Brakes
 

Bill and I were talking on the phone this evening. And we got to talking about my brakes. I can pressurize my brakes with the ignition on, and engine off. I have wires going to my MC [really to the accumulator motor], and if I remove those, or fuse x [can't remember the number], the pump will shut off, and I will lose power brakes. Now, he says that my PMV is dead, and that I still have vacuum brakes and the electronic is a backup. However, I don't see a single vacuum line going anywhere near the MC. He also talked about some federal law mandating brakes need to be vacuum controlled, or non electronic, whatever blah blah blah. Now, let's settle this once and for all. My brakes are electronic, right?


-justin

Federal law mandates that if the electronics fail, mechanical braking is unaffected. Not that it must be vacuum ;)

Brakes cannot be solely controlled electronically.

Well, like I said on the phone, the brakes still operate when the electronics are disabled.. however they aren't unaffected.


-justin

From the talk, it sounds like you have a Teves ABS (used by GM from about 1988-1991). This is an electrohydraulic rather than vacuum-hydraulic system and was designed for cars with very low or no (diesel) vaccuum sources.

Boost is provided by an electric pump and stored in an accumulator (think of it as a big spring). If the boost system fails, you still have (some) braking on the front wheels only but it requires excessive pedal pressure (same as if you lose vaccuum to a conventional power brake).

The ABS is completely seperate from the boost system other than if you lose boost, the ABS will turn off. With boost but no ABS (yellow light on, red light off), it acts like a conventional non-ABS brake.

By 1991 when it was evident that the diesel craze was over, GM went to a more conventional vaccuum-boost ABS system.

The high cost of most parts means that the usual course in the event of failure in the ABS is either to replace with a conventional system (biggest hurdle is that the Teves uses a single rear brake line whereas other current systems have two) or have the unit remanufactured (Prior is the only known source and charges about U$600 including a new accumulator).

The most common boost problem I see today is failure of the accumulator which causes the pump to run ecessively and the red light may come on while braking. It is available seperately for about U$100. The other common failure is the pump relay (U$10).

one day when i was trying to pinpoint my accumulator problem, i unplugged something electrical on the side of the master cylinder, and lost all braking ablilty. pedal went straight to the floor with little effort, and no results. i wont do that again. makes me wonder.