Very few times when I start the car will the ABS light come on. Often though, when I start to move the car and have to brake, it will then come on and stay on. Also, for the first time last night, when I was coming to a stop, it sounded like the ABS module was initializing (the sound that you hear once the car hits 3mph) and it almost seemed like my rear brakes locked up in the driver side rear...then it was ok. The other time I've noticed the light to come on is if I get on the gas pedal while switching gears between 1st, 2nd or 3rd while accelerating faster. When it was last in the shop (2 weeks ago) the codes being thrown were indicating driver side rear and passenger side front. Sensors were cleaned and looked to be operable. What am I looking at possibly? Need new sensors? New ABM module? Sigh...the $$$ keeps adding up for repairs. Car has 59k on it and I've had it over a month now.

 

:seeya

 

Unfortunately, I don't think you'll find the answers here. I haven't seen / heard any real ABS gurus, just a lot of guessers. As a matter of fact if you get this figured out, be sure to post the answers-- you'll be the ABS Guru.

Anyway, here's my guesses - (in order of cost)

1) Do a very thorough inspection of all the connections. Look for corrosion, pitting, water damage, etc. Clean all electrical connectors. Be sure you have a good clean ground.

2) Flush new brake fluid through the entire system. You'll need to have a Chevy dealer ( or someone with a TECH II ) to flush the ABS body.

3) Replace the wheel sensors.

4) Replace the ABM. I don't know if rebuilt ones are available.

4) good luck.

 

I feel your pain. Worked with my 90 coupe for several months, monitoring symtoms, trouble shooting etc. The service manual will tell you to check the sensor resistance, 900-1100 ohms. You can do this with the sensor on the car. Follow the wire from the sensor, locate the connector, disconnect, and measure resistance. If all is well, you have some choices. If you can locate someone with a good scanner and appropriate brake cartrdige, you hook it up, drive down the road, and monitor speed sensors on display. They should be within one or 2 mph of each other. This checks both sensor and connections. The service manual has you buy a new sensor before proceeding to buy the controller. In my case, all worked however kept getting RF sensor codes. A second symptom, the ABS controller would run the Built In Test Equipment (BITE) well past 11 mph. This is what made me change out the controller and problem solved. You can shop around and get a used controller for around $250, salvage yard. Most will even guarantee the thing. Still, check connectors located in the pump area, check to see if the self test procedure is consistently doing something different, ie, not running. Keep track of the codes being noted. I really suspect your module is heading south. The 86-89 module will not work in your car. Let us know.

dlmeyers 90 coupe zf6 3 speed shocks

 

fast90,

Drawing on my own experience and a lot from the Aug. 1998 Corvette Fever article
by C4 Expert Gordon Killebrew there are several checks that can be done.

Static Test #1
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.

2. Using a digital volt/ohm meter (DVOM) with a 10 megohm reisitance, 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 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
VERY IMPORTANT: Disconnect the main harness that connects directly to the ABS Controller first.

1. Set the DVOM to AC volts and you will now have to 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).

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 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. You can check with Gordon at 1-800-4YOURCAR. He charges a fee of $10 + $3/minute, so be prepared to be brief or it will cost you. Gordon was a mechanic at GM who helped write the Service Manuals for the C4 vettes.

Personal note:
I have found that the front wheel bearings have a minor amount of play 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 locate the switch behind the AC/blower controls (it will have a orange and 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. (Since you see the ABS light come on when shifting hard, it is VERY possible that this is your problem.)


Relays:
According to my GM Parts & Illustartion Guide the 90 C4 ABS system has 3 relays. The GM Part #s are:
14105967 ... RELAY, PRESS MOD VLV SOLENOID
14105968 ... RELAY, PRESS MOD VLV PUMP MTR => Replaced with GM part # 12513302
1636973 .... RELAY, ELEK CONT

The 14105968 relay was replaced with a GM unit of different internal design. It is the first relay Iwould replace and then follow that with a new 14105967 unit. Forum member Jeff Koop 800-728-8267 can get all 3 for you at a major discount to the usual GM dealer price. Approx $35 each form Jeff, $54 each from dealer.

Let us know how it goes.

Good Luck,
Mike

 

Wow! Thanks for the suggestions guys. I have printed all your suggestions and will submit them to my mechanic and request that he review them for possible solutions. I'll post what he finds out :)

 

Excellent.

I guess I was wrong (don't tell my wife I said that) about no ABS experts. Or did you read the manual :D ?

You should put this in TECH TIPS so we don't loose it.

 

John,

Yeah, I need to put this info in the Tech Tips section. I'll finish editing and submit it.

I went to 3 dealers and at $75/ hr none of them had a clue how to properly troubleshoot the early C4 ABS. They all suggested a new controller at $900. So I bought an original GM ABS test tool a Kent-Moore ABS tester from an retired GM mechanic to troubleshoot the ABS problem on my 88 and contacted Gordon Killebrew after reading his article in Corvette Fever. Turns out it was a combination of things in my case.
1) Sloppy wheel bearing in the left rear threw the side to side reading off. Replacing the wheel bearing resolved that problem.
2) Minor adjustment of the mounting of the front bearings aligned those side to side readings which were slighty off also.
3) Replaced the 14105968 relay with the new 12513302 relay.