Re: Brake light question

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Posted by Frankenbiker ( on 20:04:35 07/30/07

In Reply to: Brake light question posted by BK NorCal '91 3/4 Ton

: I tow to the shop where they replace the master, flush and bleed the system test drive starts out ok but brakes start to get flaky again. They replace the ABS modulator and brakes seem to work well and I pick the vehicle up (800 bucks poorer) I head down the road and brake light comes on (huh?) so I turn around and head back where the mechanic cycles the key and it goes back off. I head home again and close to home the light comes on again (WTF?) all the while the brakes are working fine. I call the mechanic today and he says I should be find continuing to drive. We both know the sub is going back to his shop very soon for drag link and tie rod so he says just let him know if the brakes start to feel funky.
: I do like and trust this know he would not put me in danger but he can't troubleshoot without seeing the vehicle.
: My question is, what sort of brake failure(s) cause the brake light to come on?

As I recall, the '91's are still the '73-87 body style, and use the same RWAL ABS system that my '89 did. If there's a fault in either the foundation brake system, or the ABS system, it uses the same light.

There's a diagnostic test you can use to pull limited fault codes out of the ABS computer, but full diagnosis requires a computer that can plug in to the ALDL connector (dealer item?). The limited test is similar to the test you do for pulling engine diagnostic codes: you short a specific pin to ground on the connector and watch the light flash; however, I don't remember which one it is. My Chilton's manual listed the test, though.

As I recall, troubleshooting the Kelsey-Hayes RWAL ABS controller is remarkably difficult, even for experienced brake technicians, with full diagnostic equipment. It may become relatively expensive to do the "parts replacement" on a system-wide basis, but that's often the only way to get it all fixed.

The upside to this is that you can disconnect the ABS controller all together, leaving the ABS system disconnected but not effecting the foundation brakes. Simply pull the electrical connector off the ABS module (which is mounted to the side of the master cylinder), and leave it hanging. This will also kill any ABS-related trouble light, while leaving the light useable for foundation brake system problems.

As a note here, I'll add this:

If you find yourself in a situation where you have low brake fluid, NEVER NEVER NEVER NEVER use any kind of petroleum-based fluid to fill the level. Use only DOT3 or DOT4 fluid. If you find yourself in the boonies, or on the side of the road, use WATER, not transmission or power-steering fluid.

Petroleum-based fluids tear the system seals to shreds. WATER will blend with the brake fluid. While it dramatically lowers the boiling point of the fluid in the wheel cylinders, it doesn't bother the seals since brake fluid is hygroscopic (i.e. absorbs water) to begin with. This way you can get yourself back to civilization with at least marginally functional brakes, and then need only bleed the system with new fluid, versus having to rebuild the system from the ground up.

(This tip given to me MANY years ago by a brake mechanic.)


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