If i remember correctly, a failed pressure switch would throw a code.

Now I just need to figure out how to retrieve ABS codes.

Dont rely on just the abs light coming on, in their old age these systems get lazy, my pressure switch failed and always told the system that it had full pressure even though it didnt.

Thanks. That gives me something to look for. When this latest brake issue first came up, thoughts of the last repair ever to be done to the car having already been done started taking over my brain.

You can read the codes by jumping two pins in the harness under the steering wheel. Jump pins A and B for engine codes and jump A to H for ABS codes.

Here is an exerpt from www.fullsizechevy.com, Heres the original post.
[url]
http://www.fullsizechevy.com/forums/...i-up-95-a.html

"This procedure only works on OBDI vehicles up to '95. Do NOT try this procedure on a 96 or newer vehicle!!

You want to jumper pins A and B. That should be the two upper right pins as you look at the plug.

1. With power off, jumper pin A to pin B

2. Turn the ignition to ON but DO NOT start the vehicle.

3. The Service Engine Soon (SES) light should flash code 12 (1 flash, pause, 2 quick flashes)

4. After a longer pause, code 12 should repeat two more times.

5. If there are any codes stored in the ECM they will follow Code 12.

6. If no other codes have been stored then Code 12 will continue to repeat until the jumper is disconnected.Remember all OBDI codes are two-digit."

(use the above procedure with pins A and H to read ABS codes.)

"Here'* the procedure for clearing ABS codes:
1. Turn the ign switch to the "ON" position.
2. Use a jumper wire to ground DLC terminal "H" to "A" for 2 seconds.
3. Remove the jumper wire for 1 second.
4. Repeat the grounding for 2 seconds.
5. An indication of success is when the "ANTILOCK" and "BRAKE" lamps turn on and then turn off.

All of the above is directly from my '95 FSM.

Pin A, the ground, is the topmost right corner pin & pin H is one pin over from the bottom left corner."


Another thing to remember is that not all codes will cause the ABS or SES lights to come on. Some are just recorded in the computer.

I got 2 flashes of the ABS light 3 times. Then the light came on steady. I turned the ignition off, disconnected the jumper, turned the ignition back on, and the pump started running. Took it around the block, applied the brake pedal several times in the driveway, and the Brake and ABS lights came back on.

The only thing I noticed this time is the ABS light flickered slightly while the pump was running. Not enough to go completely out, just didn't stay steady bright. It may have been doing this all along, I just never noticed it before.

The '91 shop manual only talks about using a scanner, no mention of how to jump the ALDL.

Try this procedure to get the codes.

Verify codes are stored by turning the key to the run position & see if the ABS light stays on for more than 30 seconds.

If light stays on more than 30 seconds then turn the key off & connect the jumper to the ABS terminals.
Turn key to run position.
All codes are 2 digits long.
The 1st digit is flashed followed by a 3 second pause.
The 2nd digit is flashed followed by a steady light. Do not count the steady light as a flash.
To get the next code leave the key on & disconnect & reconnect the jumper.
Repeat until all codes are read. Up to 7 codes can be stored.

Okay, that helped.

Code 21 "RF speed sensor circuit open" and Code 47 "LF speed sensor noisy".

Why that would keep the pump from running properly makes no sense.

The brake and ABS lights stay on constantly, and the ABS light still flickers slightly.

The book with my code reader lists this for the Teves Mark ll (Version A) system:
21 - Main Valve
47 - Either rear wheel speed sensor signal is missing.

It also says to fix the problem causing the 'Brake' warning light to come on before reading codes.
Have you checked the brake fluid level & made sure the parking brake is fully released?

I believe WJCollier has the 89 factory manual. He might be able to scan the section on ABS for you.

Interesting. I was under the impression the Helm manual was the official factory service manual.

It says both lights on indicates different things than just one or the other on.

Park brake fully off, brake fluid level proper when the unit is pressurized (overfull when completely unpressurized).