Pry

For some reason I don’t get power from the R53 DME and fuel pump relay to the fuel pump. The car is a ’97 993 Turbo if that makes any difference.

When I measure the voltage of the disconnected leads of the fuel pump while cranking the car (or when making a jump lead from the relay connector 3 to connector 7) I get 12 V. When I reconnect the leads to the fuel pump and crank the car (or the jump lead) I get 0 V and the fuel pump won’t work.
When I make a jump lead straight from the relay fuel pump output (connector 7/87b) to the fuel pump positive terminal and crank the car the fuel pump works fine and the car starts. The fuel pump takes 8 amperes when working.

So the relay and the fuel pump both works.
It’s not the fuel pump fuse either because I get the 12 V when the pump is not connected and the fuse seems to be fine.
Could there be something between the relay and the fuel pump positive terminal that can’t take the load?

Garth S

Make a 3-way jumper to (temporarily) replace R53 ...... connect pin 30 ( +12V) to both pins 87 & 87b: try to start the car.

If it starts and runs, the relay is defective. If that is the situation, either repair or replace the relay.

In one of your tests above, it is mentioned that +12V was measured; however, that can be misleading as a milliamp will still read 12V, even though 8A are req'd to energize the pump.

Pry

Thanks for the tip. I made the jumper and it won't start the fuel pump. The relay works fine and there's power coming to it.
I made a jump lead from the relay pin 87b straight to the fuel pump and the car works like there's nothing wrong:
https://www.dropbox.com/s/nk65m6ommq...2014.55.29.jpg
https://www.dropbox.com/s/37zwk77mbu...2014.56.32.jpg

The problem must be between the relay pin 87b and the fuel pump.

Garth S

Well done! .... in the CE panel, is there any sign of damage to the female side of relay pin 87b? ... or is that connector possibly pushed too far into the socket? Wires rarely fail in the harness verses at the connectors.

If the wiring diagram confirms that there are no other branches off of the +12V (green) wire to the pump, then your jumper wire is a very workable solution .

techman1

Had a relay pin in a panel for a 928 that was loose, and came loose in the panel when the relay was seated. Looked in the back of the panel, and you could see the pin pop out when the relayvwas seated

IXLR8

There is.

I would hook up a conventional headlight or two in parallel (~5A or 10A draw) to the relay side of fuse 26 (fuse removed). That will test the relay contacts as well as the wiring to the fuse. Note the voltage drop.

If all is OK, then I would remove fuse 26 and apply battery power to the pump side of the fuse connection and attach a load to the positive lead of the detached pump wire. That will test that lead as it has a node with another wire coming off it. Those joints have been known to fail in general. Note the voltage drop.

Pry

Ok, thanks. Have to try the headlight connection.
The wire from the relay connection 87b to the fuel pump had 3k ohm of resistance.

The problem started to happen always when the car warmed up. Yesterday morning it started, ran maybe 30 seconds, stalled and then I couldn't get it to start again without the jump lead I made.

Pry

The previous owner had done some own wiring between the DME relay and the fuel pump fuse. Once I got the wiring removed it started to work.

hkspwrsche

Interesting but, odd. Any idea what they were doing on the wiring?

Pry

Maybe some kind of kill switch between the relay and the fuse?
Didn't have time to check where the wires went so I just disconnected them, left them there and reconnected the factory wiring.