ABS Sensor Frequency Test
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Instructor
Joined: Dec 2003
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From: Crestview Hills Kentucky
ABS Sensor Frequency Test
I removed, cleaned and checked each wheel sensor. Then I checked each one with an OHM meter. They all showed around 1000 OHM, give or take 50 OHM.
I have heard the system whine at start up, but then it sets itself and shows the dreaded ABS code at about 7 miles per hour. I have heard that 90% of the time, the problem is a wheel sensor and they could still be bad, but require a frequency test to determine which one is bad.
Does anyone know what equipment is required to do this? Can Auto Zone or someone do the test on each sensor.
I have an 87 and I was told it does not store codes. :nopity
I have heard the system whine at start up, but then it sets itself and shows the dreaded ABS code at about 7 miles per hour. I have heard that 90% of the time, the problem is a wheel sensor and they could still be bad, but require a frequency test to determine which one is bad.
Does anyone know what equipment is required to do this? Can Auto Zone or someone do the test on each sensor.
I have an 87 and I was told it does not store codes. :nopity
Drifting
Joined: May 1999
Posts: 1,478
Likes: 3
From: Sacramento, CA Money can't buy happiness - but it's more comfortable to cry in a Corvette than a Yugo.
Re: ABS Sensor Frequency Test (kohlbear)
What you have heard is correct. Your 87 ABS system stores no codes. GM had a special ABS tester made by Kent-Moore that was used to test the early C4 ABS controllers and sensors. It is possible to test the sensor cycling voltage using a 10MegOhm voltmeter. I've posted this on CF in the past, but it has been a while so maybe it wouldn't hurt to do so again. Please ignore the steps you have already done.
Drawing on my own experience and a lot from the Aug. 1998 Corvette Fever article by Gordon Killebrew there are several checks that can be done.
1. Pull each sensor and clean with WD40. The sensors are essentially powerful magnets and they will accumulate a lot of road dust and debris which affects the sensor readings. The sensors are screwed into each wheel knuckle with a 10mm screw. After you clean them, use a little anitseize on the sides to keep the steel sensor from welding itself to the aluminum knuckle. See if that helps.
2. Using a 10 megohm digital volt/ohm meter (DVOM) disconnect and check the resistance across the 2 pins on each of the wheel sensors. They should read 900 to 1100 Ohms. They should also all be within 100 ohms of each other. If not in range clean again with WD40. If still not in range, replace the sensor. You can pull the sensor connector where it connects to the main wire for each sensor going into the main harness, or you can disconnect the 8 pin connector in the ABS compartment on the drivers side under the plastic tray. If you use the connector behind the seat you need to get the correct pair of wires for each sensor. Check the ABS wiring diagram for you year car in the manual. For my 88 the wire colors are
LF - yellow & blue/white stripe, RF - Green/white & brown/white,
LR - red & black, RR - brown & white.
Dynamic Test #1
This test is essentailly the same as that performed by the GM official Kent-Moore J35890 ABS Tester when testing the sensor AC voltages. Unfortunately, we can't test the ABS relays at the same time like the K-M tool does.
VERY IMPORTANT: Disconnect the main harness that connects directly to the ABS Controller first.
1. Now set the DVOM to AC volts and go to the ABS 8-pin connector in the ABS compartment on the drivers side under the plastic tray (unless you have or can borrow a Kent-Moore J35592 Pinout Box) and connect the DVOM to each wheel sensor's wire pair.
You will need someone to make AC voltage readings for each wheel sensor while you drive at 15 MPH. The left front should read .65 millivolts and the right front very close to the same. The rear sensors should each read 1.1 millivolts. If your reading is high it means the air gap at the toothed gear is too small. A lower reading would mean too much of a gap. If the air gap does not match left to right it could be caused by worn wheel bearings. If the air gap is out of adjustment it will cause the ABS light to come on. I personally called Gordon about adjusting the air gap and he said that it is not really adjustable. To decrease the gap you may need to sand the area where the sensor attaches to the wheel knuckle. To increase the gap you need to put a layer of paper between the sensor and the knuckle. A meer fraction of an inch makes for a major change so DON'T do this unless you are sure it is not due to a loose or bad wheel bearing. An air gap/voltage problem is usually a bad wheel bearing or other suspension misalignment problem.
