NeilW

Hello all. Yesterday when starting the car the master alarm, PDAS light, and low brake pressure light all stayed on. When I then pressed the brake pedal they were joined by the audible alarm. Reading Adrian's book and posts on here suggest that I should be looking for a pump failure, pump switch failure, fuse, relay, or leak.

The thing is, everything still works. No problems at all. Brakes are still boosting after many miles driven. The hydraulic pump ran for its usual 40 seconds this morning then stopped. Two or three presses of the brake pedal and it runs again for 6 seconds. I can engage traction control, and can tell it's working. The fluid reservoir still shows max. I've had the carpet, spare wheel, etc. out and there are no leaks.

So really, the only evidence of a fault is the array of red lights, which have stayed on every time I use the car.

I pulled the plug on the hydraulic pump switch and there are 5 pins. Given that the pump still operates in the normal way, I'm guessing that separate pins send the signal to the dashboard/central warning system, and perhaps this portion of the pressure switch has stopped working. I've had a look for obvious wiring problems, but everything looks OK. I did clean all the grounds near the battery just in case.

Has anyone come across this before?

Cheers,

Neil

1990 C4 Guards Red

tgage

IndyCam?

dfinnegan

Have you read the fault codes?
Have you tried 4 wheel low?

Indycam

You rang, Sir ?

tgage

Yes... I seem to recall this is your area of expertise.... or at least I relied on you in the past

I was thinking it is likely the green pressure switch 964-606-904-00-M100 or the wire running up to it, but the fact that the pump turns off would seem to indicate that it is working.

The shop manual says "if there is an electrical fault at the pressure warning switch, always exchange the pump relay (R 51) as well. There qre quite a few checks in the manual for checking pressure etc.

tom

Les Dawes

I had a similar problem to this, lights illuminated and warning horn sounding (until I pressed the cancel butgton to silence the horn). It turned out to be an internal leak in the pressure switch. A replacement switch cured it all. Regards, Les.

NeilW

Thanks for all your replies. Sounds like a new pressure switch is worth a go.

Dfinnegan, I don't have a way of reading the fault codes (must buy/build a scantool) and am not sure what you mean by "4 wheel low" other than possible engaging the diff locks, which do definitely work.

Cheers,

Neil

dfinnegan

I was thinking of the switch in front of the stick to lock the diffs.

There are a lot of things that can effect this system.

How's your battery? Alternator?

Check the ground wire behind the pressure switch plug. And other grounds in the front boot.

You've said that the hydraulic fluid level is up to max. These cars are sensitive to this level. Although, in my experience, I've seen intermittent lights with a slightly low reading. Not full on, all the time.

You might try bleeding the system. Though, again, if everything is working and you're getting 40 sec startup pump run times it seems okay.

You really should try and get the codes read. There are about 20 that relate to the PDAS system. Without the codes it's going to be a process of checks and there is a long list.

I expect the other, more experienced folks on the board can help direct you in a more expedient mode. I can provide a list of checks that I've gleaned from reading the forum and the list from the shop manuals.

Perhaps someone nearby to you could help you get the codes read.

Cheers

NeilW

Thanks again dfinnegan. You're the second person who has mentioned a ground wire behind the switch plug. Where is this exactly? I've cleaned the one adjacent to the battery and spotted the two at the front of each inner wing. I removed the pressure switch connector last night and looked behind, i.e. toward the front of the car, but couldn't see a grounding point. All of the wires from the plug stayed inside the loom and headed towards the left-hand wing (looking from the front).

Cheers,

Neil

dfinnegan

I've looked through the wiring diagram and I can not see any ground point behind the switch plug. Not sure where I got that bit of information, but I've stricken it from my notes.

The pump, abs system and valve control block are grounded at GP II (2cM) and GP X (3eP); see diagram below.

You should also pull the control unit plug to check for corrosion. These are mounted vertically and water leaking in to the boot can settle into the plug connection and cause corrosion on the pins.

NeilW

Forgot to mention originally, that was the first thing on my list, checking the large multi-plug on the control unit. Terminals all looked nice and clean. I can't see your diagram. Scratch that, now I can.

NeilW

Appreciate your reply. I think the 3eP point must be the one down by the battery on the chassis rail. Just also cleaned the ones at 2cM and 2cP. All of the points looked good to begin with though. As things stand I've got a replacement pressure switch and o-ring on order; they will be with me Saturday. It looks like I should be able to remove the switch using an open-ended spanner without removing the pump from the car. But, all the procedures I've read describe removing the pump first.

Cheers,

Neil

dfinnegan

I had to remove the pump. Just couldn't get the wrench on there.

Also, the switch was rather tight and I was very glad that I had the pump out so that I could put it into a soft sided (wood) vice to hold it. It was still a bit nerve wracking to put that much torque on the pump in order to the the switch free. Of course, it did come free.

A bit more info; sounds like you have #13 and #8 warning lights, which suggests a booster circuit fault (yours works??).

Have you tried flushing all of the hydraulic fluid; guess not as that requires the Hammer to open the lock cylinders. Though, you can flush/bleed the accumulator and valve block without the Hammer.

It seems possible that you have low pressure (could be air in the accumulator) which is causing an alarm. Though, with just 40 secs to charge and not too frequent re-charging something just doesn't seem right. Seems like it may be electrical???

All the best; hope you get it sorted.

NeilW

Yes, lights #8 and #13, on permanently. As you say, the normal charge, stop, re-charge process seems to operate the way it always has. The pressure switch must have at least 3 functions, i.e. 1) detect lower threshold pressure and trigger pump, 2) detect upper threshold pressure and stop pump, 3) detect very low pressure and trigger warning lamp. So perhaps part of the internal circuitry/mechanism, the bit that holds the pressure warning light on until we're past the critically low pressure stage, has failed.

I'll report back on progress on Saturday when I have the parts.

Cheers,

Neil

dfinnegan

Not sure if it'll help any, but here is a link to the thread I created after doing the pressure switch on my car.