fuel pump relay 1980 911SC

[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.

[Flat6pac]

You can remove the relay and jump terminals 30 and 87a giving you just the fuel pump with. The key on.
I believe on the SC the rev limiter is in the tach.

Bruce

[ossiblue]

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.

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.

[worn]

Thanks for the responses. My son lost his clutch suddenly last week with his MGB and almost was in a serious accident. It emphasizes how much a no-go situation can be as dangerous as cannot stop.

My car has had intermittent engine failures. Once a sudden cut off on I-90 (yes for those on Mercer Island, I-90 goes through Wisconsin) and another no-start in a parking lot very near my garage. I got the engine going on the freeway by swapping relays.

Would prefer to just drive. Still in the break-in phase of my DIY engine rebuild. Like L.J., I am struggling with a Porsche addiction.

[ossiblue]

Originally Posted by worn

Thanks for the responses. My son lost his clutch suddenly last week with his MGB and almost was in a serious accident. It emphasizes how much a no-go situation can be as dangerous as cannot stop.

My car has had intermittent engine failures. Once a sudden cut off on I-90 (yes for those on Mercer Island, I-90 goes through Wisconsin) and another no-start in a parking lot very near my garage. I got the engine going on the freeway by swapping relays.

Would prefer to just drive. Still in the break-in phase of my DIY engine rebuild. Like L.J., I am struggling with a Porsche addiction.

If you suspect the fuel pump relay as the problem in your sudden loss of power, you may first try spreading the pins of the relay apart, using a razor blade. That will ensure there is good contact in all terminals of the socket.

[911truther]

this worked for me

[fred cook]

Check the fuel pump relay fuse. It should be fuse #16 from the front of the panel. This fuse pulls enough power (amps) to cause the ends of the fuse to corrode and create a high resistance. While you are at it, check the wires going to each side of that fuse holder and see if they are also corroded. If so, clean them off and use a .22 caliber bore brush to clean out the holes the wires fit into. Hope this helps!

[schoward]

The rev limiter can simply be unplugged to disable it and see if that is your issue. The car will run just fine with it unplugged as it is not needed to provide power to the fuel pump. It's only function is to interrupt power once 7000 Rpms is hit.

It is the square box on the side bulkhead behind the fuel gauge with an 'x' on it and is easily accessible from the frunk simply reach in and unplug it and see if problem goes away. It is a super easy test and takes 30 seconds.

I have had two of these boxes go bad and cause power issues to the fuel pump. My original went awry and then a used one from parts heaven lasted 10 years and then went on the fritz. In both cases the car would run but would flicker power to fuel pump just enough to cause the car to run slightly 'off'. Both times (on both boxes). unplugging the box corrected the issue proving it was the relay box. When the first one went bad, the fuel pump would run but intermittent power made it sound like gravel was going through it. On the second one the car just ran a little bit off and seem to buck a little bit under light throttle but still ran and drove.

Easy test - reach in from frunk and unplug the rectangular white plug front the bottom and see if any change. There is no adverse effect you just won't have rev limiter while unplugged.

[worn]

Wow! Such a trove of good advice. I am on it with new mental tools to get at things. Out to the garage tomorrow. I now have a 911 and a 914-4 and a 914 Carrera 3.2, so I need all of the space and help I can get.

Warren

[F1LOCO]

Reviving an old thread b/c this post below nailed it and hoping it can help someone else. The 5,300 cut out gremlin struck my '78 recently. Tried all the tricks, cleaning fuse terminals, new fuse, widened fuel relay terminals (great trick by the way as mine always had a habit for working up from the plug). Rebuilt entire ignition system, including AshlockTech CDI - can't say enough good things about Bob's work. Once I unplugged the emission control/rev limit relay, smooth runnings to 6,000+. Guess that capacitor has seen better days. As an added bonus, Bob Ashlock put a rev limiter on the CDI so no worries on an excitable over-run. Given the fact the CDI upgrade/rebuild was $500 and this relay is now $750, I just saved $250 and have an updated CDI that looks brand new. Love the wealth of knowledge on this forum.

Originally Posted by schoward

The rev limiter can simply be unplugged to disable it and see if that is your issue. The car will run just fine with it unplugged as it is not needed to provide power to the fuel pump. It's only function is to interrupt power once 7000 Rpms is hit.

It is the square box on the side bulkhead behind the fuel gauge with an 'x' on it and is easily accessible from the frunk simply reach in and unplug it and see if problem goes away. It is a super easy test and takes 30 seconds.

I have had two of these boxes go bad and cause power issues to the fuel pump. My original went awry and then a used one from parts heaven lasted 10 years and then went on the fritz. In both cases the car would run but would flicker power to fuel pump just enough to cause the car to run slightly 'off'. Both times (on both boxes). unplugging the box corrected the issue proving it was the relay box. When the first one went bad, the fuel pump would run but intermittent power made it sound like gravel was going through it. On the second one the car just ran a little bit off and seem to buck a little bit under light throttle but still ran and drove.

Easy test - reach in from frunk and unplug the rectangular white plug front the bottom and see if any change. There is no adverse effect you just won't have rev limiter while unplugged.

[jking]

FYI there is a 40uf 40v electrolytic capacitor and a BD135 NPN output transistor in the rev limiter. These would be the most likely failed components and very easily replaced for a few dollars.

[jking]

Also a common failure mode for automotive electronics is a failed solder joint where connectors or large components are soldered to the circuit board. Look closely for a crack around the pin; sometimes they are hard to see.