The rear wheel bearings have no allowable play when good. I have found that the front wheel bearings have a minor amount of slack in the way that they mount to the car. Once they are bolted in, they are in solid, but they can be mounted a tiny bit more forward or aft of center. Since the toothed wheel that induces the voltage to the ABS front sensors are on the front wheel bearings, this will allow you to make a minor adjustment in voltage if your reading is slightly too high or too low compared to the other front sensor by adjusting the wheel bearing. This doesn't work for the rear.
If the Static #1 and Dynamic #1 tests do not pinpoint the problem, you can do the following:
Static #2
1. Check your laterial acceleration switch, remove the right console carpet and find the switch behind and under the AC/blower controls. It will have 2 wires. One orange and one orange/black wire. Test it by holding it in your hand in the same position as it was mounted in the car and measuring the resistance with an ohmmeter. It should read zero ohms. Then turn it 90 degrees to vertical and check for an open (or infinite reading). If it fails this test replace it. Check the junk yard for one. Nobody usually buys them there because they seldom go bad. So you should be able to get one for about $20 or less.
Dynamic #2
2. Take you car to a dealer or someone who has a Kent-Moore J35890 ABS Tester. This tool will test the entire system including the relays and some Control Module (CM) circuits. I bought a used test tool for $150 on eBay. They sold new for $500. Make sure that if you take it to a dealer, that they have a working ABS tester. The Kent-Moore tester is the GM Authorized tool, but a lot of dealers don't have them around anymore.
If the dealer suspects that it is the ABS Controller be sure to get in writing that if replacing the controller doesn't fix the problem that you don't pay for it. Most dealers will say it is the CM when they don't really know what the cause of the problem is. The controllers are over $900.00 new from GM.
If the controller really is bad you can get a replacement from a junkyard for about $300, maybe less. I know that there were some at my local Corvette junkyard for that price.
Another point of failure is one of the 3 ABS relays. According to my GM Parts & Illustartion Guide the 1986-1991 C4 ABS systems all use the same 3 relays. The GM Part #s are
1636973 .... RELAY, ELEK CONT
14105967 ... RELAY, PRESS MOD VLV SOLENOID
14105968 ... RELAY, PRESS MOD VLV PUMP MTR => Replaced with GM part # 12513302
The 14105968 relay was replaced with a GM unit of different internal design. It is the first relay I would replace and then follow that with a new 1636973 unit. Forum member Chris at Superior Chevy 800-728-8267 can get all 3 for you at a major discount to the usual GM dealer price. Approx $35 each from Chrris, $54 each from dealer.
Good Luck,
Mike
Drawing on my own experience and a lot from the Aug. 1998 Corvette Fever article by Gordon Killebrew there are several checks that can be done.
1. Pull each sensor and clean with WD40. The sensors are essentially powerful magnets and they will accumulate a lot of road dust and debris which affects the sensor readings. The sensors are screwed into each wheel knuckle with a 10mm screw. After you clean them, use a little anitseize on the sides to keep the steel sensor from welding itself to the aluminum knuckle. See if that helps.
2. Using a 10 megohm digital volt/ohm meter (DVOM) disconnect and check the resistance across the 2 pins on each of the wheel sensors. They should read 900 to 1100 Ohms. They should also all be within 100 ohms of each other. If not in range clean again with WD40. If still not in range, replace the sensor. You can pull the sensor connector where it connects to the main wire for each sensor going into the main harness, or you can disconnect the 8 pin connector in the ABS compartment on the drivers side under the plastic tray. If you use the connector behind the seat you need to get the correct pair of wires for each sensor. Check the ABS wiring diagram for you year car in the manual. For my 88 the wire colors are
LF - yellow & blue/white stripe, RF - Green/white & brown/white,
LR - red & black, RR - brown & white.
Dynamic Test #1
This test is essentailly the same as that performed by the GM official Kent-Moore J35890 ABS Tester when testing the sensor AC voltages. Unfortunately, we can't test the ABS relays at the same time like the K-M tool does.
