Direct wire fuel pump or not - 930

[e p slick]

Bought an old 1979 930 and have discovered that the fuel pumps have been directly wired to run when the key is in ON position before start-up. It was a home made job and needs to be re-done/cleaned up. My question is: What overwhelming reason is there NOT to leave the pumps directly wired. The car runs fine so I do not see a reason to alter the set up - other than cleaning up the prior owner's sloppy job. Any thoughts?

[Walter_Middie]

Slick,

On my stock 1975 911S (N/A with CIS), the fuel pump is wired directly to the ignition. I assume it is that way with earlier cars also. Once the key is in the run or start position, +12 V is sent through fuse #12, and then directly to the fuel pump, and the fuel pump runs - even if the engine is not running. My wiring diagrams also show that in 1977 they added a fuel pump relay, that was also carried over to the CS's. I'm not familiar with the 930 wiring, but it seems like a lot of systems were not updated on the turbos, and may not include a relay. Or the relay gave the PO some problems so he got rid of it.

[Grady Clay]

I guess my first question is “what is the engine?” Is it stock ’76 930?

A stock 930 uses two CIS fuel pumps plumbed in series and wired in parallel.

Even ’75 930 have two fuel pump relays to handle the current. They are the two left-most relays above the fuses. The SC got a pump relay as a safety device. In a crash, both SC and 930 the engine stalls and the pump(s) stop.

To check the CIS pressures you can use the standard P378 gauge or equivalent. To check the pump pressures you will need the two 9114 adaptors (easy to make from old filter fittings.)

The pumps flow is specified at 1500 ml/30 seconds minimum. The fuel pressure should be between 6.0 & 6.7 bar bar. If less than 2 bar, the front pump is probably defective. If more than 4 bar, probably the rear is defective. If I found one defective pump and the other is at all noisy, I would replace both.

Always check the voltage AT the pumps. Compare to the voltage at the battery. It shouldn’t be more than 0.5 Volts less than battery Voltage. Relays and relay sockets are always suspect as are the screw-on terminals on the pumps (fast-ons won’t handle the current.)

Best,
Grady

PS, thanks again Chris.

[dean]

Like Grady said, a crash is the reason you don't want the pumps to run all the time.