From time-to-time, when I start up my 87, the Anti-Lock light stays on when I start up the car. Sometimes when I turn off the ignition and start it up again, the light will go out... other times, it will stay on... so I'll just drive it with the light on, assuming my anti-locks aren't working (no big deal around town in dry weather). The light might stay on for a day, or two, but then will go out (meaning it will behave like normal, where the light goes out a few seconds after starting the car). I can go weeks or longer before it happens again.

Anyone else had this problem?

 

It happened to me over a year ago in my 90. I did what you did, turned the ignition off and then started it back up and no light. Its worked fine ever since, no more lights.

 

Hello there,

The system is likely sensing some type of circuit failure. If the light activates while driving, likely suspect is wheels sensors. This system runs a Built In Test (BIT or BITE, E is for Equipent) and should it fail any test, you get the light and no ABS. The 86-89 are harder to trouble shoot since you don't have the code setting module. When this gets to 100% failure, I'd check the solenoid and pump relays. Intermittants are hard to catch, and can be expensive. You may be able to find and pump and controller assembly, used, for around $50-100 bucks, ebay or some salvage yard. Let me know if you have any questions. I spent 7 months trouble shooting my 90's ABS system. Spent lots of time studying schematics for the ABS.

Keep the greasy side down.

dlmeyers 90 coupe zf6 3 speed shocks

 

I haven't had the light come on while driving. Instead it just fails to go out after the BITE test which (I presume) is what runs at start-up. I had heard that the ECM didn't capture the codes; so does that mean the only way to read them is when the light is on? Can I read the codes using an ALDL cable and a laptop? Does the ABS system even set codes? Are the ABS codes set in a different module?

From your experience, it sounds like replacing parts is the way to go... but only after it becomes a permanent problem.


Hey, I'm tryin'... but sometimes it's hard to do! :lol:

Thanks!

 

OK,

For the 86-89 ABS computer, GM has a trouble shooting computer set, forget the number. This helped dealers with the 85-89 circuits. Saw one on ebay couple weeks back. The module is located behind the drivers seat near the pump.

For the 90 and above, the ABS module will store codes. The ECM does not interface with the ABS, (lets forget about ASR for a little bit). The ABS module is self contained, unplugs with a simple little clip, couple of 7 mm headed screws, and is pretty easy to remove.

OK, if the light comes on at start up, no vehicle movement or little movement, I would suspect module, solenoid relay, or pump relay. Look back in the ABS pump area, the little area behind the drivers seat. The pump is pretty easy to identify and there will be two fairly small square relays located on the top of the pump. The smaller relay is for the solenoids, larger relay is for the pump. They can go bad. It could be your module is slow to wake up or has some type of circuit problem. The module checks both the solenoid and pump relays for power output, meaning it is looking for power to reach a ground, and assumes all is well when it sees power making it through the system. At least that's the way I read the schematic in the Bosch training manual. My problem was the module. Fairly rare. Now, assuming the module, relays, and connections are fine, that leaves one or more sensors. The sensors are nothing more than a small magnet with wires wrapped around the magnet. The reluctor (wheel with square teeth) located on the wheel bearing will pass by the magnetic field, modify the electrical/magnetic field and will pulse based on speed. Follow the cable out of the sensor (located on wheel "knuckles" and should find a connection. Two wire leads, stick ohm meter test leads into the sensor pig tail (side going to wheel) and take reading. It should be around 900-1100. Check the appropriate service manual. If you get lucky and find one out of spec, good start. I have read where the right side seems slightly more prone to failure. Maybe more dirt on right side of road, rougher, who knows. Sometimes removing the sensor and cleaning will do the trick. Check for greasy film, dirt, etc. Also, shine light into hole and check out the reluctor.

After you check all the sensors, assuming all are in spec, you could focus more on relays and controller.

I have seen the 85-89 pumps and controllers for sale around $35-$95. The modules may be harder to find but I know where some are. Let me know what you want to do and I will pull some old emails.

Personally, I really had to work at trouble shooting my system. With the 90, intermittant code for Right Front sensor. Sensor checked good but chart said replace anyway. All connectors good. New controller, $1100 from GM. GASP. No way. Finally got pump and controller off ebay, under $200. One guy on ebay, Beachvette, still owers me $50 for a controller never sent but will probably pay, I hope. It can be tough getting computer parts off ebay but I may be able to connect you with someone who has a pump and module for an 86-89, cause he sent this one to me by mistake. I needed the 90. If you try ebay, search - Corvette ABS- and be patient. The 86-89's are quite common. The trouble with occasional ABS 85-89 problems is it's tough to check a relay when it works most of the time. If you get a pump/controller assembly, I would replace one relay, then wait and see. If not fixed, replace second relay. If that doesn't do it, replace module, or start with module and continue. Point is, I would not replace everything at once cause you could be introducing a newer problem into the system. Just my opinion. Let me know what you need. Hope this helps.

dlmeyers 90 coupe zf6 3 speed shocks. dlmeyers@hotmail.com

 

Thanks, man... this is VERY helpful! The next time that light stays on, I will have some good places to start. I've taken the wheel mounting apart before, and checked out the reluctors. Like you've said, it's probably not a wheel sensor. So, I'll probably focus on those relays when the problem happens again. Thanks for sharing your experiences. :cool:

 

Hello again,

Just saw a unit on ebay-pre 85-89-pump (includes relays and controller) currently around $28.

You should be able to get an entire unit very cheap. Not sure even how much those relays are new. Sometimes they are not expensive, like around $10. Just don't know for sure.

Let me know. I have a relay tester for corvettes.

dlmeyers 90 coupe zf6 3 speed shocks

 

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 for 86-91 ABS systems.

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. Use a small amount of anti-seize when putting it back in the wheel knuckle.

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 (and probably your 87 as well) 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.
Note: this test is essentially the same as what the original GM service tool Kent-Moore ABS tester tests in the controller, but we aren't testing the relays at the same time like the K-M tool.

1. Next 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). Again, for my 88 the wire colors are
LF - yellow & blue/white stripe, RF - Green/white & brown/white,
LR - red & black, RR - brown & white.

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 the head C4 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.

Dynamic #2
2. Take you car to a dealer or someone who has a Kent-Moore J35890 ABS Tester. It will test the entire system including the relays. I bought a used one 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.
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. They are over $900.00 new from GM so EBay is a nmuch cheaper option.

The 3 ABS relays used on 86 - 91vettes are:
RELAY, PRESS MOD VLV PUMP MTR - GM # 12513302 (replaces 14105967)
RELAY, PRESS MOD VLV SOLENOID - GM # 14105967
RELAY, ELEK CONT - GM # 1636973

Good Luck,
Mike