ft sensors and code cleared. ABS still malfunctioning. Around 9mph pedal dropped and wheels shuddered heavily with weak stopping power. From stopped, front sensors don't begin to read until 9mph. Rear reading from outset. Something amiss in ABS module. No fault code. Tahoe computer smarter than ABS computer, it seems.

A new Kelsey Hayes module is $2900. I found no one rebuilding these modules, but lots of places selling supposedly good used units. I've already spent $300 having brakes flushed, refilled, and trouble shooting ABS problem. If a used module fails the cost to R&R and then replace my old module may soon exceed vehicle value. All used modules I checked had only 30days to return a faulty module.
Brake pedal now feels a bit more solid than w/ABS. I love the Tahoe and would upgrade to newer GM brakes if I could afford it, but for now the brakes are at least dependable. My wife and I are used to cars w/o ABS, so it's not a big problem beyond the light on the dash. All our trouble lights have been off since we owned the Tahoe.

Last month the shop rebuilt the differential @ $2500 to fix the sound of a failing bearing. Sound is unchanged and Mechanic says he doesn't hear it and both rear axle seals are now leaking. I once owned a Towing, Recovery, Storage, and Repair business in AZ. I'm use to doing everything on our vehicles, but @ 87yrs I'm unable and dependent on commercial help. A particularly bitter pill.

 

[Fless]

FWIW Based on the front sensors not reading until 9 mph, I'd suspect faulty hubs (tone rings in the hubs) as @Eman85 suggested. Sometimes new sensors aren't 100% compatible with worn hubs.

EDIT: it might help to graph all four wheel speed sensors with a scanner, and do a short drive at lower speeds.

 

[Eman85]

FWIW my Tahoe has the differential gear noise also. It did have bad bearings so when I pulled it apart I replaced every bearing and seal, gear noise is still there. If I get excited I might buy a set of gears but for now it's fine. You've got a few years on me and I think about how the heck I'm going to keep all of these older vehicles going, hope I'm still going too.
I still believe your problem is in the front hubs/sensors.

 

[K7KY]

FWIW Based on the front sensors not reading until 9 mph, I'd suspect faulty hubs (tone rings in the hubs) as @Eman85 suggested. Sometimes new sensors aren't 100% compatible with worn hubs.

EDIT: it might help to graph all four wheel speed sensors with a scanner, and do a short drive at lower speeds.

We ran 3 road tests at various speeds above and below 9mph. Rear sensor incremented up/down correctly. front sensors don't begin to read until 9mph, then track together perfectly until speed drops below 9mph. When braking and speed drops to 9mph, pedal drops and ft wheels shudder harshly with dangerously reduced braking. Again, with vehicle at rest, there's a small reading on all 3 sensors, all exactly same value. I believe they should all read 0 at rest. Ft wheel bearing & seals have less than 12k/mi. I don't think they're a problem. The fault code indicating intermittent ft/rt sensor occurred b4 ABS issue and was resolved when both ft sensors were replaced. I think the small reading from all 3 sensors at rest indicates a fault in the ABS module. In any case, I don't have funds for trying a used module that may result in 2 R&Rs if it isn't sound. Although the main computer doesn't show a fault, I don't think this gives the ABS module a clean bill of health. If I could, I'd buy a new Kelsey Hayes ABS module. I really like the Tahoe, but I have to be realistic with expenses. The brake pedal is a bit firmer w/ABS disabled. I don't think the Tahoe is unsafe. We have to be mindful there's no ABS. Both of us are used to early vehicles w/o ABS. I appreciate everyone's experience and suggestions. TYF is an important asset to me for keeping the Tahoe on the road. TU

 

[GMCChevy]

We've been running our 99 for years with the abs fuse pulled. Never had the pedal drop problem but it would activate unexpectedly when not needed usually increasing stopping distance in the process. Pulled the fuse and never looked back.

 

[K7KY]

TU I expect ours will work out the same...