VERY IMPORTANT: Disconnect the main harness that connects directly to the ABS Controller first.
1. Now set the DVOM to AC volts and go to the ABS 8-pin connector in the ABS compartment on the drivers side under the plastic tray (unless you have or can borrow a Kent-Moore J35592 Pinout Box) and connect the DVOM to each wheel sensor's wire pair.
You will need someone to make AC voltage readings for each wheel sensor while you drive at 15 MPH. The left front should read .65 millivolts and the right front very close to the same. The rear sensors should each read 1.1 millivolts. If your reading is high it means the air gap at the toothed gear is too small. A lower reading would mean too much of a gap. If the air gap does not match left to right it could be caused by worn wheel bearings. If the air gap is out of adjustment it will cause the ABS light to come on. I personally called Gordon about adjusting the air gap and he said that it is not really adjustable. To decrease the gap you may need to sand the area where the sensor attaches to the wheel knuckle. To increase the gap you need to put a layer of paper between the sensor and the knuckle. A meer fraction of an inch makes for a major change so DON'T do this unless you are sure it is not due to a loose or bad wheel bearing. An air gap/voltage problem is usually a bad wheel bearing or other suspension misalignment problem.
The rear wheel bearings have no allowable play when good. I have found that the front wheel bearings have a minor amount of slack in the way that they mount to the car. Once they are bolted in, they are in solid, but they can be mounted a tiny bit more forward or aft of center. Since the toothed wheel that induces the voltage to the ABS front sensors are on the front wheel bearings, this will allow you to make a minor adjustment in voltage if your reading is slightly too high or too low compared to the other front sensor by adjusting the wheel bearing. This doesn't work for the rear.
If the Static #1 and Dynamic #1 tests do not pinpoint the problem, you can do the following:
Static #2
1. Check your laterial acceleration switch, remove the right console carpet and find the switch behind and under the AC/blower controls. It will have 2 wires. One orange and one orange/black wire. Test it by holding it in your hand in the same position as it was mounted in the car and measuring the resistance with an ohmmeter. It should read zero ohms. Then turn it 90 degrees to vertical and check for an open (or infinite reading). If it fails this test replace it. Check the junk yard for one. Nobody usually buys them there because they seldom go bad. So you should be able to get one for about $20 or less.
Dynamic #2
2. Take you car to a dealer or someone who has a Kent-Moore J35890 ABS Tester. This tool will test the entire system including the relays and some Control Module (CM) circuits. I bought a used test tool for $150 on eBay. They sold new for $500. Make sure that if you take it to a dealer, that they have a working ABS tester. The Kent-Moore tester is the GM Authorized tool, but a lot of dealers don't have them around anymore.
If the dealer suspects that it is the ABS Controller be sure to get in writing that if replacing the controller doesn't fix the problem that you don't pay for it. Most dealers will say it is the CM when they don't really know what the cause of the problem is. The controllers are over $900.00 new from GM.
If the controller really is bad you can get a replacement from a junkyard for about $300, maybe less. I know that there were some at my local Corvette junkyard for that price.
Another point of failure is one of the 3 ABS relays. According to my GM Parts & Illustartion Guide the 1986-1991 C4 ABS systems all use the same 3 relays. The GM Part #s are
1636973 .... RELAY, ELEK CONT
14105967 ... RELAY, PRESS MOD VLV SOLENOID
14105968 ... RELAY, PRESS MOD VLV PUMP MTR => Replaced with GM part # 12513302
The 14105968 relay was replaced with a GM unit of different internal design. It is the first relay I would replace and then follow that with a new 1636973 unit. Forum member Chris at Superior Chevy 800-728-8267 can get all 3 for you at a major discount to the usual GM dealer price. Approx $35 each from Chrris, $54 each from dealer.
Good Luck,
Mike
Thread Starter
Instructor
Joined: Dec 2003
Posts: 137
Likes: 0
From: Crestview Hills Kentucky
Re: ABS Sensor Frequency Test (Mike_88Z51)
WOW, YOU KNOW YOUR STUFF!
THANKS SO MUCH FOR SUCH A DESCRIPTIVE EXPLANATION!
