RPM Limiter Switch

[Charlie V]

Doing a little more electrical gremlin killing........ Specifically fuel pump electrical gremlins on my 1982 911 SC Targa.

Need some advice on the RPM Limiter Switch so I can understand how it works. Studying the wiring diagram, I see it is connected to the fuel pump relay curcuit. My questions are:

1. Is the RPM Limiter Switch for upper RPM only?
2. At what RPM?
3. Assumption is that it hits the RPM limit and turns off the fuel relay and stops the fuel pump. Is this correct??
4. Where the hell is it hidden so I can test it?

[OSI930]

1. Yes
2.Around 7K, + or -. You can do a search on this and get many different answers.
3. Yes, it interupts fuel flow by stopping the fuel pump through the fuel pump relay.
4. Black box, 2.75" x 2.75" x 1" just right and behind the left hood strut assy.(thank J.W. for this). The box also has an"X" molded into its' cover.

What is your car doing?

[KNS]

OSI930 is correct. I had a problem with my limiter kicking in at around 5500 RPM(??). It turned out it was my Tach reading incorrectly. The limiter is also called a Speed Relay in some wiring diagrams and you'll find it right where OSI mentioned.

[Charlie V]

Originally posted by OSI930

What is your car doing? [/B]

Trigger side of the fuel pump relay does not seem to work.

I was at the Whistler Weekend with the CWR and PNWR PCA folks this past weekend. Went to go on another road run with some of the great people attending the weekend. Car would not run.

Note - Air Flow Sensor switch is disconnected as part of my 5 month challenge in finding my intermittent hot start problem.

On Saturday morning the engine would flash up when the ignition key is in the start position, but immediately dies when it is in the run position. To get the car home, I hot wired the fuel pump with a jumper wire (w/ 15 amp fuse) across pin 30 and 87A on the fuel pump relay socket. Car starts and runs better than it ever did.

I want to do a proper permanent repair and correct the root problem.

So far I have done the following in my 5 month quest.........

1. replaced the fuel pump
2. replaced the check valve
3. replaced the electrical component on the back of the ignition switch
4. replaced fuses and relays in fuse panel
5. cleaned all fuse, relay and wire connection on the fuse panel
6. replaced some tired wires within the fuse panel
7. cleaned all major ground connections in car
8. cleaned multi pin connector on OXS "brain" under passenger seat
9. tested fuel pump circuit per Bentley manual - all OK

So - this leads to my question about the RPM Limiter Switch. This switch looks like it is connected to the trigger side of the fuel pump relay. I therefore would like to test it to confirm it is not part of the problem. But, the Bentley manual does not have any information on it.

Thanks for the assistance so far. Any suggestions??

[johnsjmc]

I had a similar problem with a 2300rpm surge. I accidently eliminated the overspeed relay when I moved the engine to a 1977 ROW body (rev limited with an ignition rotor) The surge dissapeared. You can disconnect the relay at the back of the tach,or at the pump relay or at the relay itself . Then if your start/run problems change then your rev relay is bad. I used a rev limiting ignition rotor original to a 3.0L turbo( I believe)I don,t think the 2.7 one will work because the SC dist is physically bigger.Bosch part#1 234 332 223-850 6800RPM
The factory went to a fuel cutoff in the States because it was better for the catalyst to not send it unburned fuel by cutting ignition. Therefore test is by removal BUT be careful not to overspeed until you replace with either system. They can go bad and affect lower speed running.

[Charlie V]

After tearing apart most of the dash and removing the fresh air blower to access wiring from the front, I think I have finally narrowed down the problem.

It looks like the fuel pump relay socket needs to be replaced.

The RPM Limiter Switch and factory alarm test OK. All wiring from the ignition switch to the RPM Limiter and alarm to the RPM Limiter tested OK. But, when I put various relays into the socket I get random success in the fuel pump working. The relays always work in other sockets.

Took me better part of 5 hours to get to here. Plus a couple of hours on another day to put everything together. I ran out of patience for today.

So - now I need to cut deep into the wiring harness. Something I really did not want to do when I'm not 100% sure this is the problem. However, everything else seems to be eliminated.

I will update with results of replacing the socket.

[johnsjmc]

Replacing that socket doesn,t go very deep Power comes off the fusebox and only 4 or 5 wires to splice. I suggest solder not crimp splices