M. Requin

A week ago, stopped at a drive-through ATM, car in gear and foot on the brake, the car suddenly gave a jerk which I felt through the brake pedal, and in less than a second (but not simultaneously), the brake pad warning light came on. I drove it home gently and parked it. Then I put it up on the lift to make sure all suspension components were OK. (Felt almost like a ball joint rolling out of its socket.) Since I have taken it out several times, and this has happened each time, always at a stop light or other stationary situation. The brake pad light usually, but not always, comes on. The jerk feels as if the brakes release for an instant and then operate again.

Background:

• Brake pads front and rear are PBR, 10-11mm thickness. New at 180,200.
• Rotors, Zimmerman drilled, replaced last August with 189,750 on the clock. Current mileage is 194,650.
• Front suspension rebuild also done in August, included new shocks, wheel bearings, lower ball joints, (upper ball joints not great but ok, replaced boots), outer tie rod ends.
• Brake fluid replaced in August with Valvoline DOT 3,4.
• ABS wiring is ratty.

Because of that last item, it is possible that when this occurs a brake pad sensor circuit fault is caused. Just a hypothesis, because I can't otherwise link the two events, jerk and light.

I have gone through the threads I found under "ABS" and "brake booster", and found that there have been vaguely similar incidents caused by the booster, nothing exactly the same.

I could really use some help with this, so if anyone has any ideas at all, please post them!

TIA!

M. Requin

I guess I need to bump this with my own theories:

It feels like the brakes momentarily release, as if 1), the ABS has kicked in for second, or 2) the brake booster has gone off for a second. Is either scenario plausible? How? How to diagnose?

Guys, I really need help with this one - this car is my DD and I do not feel it is safe to drive in its current state.

huskeric

it does sound like an ABS matter. unfortunately, I don't have an ABS troubleshooting data with me.

M. Requin

Thanks, huskeric - I'm going to pull the ABS relay and test drive it. It's about all I can think of in terms of diagnosis at this point. And stay safe, man!

M. Requin

Folks, I have participated in this forum to the extent my skills and experience have allowed. So far I have had one response to this - for me - major issue with my car, and it was from someone who likely has more important preoccupations.
If anyone has any ideas about this problem, I would really be appreciative of their input.

The Forgotten On

Check your ABS wiring as it sounds like it could be giving incorrect values to the ABS computer.

Check all wires with an Ohm meter to be sure they are providing the correct resistance.

Also, inspect your ABS pump to be sure it is working correctly because that can cause jerking when braking if it has gone bad.

rnixon

Sorry, I don't have experience of the 928 systems, but I'll have a go.

I don't know how the brake pad sensor is activated, but I'd suspect that there's an earthing issue causing both symptoms. I'd be looking at the circuit diagrams for both of those systems, especially where they earth.

M. Requin

Will do- and thanks!

M. Requin

I think that this points in a direction I have been wondering about- the possibility that there is an intermittent short between the ABS and brake pad sensor wiring- back on the lift, wheels off! And thanks!

Mrmerlin

I would be checking the ABS fuse and the relays for the ABS.

also check that the ABS connection is tight at the PS reservoir (the big phillips screw,)
NOTE check this with the battery disconnected .

Pull the ABS sensors and clean them, check the wire connections for corrosion.

Check the battery connections clean all of the terminals.

Check the adjustment of the front wheel bearings , if the hubs have play make sure that the adjustments are properly done its better to have them loose than tight.
trying to reduce play by snugging the nuts will only damage the bearings further.

The wear on the spindles will be on the bottom of the inner bearing seating area, the spindles are made of a softer metal and this part does wear down

M. Requin

Will follow up on these points.

All the above were done in August, and wheel bearings were adjusted and double checked as you indicate (have some experience with this). Removed the ABS relay in the CE board and took it for a test drive. Pad wear light came on, but no "jerk" at stops like before. Needs to be repeated, but IF this is consistent it indicates it is definitely an ABS system problem, and I will pay particular attention to your suggestions to start with.

Thanks, Stan.

Mrmerlin

Your welcome,
I hope these suggestions fix your car,
Oh you also have a worn brake sensor or the lines are corroded,
deoxit spray at every ABS and brake sensor connection would also be highly recommended.

ramcram

The pad warning system is a continuous single electrical circuit that runs from the Central Warning Unit, to the front left pad sensor, through the CEB, to the front right pad sensor, back through the CEB to the rear right pad sensor, across to the rear left pad sensor, them home through the CEB once again completing the circuit at the Central Warning Unit.
The system works by wearing through one of the wires pushed into the pad, once the pad material has worn away. This breaks the continuity of the circuit and the alarm is triggered.
So any loose connection, or dirty connection, or broken wire INSIDE of its insulation, will open the circuit and trigger the warning. Even if only momentary open circuit is detected.
A multimeter working systematically from point to point will find the open but only if it's there when you are testing! The joy of intermittent faults.

ramcram

Thinking about what is common with the ABS and the pad wear warning system and the Central warning module starts ringing bells, I kept getting ABS warning alerts but the ABS worked and after checking and changing everything with no joy, I changed the Central Warning Module and the alert was fixed.
Both systems go through the same big plugs near the wheels but are very well separated from each other. I have seen the wires broken and frayed where they are moulded into their plugs that push into the big round connectors under the fender. An easy thing to check.

M. Requin

Thanks for all the suggestions, gentlemen. I disabled the ABS system yesterday by pulling the relay, it may take a day or two of driving to see if that isolated the ABS as the source. Stan, I'll check those points out today, ramcram- that is a very concise description of how the pad wear warning system works. Very familiar with it from other cars, esp. my old XJ40 (and ABS somewhat, come to think of it). And my intuition (ha, typed "untuition", Sigmund) tells me it is a situation somewhat like you describe in your second post. I do have another Central Warning module I can swap in when I get to that step.

One thing is clear, age is not kind to this system, it may become a largish project.

My payback for this help will be to post the solution in this thread - or if necessary in another thread titled for search results - when I find it.