Brake Warning Light Coming On At 35 Mph

darisb · March 31, 2008

I'm working on my 92 K1500. The brake warning light (not ABS) was coming on when I bought the truck six weeks ago, and the pedal seemed soft to me. I figured it was the master cylinder and put a rebuilt one on it. While I was getting ready to bleed the brakes, I decided to take a look at the rear brakes. The axle seal was leaking on one side, so we replace that and the shoes on both sides. The wheel cylinders did not appear to be leaking. With everything back together the brakes felt like I thought they should. When I took the truck for a test drive, the brake warning light came on again. This time I noticed that it was coming on and staying on everytime I hit 35 MPH. It never comes on when I'm crusing at 25 in the housing track. It's definitely speed related. We had the rear axle out to fix the leaking seal. I never saw any electrical wires going to the axle or backing plates. I'm not sure whether that truck has ABS or not. I don't have an ABS light on the dash, just a brake warning light. There is a antilock pressure valve, combination valve, and an electronic brake control module next to the master cylinder (as referenced in the Chilton manual).

I tried pulling the codes, but get nothing. Perhaps I'm doing it wrong. The brake warning light always goes off when you turn the key off. If you turn on the ignition and restart the truck, the light will stay off until you reach 35 again. The Chilton guide says that you must jump H to A when the light is on. Other wise you will get a false code 9. I never get any codes.

Any ideas on what is cause the brake warning light to come on at 35 MPH?

Thanks,

Daris

darisb · March 31, 2008

I did some online reading on the GM RWAL antilock brake system this morning. I read that in 1990 GMC made some software changes that made three fault codes "soft codes". Soft codes only illuminate the warning light as long as the fault is present. The article states that soft codes can only be read with a scan tool. This explains why I can't pull the codes with a jumper wire after the ignition has been turned off. It turns out all three soft codes are for speed sensor faults. I'm going to pull the three speed sensors and test the resistance to determine which one is bad.

Daris