I KNOW I HAVE A SLIGHTLY LOOSE REAR RIGHT WHEEL BEARING. SEEMS LIKE THAT COULD BE THE CAUSE.
DO YOU THINK IT MIGHT BE POSSIBLE TO FIT THAT SENSOR THERE WITHOUT RREPLACING THAT BEARING RIGHT NOW?
IF SO, HOW WOULD THAT BE DONE? WITH PAPER OR SANDPAPER TO WIDEN OR LESSEN THE GAP BETWEEN THE SENSOR AND KNUCKLE?
THANKS SO MUCH FOR SUCH A DESCRIPTIVE EXPLANATION!
I KNOW I HAVE A SLIGHTLY LOOSE REAR RIGHT WHEEL BEARING. SEEMS LIKE THAT COULD BE THE CAUSE.
DO YOU THINK IT MIGHT BE POSSIBLE TO FIT THAT SENSOR THERE WITHOUT RREPLACING THAT BEARING RIGHT NOW?
IF SO, HOW WOULD THAT BE DONE? WITH PAPER OR SANDPAPER TO WIDEN OR LESSEN THE GAP BETWEEN THE SENSOR AND KNUCKLE?
Drifting
Joined: May 1999
Posts: 1,478
Likes: 3
From: Sacramento, CA Money can't buy happiness - but it's more comfortable to cry in a Corvette than a Yugo.
Re: ABS Sensor Frequency Test (kohlbear)
kohlbear,
>> I KNOW I HAVE A SLIGHTLY LOOSE REAR RIGHT WHEEL BEARING. SEEMS LIKE THAT COULD BE THE CAUSE.
Yes, that could very well be the cause. A bad left rear bearing was the cause on my 88.
>>DO YOU THINK IT MIGHT BE POSSIBLE TO FIT THAT SENSOR THERE WITHOUT REPLACING THAT BEARING RIGHT NOW?
It is possible. However, a bad bearing will get worse and the air gap amount it is out may vary during use just enough to make any change you make useless over time or at a different speed. Also, if you make physical changes to the sensor mounting to correct for the bad bearing, you will then have to make additional changes to get it back in spec when you replace the bearing with a good one.
IF the voltage reading is too HIGH due to the sensor and toothed ring on the bearing being too close, then maybe you could put some stickers or tape layers underneath the sensor mounting plate where it contacts the wheel knuckle to raise the sensor end away from the toothed wheel enough to make the ABS work correctly again. At least then you can easily remove the tape when you get a new bearing. Only a few millimeters makes a BIG difference so add carefully and retest the voltage. If one side is too low due to a too big gap, it is probably best to try to make the other side the same voltage by increasing its gap with tape. You want .65 left to right in front and 1.1 left to right in the rear, but you don't have to be exact. It is more important to have the two sides L-R match than it is to have the exact voltage.
Assume you have 1.1 in the LR and .95 in the RR. The RR bearing is bad and has shifted away from the sensor just enough to affect the volatge. Don't remove knuckle material to get it back in spec. Try adding layers of electrical tape or thin scotch tape to push the good sensor out to where it reads .95 also. That might do it. You still have to have readings that are in the ballpark to begin with. If you have readings of 1.1 and .90, it's worth a shot to move the good sensor out until it matches the bad one at .90, but if either sensor is reading more than 25% off of 1.1 in the rear or .65 in front forget it.
If the voltage reading is too LOW due to the sensor and toothed ring on the bearing being too far apart, I STRONGLY ADVISE AGAINST removing any wheel knuckle material to make the gap less. Only after replacing the wheel bearing with a good one and double checking all other options would I remove material to get the volatge in spec. Remember, it came from the factory correctly toleranced and if you modify it you will have an impossible time getting it back in spec when the rest of the suspension is corrected.
Having the ABS light set is not that big a deal. It only means the ABS controller will not cycle the ABS system to prevent a skid. Good braking and defensive driving on your part can compensate for the ABS loss until you get a chance to get new bearings. The brake system itself will still work fine, only now instead of a computer doing the work to avoid a skid condition the responsibility is all yours.
