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That's the new pressure switch I bought. There's a black o-ring in the end. Never thought of that a solenoid. Interesting. I thought it's job was to turn off the motor via a switched ground to the pump motor.
The green item is the pressure switch. The gold item in your picture is the magnetic valve that either sends fluid pressure from the accumulator to the slave cylinder, holds pressure on the slave, or reduces fluid pressure on the slave by allowing fluid to return back to the reservoir. The pump will not build pressure in the accumulator and activate the pressure switch, shutting off the pump, unless the bleed nipple on the mag valve is closed. I don't think fluid should leak through the discharge port on the mag valve as this would result in the slave slowly extending due to fluid pressure from the accumulator filling the slave cylinder. Perhaps there is a bit of dirt stuck in the mag valve causing the bleed through? The spool in the mag valve has different positions depending on the amount of current applied just like the valves in the ABS unit (same type valve.) Try flushing fluid through the mag valve and slave cylinder to stop the leaking of fluid to the discharge port of the mag valve. You might need a Bosch Hammer to activate the "Pressure Reducing Phase" of the mag valve and force fluid pressure to be relieved from the slave back to tank via the mag valve. If the valve will not shut off tight it will need addressed.
https://rennlist.com/forums/928-foru...procedure.html
If air is in the PSD-system priming of the system can be kind of tricky as mentioned on this forum before. The suction function of the pump is very limited making it difficult to get the fluid into the system. The Workshop Manual tells you to put some air pressure 1,5 to 2,0 bar (22 to 29 psi) into the top of the fluid container.
I tried another method which worked very good. Connecting a Mityvac vacuum-pump to the bleeder valve on the locking solenoid, the vacuum immediately made the fluid to enter/prime the system. When the system was primed the flushing/bleeding procedure was no trouble at all to perform.
Åke
Thanks for the suggestions. I get clear brake fluid free of bubbles at the bleeder valve on the discharge unit. When I put power to the motor on the bench all the air in the system was expelled.
If the motor is cavitating would I get clear fluid at the bleeder valve?
Finally, gave up and sent the 90's PSD off to Roger. While I wait I decided to pull my 90 GT's PSD and see if it work in the 90 S4 - it didn't. Pump never shut off - same as 90 S4. What were the odds?
It was full of brake fluid foam that shot out of the bleeder valve when I loosened it - shot across the shop - reminded me of an 80's **** film.
So, I built a PSD purposed test bench - complete with PSD stand and place for battery, power bleeder and brake fluid catch bottle.
I used another bleeder screw to plug the orifice to the slave, pumped up the power bleeder to 15 pounds tossed my battery jump box on the battery and applied 12 volts to the motor.
Lots of foam, had to keep adding pressure. But finally it cleared. I tightened the bleeder valve and when I ran the motor within seconds you could hear the motor start to labor. That is the first time I've heard that happen.
I'll put it into the 90 S4 today and see if I finally have a functioning PSD unit free of error messages. Note, I never got any faults on the Theo tool.
Kevin
Released the bleeder valve and more bubbles and finally clear fluid.
Earlier and maybe in another thread Greg mentioned having to put some pressure on the reservoir and it didn't occur to me that he meant a power bleeder or something like that. The ah ha moment was the realization that the power bleeder fit the reservoir. ( d'oh! ). That in concert with stepping up the voltage with my jump box on the battery seemed to yield the results.
11-06-2017, 10:47 PM
