964Luftballoon

Hello Gents,

I'm not quite sure where to start trying to repair/resolve my issue. My 91 C4's hydraulic pump might be on it's way out. As you may know when turning the ignition to the on position the hydraulic uint will prime the system for about 30-40 secs and then stop. From there you can start the car. As I daily drive my car I recently noticed the pump didn't kick on right away after sitting for long period (after a work day or overnight) When it did ultimately start, the pump seemed to be going on then off as if the power plug was loose.

Naturally, I checked the connection to the pump which was fine, Checked brake fluid levels and that was fine. Looked for any leaks and did not find any. Doing some research on the forum and texting Goughary I found old threads mentioning the pump pressure switch and relay. Or the possibility of rebuilding it with a Jaguar pump. Since the relay was a low hanging fruit. I pulled a working relay from another local 964 c4 owner but sadly that did not resolve my issue. So that leaves the horrendously expensive pressure switch.

At this point you're probably asking what about the "bomb". I tried testing the pressure by pressing the brake pedal about 20-25 times with the ignition off. And while the pedal gets somewhat firm there is some play for me to push down with very little resistance. Once turning the ignition on still nothing. I will say if I turn the car over and press the brake pedal I would be able to get the pump to kick on but when I press the pedal it would sink down and pump would stop. This leads me to believe that it is possibly the "bomb" since it's not building pressure it doesn't allow the pump to detect pressure in order to turn on. That being said, how do I bleed the bomb in order to swap it out, if I can't get it to pressurize?

Sorry for the long post but I figured I had to give as much info and troubleshooting on my part for you all to understand where i'm at. Thank you in advance

Goughary

To bleed the bomb, for this purpose, get the pump to run and pressurize the bomb, then crack the bleeder very slowly and let some pressure out- slowly- that will force any air out immediately. Let it repressurize and see if anything is different.

I'm going to say the pressure switch is probably your issue.

If that is the case- don't buy a jag unit. You don't need the whole pump- you are just changing the green thing - which is easy to do- you need to get a gigantic socket (27mm?? Can't remember) and put it on a bench grinder and grind it thin enough to fit. Then crack the switch loose and replace with new.

Well - just saw the switch is now 700 dollars....so much for cheep!

Used jag bits here we come...

964Luftballoon

Yep, horrendously expensive....

Ok, so there is no way to bleed it without the pump. Gotcha, It may have to sit and not driven for a bit then. Oddly enough as part of my planned winter maintenance over Thanksgiving holidays involved doing another brake fluid flush/bleed

budge96

Last summer I had a similar occurrence with my new to me '91 C4 the pump wasn't activating and the pressure light or ABS remained illuminated .. I was able to remove the switch and replace with I think a Range Rover unit from a buddy and once swapped the pump came back to life and operated as it should .
im still getting and intermittent ABS but haven't returned to my vaca home to address this year as of yet , Bert

964Luftballoon

Thank you for your input. I had not heard about the Range Rover switch only the Jaguar pump. Would you happen to have any details/part number on this by chance? I didn't get any pressure or ABS lights. I just noticed the lack of startup and the brake pedal hard as a rock when trying to drive.

Also, for the pump did you have to reverse the power by chance? I'm waiting on a reply from the person who swapped with a Jaguar pump and he stated the power had to be reversed. Even though I do need to get the car running. I'm trying to gather as much info as possible to make the swap a breeze. Thanks again,

- Ryan

CRG

if your pump is bad think about rebuilding it yourself, its not hard.
https://rennlist.com/forums/964-foru...p-rebuild.html



also you can test the pressure switch (green thing) if you have a meter, look at the bottom pic
https://rennlist.com/forums/964-foru...-question.html

964Luftballoon

Thank you so much for the reply and sharing your threads. In my search I'm not sure how I missed those.

Great advise on taking the pump to a local armature shop to have them refurbish. Better to refurb the oem one to new versus buying a different model all together. As I haven't had much time as of late I follow your instruction to test the continutity on the switch while it's still in the car. After which I'll do as Goughary advised and removed the pressure switch and give it a good cleaning. Hopefully that is the issue if not I'll have to remove the pump from the car and visit the armature shop. Thanks again.

By the way, dumb question. when you disconnect the hard brake line from the pump how did you cap it off?

-nick

There needs to be a dedicated thread on C4/PDAS parts & cross references. The electric-hydraulic pressurized braking system was the de facto approach for lots of 1990s forced induction models, foreign & US domestic. The lack of consistent non-blown engine vacuum being the motivation. The electro-hydraulic differential locks on the C4 and 928 LSD were, I suppose, just a creative application of the new idea.

Attached some notes I’ve made over the years. No doubt that a lot of these parts are interchangeable-



screenshot from my compilation

CRG

just a though before you remove the pressure switch, test it. if it tests ok there is no need to mess with it and in doing so you may jack it up.
as far as capping off the brake line- just need to elevate it high enough and tie it to something with a zip tie. have shop towels handy.

I dont know how many others have actually rebuilt thier pumps but this is not a hard project and shouldnt be intimidating. I would advise folks to seriously consider NOT using an old /used/junkyard pump from another vehicle e.g. Jag, etc. The motor has a copper commutator and 2 brushes, these wear over time. My orginal (89 C4) commutator and brushes were very very thin, it would have failed soon. You'll end up paying more for a used pump thats reliabilty is questionable and you'll have to modify to fit. Or you can just rebuild the the orginal with all new parts and no mods for less and be sure of its reliability.

From having done this I see only two failings of the pump itself- the motor goes TU or it leaks.
Motor which is the armature, commutator, brushes (all can be rebuilt/replaced for $2-300.
The O rings and one flat seal/gasket- buy the O-rings and you can cut your own flat gaskets/seals with sheet EPDM (McMaster-Carr)
The EPDM rotary shaft seal- good luck, i think im the only guy on the planet who has any because i had 50 manufactured.

The pressure switch- havent heard of anyone repairing one of these nor do i see how someone could. But places do sell them used for less than 700

good luck..if i can help you pm me and we can talk.. assuming its the pump that is the issue

964Luftballoon

UPDATE for anyone watching or happened to have an issue.... THANK YOU to everyone that chimed in and provided advise and provided a different lens to tackling the issue.

Turns out it was my Pressure Switch. After pulling the pump from the car I tested as mentioned above and lo and behold the pump worked fine. I went ahead paid the Porsche Piper for the stupidly expensive switch and the car is running and back on the road.