If you are handy with tools and have a garage to work in you can do the wheel bearing replacement yourself and save a ton of money. PepBoys sells wheel bearings by Rawhide which have the letters "NDH" molded or cast into them. They look cheap (and are cheap cost wise), but NDH was the OEM supplier to GM for the Vette wheel bearings. Buying an NDH bearing from Kragen, PepBoys, etc., for $139 is the same as buying a $250 GM bearing. Of course you get a $100 cardboard box with the GM dealer unit.
-Mike
>> I KNOW I HAVE A SLIGHTLY LOOSE REAR RIGHT WHEEL BEARING. SEEMS LIKE THAT COULD BE THE CAUSE.
Yes, that could very well be the cause. A bad left rear bearing was the cause on my 88.
>>DO YOU THINK IT MIGHT BE POSSIBLE TO FIT THAT SENSOR THERE WITHOUT REPLACING THAT BEARING RIGHT NOW?
It is possible. However, a bad bearing will get worse and the air gap amount it is out may vary during use just enough to make any change you make useless over time or at a different speed. Also, if you make physical changes to the sensor mounting to correct for the bad bearing, you will then have to make additional changes to get it back in spec when you replace the bearing with a good one.
IF the voltage reading is too HIGH due to the sensor and toothed ring on the bearing being too close, then maybe you could put some stickers or tape layers underneath the sensor mounting plate where it contacts the wheel knuckle to raise the sensor end away from the toothed wheel enough to make the ABS work correctly again. At least then you can easily remove the tape when you get a new bearing. Only a few millimeters makes a BIG difference so add carefully and retest the voltage. If one side is too low due to a too big gap, it is probably best to try to make the other side the same voltage by increasing its gap with tape. You want .65 left to right in front and 1.1 left to right in the rear, but you don't have to be exact. It is more important to have the two sides L-R match than it is to have the exact voltage.
Assume you have 1.1 in the LR and .95 in the RR. The RR bearing is bad and has shifted away from the sensor just enough to affect the volatge. Don't remove knuckle material to get it back in spec. Try adding layers of electrical tape or thin scotch tape to push the good sensor out to where it reads .95 also. That might do it. You still have to have readings that are in the ballpark to begin with. If you have readings of 1.1 and .90, it's worth a shot to move the good sensor out until it matches the bad one at .90, but if either sensor is reading more than 25% off of 1.1 in the rear or .65 in front forget it.
If the voltage reading is too LOW due to the sensor and toothed ring on the bearing being too far apart, I STRONGLY ADVISE AGAINST removing any wheel knuckle material to make the gap less. Only after replacing the wheel bearing with a good one and double checking all other options would I remove material to get the volatge in spec. Remember, it came from the factory correctly toleranced and if you modify it you will have an impossible time getting it back in spec when the rest of the suspension is corrected.
Having the ABS light set is not that big a deal. It only means the ABS controller will not cycle the ABS system to prevent a skid. Good braking and defensive driving on your part can compensate for the ABS loss until you get a chance to get new bearings. The brake system itself will still work fine, only now instead of a computer doing the work to avoid a skid condition the responsibility is all yours.
If you are handy with tools and have a garage to work in you can do the wheel bearing replacement yourself and save a ton of money. PepBoys sells wheel bearings by Rawhide which have the letters "NDH" molded or cast into them. They look cheap (and are cheap cost wise), but NDH was the OEM supplier to GM for the Vette wheel bearings. Buying an NDH bearing from Kragen, PepBoys, etc., for $139 is the same as buying a $250 GM bearing. Of course you get a $100 cardboard box with the GM dealer unit.
-Mike
Thread Starter
Instructor
Joined: Dec 2003
Posts: 137
Likes: 0
From: Crestview Hills Kentucky
Re: ABS Sensor Frequency Test (Mike_88Z51)
I found a mechanic with the Kent-Moore ABS tester for my 87 vette. He identified the right rear sensor as the problem. I have one ordered from Advance Auto. Its a Raybestos brand. I tried dealers, online and everywhere, to get an oem, but they are not available.
If the new sensor does not correct it, I guess I will try your instructions for adjusting the air gap and checking the voltage. :cheers:
If the new sensor does not correct it, I guess I will try your instructions for adjusting the air gap and checking the voltage. :cheers:
