The Anti lock light is on in my 86. Will that throw any codes like the check engine light? Can it be read by jumpering the ALDL, or does it take a special code reader. I was told by Autozone that their reader doesn't do ABS. How do I start out trying to diagnose the problem? I don't just want to start replacing parts until I find the right one.

 

No ABS codes are stored on early years. You need a GM shop manual and follow the ABS troubleshooting charts to pin-point the problem. Ask Agent 86 to send you that section. He has it in PDF.

You can do the following:

Top of your brake fluid.

Check "Brake", "Guages", "Stop/Haz", and "INJ 1" fuses

More:

http://www.corvettefever.com/howto/19239/

 

There are no codes available via the ALDL for the 86-89 C4 ABS systems. The only tool that can scan the early C4 ABS is the Kent-Moore J35890 ABS test tool or the use of a 10 MegOhm multimeter and the Kent-Moore pinout box. Most dealers no longer have these tools as the early C4 is not brought in for ABS problems enough to warrant the expense, so they sell them on FleaBay. That is how I got my ABS Tester and pinout box. Drawing on my own experience and from the Aug. 1998 Corvette Fever article by Gordon Killebrew there are several checks that can be done.

First: 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 held in each wheel knuckle by a single screw with a 10mm head. After you clean the sensors, use a little anti-seize on the sides to keep the steel sensor from welding itself to the aluminum knuckle after you put it back in. See if that helps.

Static Test #1
Using a digital volt/ohm meter (DVOM) with a 10 MegOhm resitance, 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.

Static Test #2
Check your laterial acceleration switch, remove the right console carpet and locate the switch behind/under the AC/blower controls. It will have an orange and an 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 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 Kent-Moore J35890 ABS test tool tests in the ABS controller however, we aren't testing the relays at the same time like the K-M tool does.

Set the DVOM to AC volts and go to the 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 spec it will cause the ABS light to come on. The air gap 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.

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. A bad wheel bearing can cause the voltage to be off enough to set the ABS light. That was the cause in my case.

Dynamic #2
Take you car to a dealer or someone who has a Kent-Moore J35890 ABS Tester. It will perform a dynamic test of 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 the correct and working ABS tester for the early C4. 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, you don't pay for it. They are over $900.00 new from GM. If the controller really is faulty, you can get a replacement from a junkyard for about $300 or 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 1986-1989
C4 ABS system 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. Superior Chevy 800-728-8267 can get all 3 for you at a major discount to the usual GM dealer price.

If you do all of the above except Dynamic #2 and still cannot resolve the problem, drive your Vette to CA and I will test your controller for you with my Kent-Moore ABS test unit.
Good luck.

Mike

 

Thanks Pacer and Mike, Now I have a good starting point to figure this out.

 

You got all the great info there..DO check the Brake fuse first to see if it blew.I think it a red 10 fuse in the box.I cant recall if it says brake or ABS on there.

 

Thanks, I will try this on my '87. It has a weird ABS problem too. When you come to a stop, the brake pedal will continuously pulsate and all kinds of whirring and clicking will come from the ABS equipment behind the drivers seat. It does this every time you stop, not just once after the car is started. Eventually, on a long enough trip, or during an extended stop such as a long red light, it will stop pulsating and the ABS light will come on. That seems to shut the system down until the vehicle is restarted.

Any ideas on anything specific to check first, or should I just follow the above procedures. I have a meter and capable helper, but no Scan Tool.

Thanks,
Dan

 

Dan, my car did the same thing a couple of times the first few times I drove the car. I almost ran thru the stop lights because the car didn't want to stop with the ABS pump cycling. Since then I put new rotors and Hawk pads on the car and only use it for autocrossing. Now the ABS doesn't do anything. I don't know why it would cycle the pump when the wheels were not even close to locking up.

 

Dan,

The 87 is the same as the 86, 88, and 89. Put very simply, the ABS controller basically compares voltage levels in each sensor with the value from the other wheel sensors. The values on the same axle should be especially close together, since people seldom use different size wheels on the same axle. Based upon the values it sees in ACVolts, it cycles the ABS pump or it assumes that there is a fault in the system and stops completely. Each wheel has its own sensor, but the fronts brake lines are controlled seperately, while the rears are controlled together.

In your specific case, I suspect that the ABS controller is getting a signal from one or more sensors that looks like a bit of skid in progress at that wheel. Anything that affects the voltage level appears as a potential skid at that wheel to the controller. The pulsing you hear and feel is the ABS controller activating the ABS valves to avoid the wheel skidding. Since it isn't off spec enough for the controller to assume there is a fault, I would bet that your problem is due to one of three conditions. 1) Road debris affecting induced sensor voltage, 2) Flakey accelerometer, 3) Sensor going bad.

A) One (or more) sensors is covered with road debris or is loose in the wheel knuckle. Do the easy cleaning listed in the First: section.

B) Take it out for a drive afterwards and if still experiencing a problem, consider Static Test #2 and then Static Test #1 in that order.

A normal automotive scan tool is useless for the early C4 ABS. The only tool that can fully test the early 86-89 C4 ABS system is the GM specified Kent-Moore J35890 ABS test tool.
Later C4s have the ABS system diagnostics integrated into the Powertrain or Body Controler Modules and can be tested using many standard automotive scan tools.

 

Mike is correct...when my former 86 did the same thing,pulse everytime I came to a stop,one of the wheel sensors was bad.It had corrosion on it.The otheres were just dirty and needed some cleaning.I replaced the bad sensor and all was well!

Btw,if you pull the fuse,you can shut off the ABS system and it wont pulsate anymore.You can drive with regular brakes.Only annoying thing is the anti lock light will be on,but thats also a bulb awau from removal.

Im not encouraging this,its just to let you know,you can bypass the ABS and annoying pulsating problem until you get your car fixed.It will be like driving an 85 with no abs.Just remember its off if you do bypass!

 

Mike gave you excellent info. But for a simpeton like me, try this. I've had my '93 for five years and I've only heard the ABS do its thing one time after start-up. Hammer it up to about 60 mph and slam on the brakes with both feet. The ABS light should kick in and you'll feel the pulsation in the brake pedal. I do it about once a month just to see if it's working. These old cars don't necessarily throw a code if something is wrong. Repeat as necessary.