Quote:
Originally Posted by worn
I have had intermittent problems with power to the fuel pump. I have three questions;
1. The wiring diagram from Porsche shows the fuel pump relay as a different beast from the other round relays, having a diode across the coil wires. Is that to deal with inductive voltage spikes from turning off the relay? Or is for the attached rev limiter in the diagram?
2. Are these relays still available? Now our host says they are all "multipurpose" relays. Does that mean with or without the diode? I guess I could buy one and cut it open. Does the absence of the diode make them fail, i.e. does it matter?
3. Anyone know how the rev limiter works? The late and revered Warren has some comments, but he is gone and I need another engineer. It looks like it would just short the coil windings of the relay, turning off the fuel pump. Is that correct? BTW, I know there are other modes of rev limiting - for this year car they shut off the fuel pump using tach signals - at least for U.S. cars
Finally thanks and I know it was more than 3 questions.
for reference here is the diagram.
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1. Yes, the diode is to prevent voltage spikes.
2. The round relays are multipurpose relays, but only the
red ones have the diode and are a multipurpose relay. A relay without the diode will work temporarily, but will likely fail, eventually. If you talk directly to our host, specify the
red relay with the diode. I believe the diagram imprinted on the plastic cover of the relay will show a diode if it is wired into the relay.
3. I have no experience with the rev limiter, buy my understanding is as yours in that it will ground terminal 85 of the relay causing the coil to energize and the relay to switch the fuel pump to terminal 87, the starting circuit, thereby shutting down the pump until the revs drop, the ground connection is lost, and the relay switches back to 87a and the fuel pump resumes.