Brake failure.

[gr8scott]

I was lucky in that I was only three blocks from home and heading home. Slowing for a gentle curve, I heard a low rumble, something akin to a muffled raspberry, and I sensed the need for more pedal pressure. A block further I came to a stop sign. This time the pedal was even softer, the noise a bit louder, and the yellow ABS light came on. I nursed it slowly home in the right lane, and just before arriving the red brake light came on as well, and was immediately followed by the CRT lighting up with "Insufficient pressure..." and something else, but I was paying attention to the road, and didn't catch the rest before it went off. Easing into the driveway and the garage, I came to a controlled stop.

This was late last night, so I waited till daylight to do a visual check. The reservoir fluid level is normal. I se no leaks. If I start the engine, system check claims everything to be OK, and the brake indicators both extinguish in a few seconds as usual. Pedal pressure is still spongy. I didn't drive it to see what the full text of the warning was, as I live on a hilly street and don't want to stretch my luck.

What should I suspect?

[padgett]

Suspect it is heat related. Motor bearing maybe ?

[gr8scott]

I had time to take it for a spin today. I only drove it for a couple of miles, bu I did get up to speed, and there was no indication of the original problem.

Question to Padgett: Did you mean the brake problem was caused by engine over heating, and/or a main bearing failure?

[padgett]

I was considering that the ABS pump motor could be getting hot and having a bearing seize. This may take a while to happen but would fit the description. The sounds you describe are probably mechanical and the only mechanical things that would logically make noise are the electric motor and the dump valves in the ABS.

On reflection, a leaking dump valve in the ABS could easily make a sound such as you describe and the pump might not be able to keep up with it. When was the last time you flushed the system ?

[gr8scott]

The last time it was flushed was June 2008. It would probably be a good idea to do it again.

[padgett]

Can't hurt.

[Jim]

When you had the problem you mentioned the yellow light came on followed by the red light. This tells us the pump was not running and that is why you had low pressure. Either the pressure switch is bad or the pump motor brushes or commutator is bad. The most common problem when the pump does not run is the pressure switch. In early stages of going bad it can be intermittent. Remove the connector on the pressure switch and look inside next to the pins. If you see even the tiniest drop of liquid there the switch is bad. If the problem happens again, sometimes tapping on the switch will make the pump start. The only problem with tapping on the switch is the switch is mounted in the pump housing and tapping on the switch will send the vibrations to the pump motor and if the motor is bad it might start. In this case you do not know if the tapping caused the contacts in the pressure switch to make or the vibrations caused the motor to run.

I have a good pressure switches and good pump/motors available.

Jim

[gr8scott]

Thanks Jim. I'll look at the pressure switch.

[Rawja]

I went through the same thing with my old '88. Tap tap tap on the master would get the thing to run, though it would later fail again. Replaced the pressure switch, problem returned, ordered a new pump from GMPartsDirect (which unexpectedly came with another new pressure switch pre-installed in the assembly) and the problem was cured. Both swap-out procedures were exceedingly easy.

[gr8scott]

Not being a mechanically experienced owner, I attempted to look at the pressure switch as suggested... but the shop manual's illustration is of an exploded view so I was unsure. I see three things that look like switches and didn't want to remove the connector from the wrong one. Speaking of which, to remove it, do I pull up on the white tab that looks like a retainer of sort, and then the thing will slide free, or what's the correct procedure? Here's a picture looking at it from the right front.

[harry yarnell]

The pressure switch is the one closest to the pump motor (not the two pin connector, that's the motor, and not the red connector one). pull the white locking tab, and lift the tab on the connector and the connector should pull off.

[padgett]

a) looks like an 88

pull white tab to remove competely, then can unlock connector to remove.

c) see picture

- flat multi pin on top is to ABS block

- two wire (orange lock) below is to main valve

- multi pin below that is pressure switch

- two wire on bottom is pump motor (thickest wires)

Edited October 8, 2010 by padgett (see edit history)

[Barney Eaton]

Here is a picture with the major parts identified.

[gr8scott]

Thanks for all the pointers. It'll be my weekend's wild adventure to explore this.

If, as Rawja indicated, I'll need to get a new pump from GMPartsDirect, does anyone happen to have the part number?

[Rawja]

A few things you can try...

Swap around the three firewall-mounted relays. They are all the same part, see if switching them around fixes it. If it does you have the cheapest possible of fixes.

Pull the pump lead off and check with a voltmeter if you are getting 12 Volts (while system is below pressure) While the connector is off, you can create low pressure by applying the brakes a few times.

Then you can check the pump directly by running a couple of wires to it from the battery. Does the pump run when voltage is applied to it?

In my case the pump was "tired" it would pressurize the system to some extent, but intermittantly it would quit early or not start up at all. I found that tapping it with a pair of pliers would get it to pump for a bit, but towards the end I could only coax it to run for a few seconds at a time, less and less as the pressure built and it was not able to reach full pressure without almost constant tapping at the end of the pressurization cycle.

Here's all the pertinent part #s:

25533700 = pressure switch

25528382 = accumulator

25535686 = pump assembly

[gr8scott]

I checked GMpartsdirect.They listed the pressure switch at $110 but drew a blank on the pump assembly. It correctly identified by part number 25535686, but listed the price as $0.00, which I took to mean they do not have it.

[Rawja]

Try GMPartsCenter.net It shows as available. I haven't used this vendor, but others have indicated that they're good.