'90 ABS Trouble Codes

[cash's90]

I just bought a nice but high mileage '90 coupe after 40 years driving and maintaining much simpler British sports cars.
All's well except the ABS warning light (usually comes on after driving a short distance but not always). Brakes work well, disks are evenly worn and pad thickness is fine. Local dealer found trouble codes 63 & 76. Worst case estimtes by dealer for additional diagnostics and repair could total 3K.
Does anyone have experience with a similar set of problems?

[Ken]

First off Gary, I can't find those codes as being associated with the braking system. Code 63 according to one source is listed as wiring and connections to the oxygen sensor. Code 76 cannot be found in any of my reference material.

As for the ABS warning light remaining lit after the initial self-test upon start-up, the Haynes manual states:

If the dashboard light comes on and stays on while the vehicle is in operation, the ABS system requires attention. Although a special electronic ABS diagnostic tool is available, the home mechanic can perform a few preliminary checks before taking the vehicle to a dealer who is equipped with this tester.

1. Open the rear compartment lid and check that the control module is securely connected.
2. Follow the wiring harness to each wheel and check that all connections are secure and that the wiring is not damaged.
3. Check the condition of the Brake fuse at the fuse box.
4. Check the condition of the Gauge fuse at the fuse box.

If the above preliminary checks do not rectify the problem, the vehicle should be diagnosed by a dealer service department. Due to the rather complicated nature of this system, all actual repair work must be done by a dealer.

The GM service manual for my '87 adds:

The control module also constantly monitors its own supply voltage. If the supply voltage drops below a specified value, ABS will be switched off and the amber ANTI LOCK light will come on. When the supply voltage returns to or exceeds the specified minimum value, the light will go off. If a fault occurs in ABS, the ANTI LOCK warning light comes on and ABS will be switched off. The system will remain off until the vehicle is restarted at which time the funtional check is repeated. The conventional braking system remains fully operational provided it is not faulty.

Hope you find the solution soon.
_ken

[cash's90]

Ken,
Thank you for your response. I could not find the codes either. The service writer pointed out the diagnostics in the dealer's manual and the codes are correct for ABS. My guess is that they do not trust us civilians with ABS trouble codes.
I'll give the trouble shooting suggestions a try.

[Rob]

From Page 5E1-2 of the 1990 Service Manual:

Code - Definition

(* = Requires Tech 1 Daignostic Tool)

12 - Diagnostic System Operational

21 - RF Wheel Speed Sensor Fault
22 - RF Toothed Wheel Frequency Error
25 - LF Wheel Speed Sensor Fault
26 - LF Toothed Wheel Frequency Error

31 - RR Wheel Speed Sensor Fault
32 - RR Toothed Wheel Frequency Error
35 - LR Wheel Speed Sensor Fault
36 - LR Toothed Wheel Frequency Error

41* - RF Solenoid Valve Fault
45* - LF Solenoid Valve Fault
55* - Rear Solenoid Valve Fault

61* - Pump Motor or Motor Relay Fault
63* - Solenoid Valve Relay Fault
71 - EBCM Fault
72 - Serial Data Link Fault

75 - Lateral Accelerometer Fault (Short to B+ or Ground, or Open Circuit)

76 - Lateral Accelerometer Fault (Signal Out of Range or Incorrect)

Clearing Codes

The fault codes in the EBCM's memory are erased in one of three ways:

1. Diagnostic Enable Line Procedure
2. Tech 1 "Clear Codes" Selection
3. Igniton Cycle Default

Whichever method is used, be sure to verify proper system operation and absence of codes when clearing procedure is completed.

The EBCM will not permit code clearing until all of the codes have been displayed. Also, codes cannot be cleared by unplugging the EBCM, disconnecting the battery cables, or turning the ignition "OFF".

[cash's90]

Rob,
Thank you for the info.
The vette is at the A/C shop getting a new compressor installed so I have not tried out the suggested fixes as yet.
Cheers,
Gary

[cash's90]

Got the car back from the A/C shop. Blows cold air with new compressor.
Back to ABS, I checked the connections at the control module. All OK.
I pulled the ABS module relay and found that the resistance across the coil (pins 15 & 31) is infinate i.e., coil open.
Dealer wants $40 and must order one.
Could not find replacement at any other parts site.
Does anyone have a better source?
Cheers,
Gary

[vigman]

1-818-765-3333 Sport Imports ( North Hollywood Ca)

Vette wrecking yard
Ask for George


Vig!

[cash's90]

Thank you Vig.
I looked at the relay again and realized that one leg of the coil power circuit has a diode. I applied 12VDC in the proper direction and the solenoid closed the switch as it is supposed to.
I'll keep looking for the solution.:s