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964 1990 C4 brake booster question

S

snb993

Spa-Francorchamps
Joined
11 Mar 2010
Messages
364
Hi All

we haven't used our 964 C4 much this year unfortunately so we now have a big trip planned for it in a fortnight's time. 2 weeks ago, we did 200 miles in a day as a test and it ran perfectly

However on picking the car up from its oil and filter change yesterday, the dreaded "!" light was up and the brake booster is definitely not working... scary!

I changed the relay R51 about 4-5 years ago when we had a similar problem

This morning, i checked that fuse #15 is ok (40A!) which it is, I then jumped terminals 30 and 87 and the booster pump runs... and runs

my question is: if 30 & 87 are jumped, does this automatically mean that it is the relay? i.e. the pressure switch on the end of the pump is ok?

hope you can help please and look forward to hearing

many thanks


:thumb:
 
You have bypassed the pressure switch which allows power to the relay coil. you are simply putting power straight through to the motor on the switch side. Could be the accumator has gone duff or can you measure the switch?
 
thank you that's great and very useful - I'll have a look tonight to see what I can find out

Thanks :thumbs:
 
Assuming the accumulator pressure is low then you should see continuity between earth and r51 pin 86 (2). When the accumulator is full pressure there should be open circuit (takes away power to the coil on relay 51).
 
Brilliant thank you In the meantime while I have some time today at work, I have just asked my OPC about the R51 to see if my wife could pick it up tonight on her way home so I can rule that out (or carry a spare) to hear that they are on back order :what:

I'm not sure how much current the pump draws but a 12V/20A relay on a 40A fuse suggests that it is pretty high? :? maybe that is why some Indies suggest that they are changed regularly :dont know:

I am wondering if another 5 pin relay can be used safely instead that might be easier to source - with the same circuits and layout inside but higher duty. this could be a bit more involved

:?:
 
As the relay is fairly new its unlikely failed. Reseating it or looking for dry joints inside the cover but unlikely. Apply 12v across the coil and see if it operates.
 
great thanks - will do :thumb:
 
Well that was fun
:D
Having tested the relay and connections, I was getting a bit frustrated with it so I decided to check the 'bomb' in case I could see anything wrong but it all looked good to me. I traced the pipe work round to the pump which is behind a black cover. Disconnected both multiplugs, cleaned the contacts and made sure they clicked into place. Cleaned up both battery terminals... trying anything! And hey presto it now works! :D

However, it did strike me that the green (pressure switch?) is fitted into the pump at a slight angle. Not sure if that's correct. It has a series of flats on it so guess it is threaded in so maybe it's ok? No signs of any leaks anywhere

Also, stationery with the engine off but ignition on, after only 3 presses on the brake pedal causes the pump to start up for 2 or 3 seconds ... :dont know:
Time for a gentle test drive in the morning but I'm a bit happier with it for the moment

Thanks again for your help on this which is v much appreciated :thumbs:
 
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