A while back, I posted an issue with the ABS system on a 2001 Grand Am I had purchased. When I first got the car, the TRAC and ABS lights were on. I found a broken wire leading to the RF ABS wheel sensor. I repaired it and that should have been the end of the story BUT I had one or more wheel bearings that were making some noise, so I bought a set of four wheel hub assemblies on eBay - I know, but Oreilly wanted $60.99 and up for just one and I got this whole set for $102.97.

I never really drove the car before I replaced the wheel hubs. There were some other issues with the car so I repaired them as well, including new brake pads on all four corners. The issue that arose after all this was done was the ABS coming on too soon. Any time I get to half-pedal, no matter what speed I am traveling, you can hear the ABS solenoids and motor kicking in and the pedal start to bounce. The wheels will momentarily lock-up, then release making the car jump back and forth from side to side in a panic stop.

Knowing that the hubs and sensors were knew, I thought it was a hydraulic or mechanical issue. I bled the brakes, and had the front rotors turned. I bought a kit to rebuild the parking brakes on the rear discs. None of this helped, so I went back to diagnosing the ABS system.

I checked for an AC signal at all four wheels - good. I checked for an ABS signal at the ABS controller for the front wheels - good. I couldn't find the wires for the rear wheels at the ABS controller so I didn't check those. I checked for power at the ABS controller - one large terminal is battery hot, and I found a key hot wire on one of the other pin terminals. Plug looked clean and undamaged.

There is no ABS or TRAC light illuminated. It is not generating any codes (DTCs). So I sat down last evening and began searching the internet. I found about six different sites and you tube videos that all pointed to the same thing - excessive air gap between the reluctor and magnetic pick-up. In their cases these were external sensors and rust buildup had pushed the senor away from the reluctor ring on the axle. The excessive air gap gives a false reading to the controller which mistakenly thinks there is a wheel spinning (or locked?) so it starts applying (or releasing?) brake pressure to the wheel(*).

Some of the other sites mentioned integral hubs/sensors such as on my Grand Am and how they found that their "cheap eBay" units were no good. This got me to thinking. Even though I can see an ABS signal coming from each sensor, is it the proper AC voltage? I couldn't find any specs on what the AC voltage should be, and it would be difficult if not impossible to check without a fancier scan tool than I have, as the AC output would undoubtedly have to read at specific RPM of the wheel. If the air gap built into the internals of the sensor are not to spec, it could be sending a false reading to the controller, just like the examples I found on the net.

Unfortunately, I threw away the old hubs so I cannot re-install them to see if the problem goes away, but tomorrow I am going to a salvage yard and try to find four hubs to replace the new ones I installed. I will keep you posted on the results. I am going to try used ones first to see if it corrects the problem rather than spend $300.00+ to buy another set of new hubs.

Well, first things first, we are going to need to know of any active/inactive codes that are stored. Otherwise, we are gonna play guessing games all day.

For the module to activate it first sees a problem with at least one wheel that is starting to lock up.

No codes, no ABS light. I am pretty sure the sensors are sending the info to the module, but it is the wrong information due to improper air gap inside the sensor or just ****-poor manufacturing quality. That'* why I am going to try some OEM units from a donor car. I back-probed the wires at the ABS module plug and got AC voltage readings from both front wheels with the car in gear and the wheels in motion. I couldn't find the wires for the rear sensors, but did test them at the connector from the sensor and got AC readings on both by turning the hub by hand. I don't know if the readings were correct, as I cannot find any spec.* showing AC voltage vs wheel RPM. My suspicion is a wide air gap will provide a weak signal to the module. I have seen four posts regarding problems with cheap eBay units that had to be replaced with better quality items.

Sounds like you're on the right track.

What amplitude is the AC signal? I doubt it'* bad as we really need to test under actual conditions that cause the problem. It'* either bad sensor(*) or wheel bearings. It could be wiring but I doubt it.

ABS problem solved. I replaced the new hubs/ABS sensors that I purchased on eBay and put on four from a donor car at the salvage yard. My early ABS symptoms have gone away. It now stops smoothly and there is no pulse or vibration in the pedal during light to moderate braking. In full panic mode from 50 mph you can feel the ABS kick in but it feels more normal and the car stops smooth and straight, no jumping or momentary lock-ups of the wheels.

So there you have it, I will add my experience to others who have bought cheap units on eBay. Even though each sensor was putting out an AC signal, I suspect the quality of the internals was poor and/or the air gap between the internal sensor ring and sensor is probably too wide, in other words, these units are not up to specification and therefore do not give correct information to the ABS module.

During the course of this mystery, I also spent extra money having the front rotors turned and rebuilding the emergency brakes on the rear discs, not to mention time. It always pays to go back to the last thing you did before the symptoms began.

I also noticed a small water leak coming from the metal heater tube where it goes in to the thermostat housing. There is a section on the end of the tube that extends inside the housing which had rotted away, so I found a metal bushing that fit quite snugly into the end of the tube and I brazed it onto the tube, leaving about an inch sticking out. Any more than that and you might hit and damage the t-stat. I ground the welds smooth on my one inch wide belt sander and then slid two o--rings over this extension. It was a tight fit, I had to persuade it into place with a couple light taps of a hammer, but so far no leaks.

Cool, glad you got it sorted out!

Thanks for letting us know what fixed it.