ABS Troubleshooting
Thread Starter
Instructor
Joined: Apr 2011
Posts: 218
Likes: 16
From: Littleton CO
ABS Troubleshooting
OK, so I have an issue where my ABS falsely kicks in every once in awhile while I am cruising or just braking lightly. Here is what I have done so far:
1. Measured the impedance of all the wheel sensors. All are within spec except for the left front which is BARELY out of spec (1070-1170 ohms, and it is reading 1057). If the front wheel speed sensors had the same +/- 100 ohm tolerance as the rears instead of +/- 50 it would be in spec.
2. Looked at the voltage waveforms on an oscilloscope while spinning the wheels. All look good from a waveform perspective - nice sine waves that increase in amplitude as the speed increases. However, the left rear is about 1/2 to 2/3 the voltage of the right rear.
It's very frustrating because when I am at the track the ABS will kick on prematurely and sometimes really lengthen my stopping distance. What path would you guys follow?
--Calvin
1. Measured the impedance of all the wheel sensors. All are within spec except for the left front which is BARELY out of spec (1070-1170 ohms, and it is reading 1057). If the front wheel speed sensors had the same +/- 100 ohm tolerance as the rears instead of +/- 50 it would be in spec.
2. Looked at the voltage waveforms on an oscilloscope while spinning the wheels. All look good from a waveform perspective - nice sine waves that increase in amplitude as the speed increases. However, the left rear is about 1/2 to 2/3 the voltage of the right rear.
It's very frustrating because when I am at the track the ABS will kick on prematurely and sometimes really lengthen my stopping distance. What path would you guys follow?
--Calvin
Team Owner
Joined: Sep 2003
Posts: 20,179
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From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
Without your cars info on your profile its difficult to tell what year Vette you have.
Assumed you have cleaned the iron dust off the sensor tips
Assumed you have checked and cleaned the ABS module connectors as required.
Assumed you have pulled the codes. Oops we don't know what year yet.
Assumed you have cleaned the iron dust off the sensor tips
Assumed you have checked and cleaned the ABS module connectors as required.
Assumed you have pulled the codes. Oops we don't know what year yet.
Thread Starter
Instructor
Joined: Apr 2011
Posts: 218
Likes: 16
From: Littleton CO
It's a '91 base version. I have cleaned the two rear gears, the front are sealed and integral to the hub because it is a '91. No codes - the ABS unit doesn't see it as a malfunction, it just thinks that the wheel speeds aren't matching so it activates the ABS.
Team Owner
Joined: Sep 2003
Posts: 20,179
Likes: 673
From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
If no codes, I'd pull the rear sensors, check condition and ohms.
I have a test set that sees the sensors and checks for operation. You've already accomplished that. With little options for troubleshooting the control board, you might try picking up another sensor to find a matching set.
Maybe someone out there has an ohm measurement for the rear/front sensors.
I have a test set that sees the sensors and checks for operation. You've already accomplished that. With little options for troubleshooting the control board, you might try picking up another sensor to find a matching set.
Maybe someone out there has an ohm measurement for the rear/front sensors.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
If you can look at the waveforms at the ABS unit (CCA board) and have someone spin each wheel. While wheel is turning move wiring at connectors and wheel sensors. Tap each sensor area with a rubber hammer to induce some vibration. Perhaps a broken wire, bad connection or wheel sensor might show up and drop out the waveform.
Just a thought, but you might have to get creative on a problem like this or just throw parts at it which we want to avoid as much as possible.
Just a thought, but you might have to get creative on a problem like this or just throw parts at it which we want to avoid as much as possible.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
I'd also do a good bearing check. Loose hub bearings will cause ABS issues because it makes the sensor tip to inductor ring distance vary under a load.
The other abstract possibility is the Lateral Accelleration Module. It is wired into the ABS system and helps control ABS while cornering so the system does NOT see different wheel speeds and activate in long turns/corners at speed.Its just mercury switches...so its semi-mechanical and prone to mechanical fatigue.
The other abstract possibility is the Lateral Accelleration Module. It is wired into the ABS system and helps control ABS while cornering so the system does NOT see different wheel speeds and activate in long turns/corners at speed.Its just mercury switches...so its semi-mechanical and prone to mechanical fatigue.
