Dash01

Inserting a circuit amperage tester into the fuel fuse socket, it shows no juice to the fuel pump circuit when the key is turned to ON (not crank) position. In other words, turn the key to ON and no juice flows to the fuel pump, which remains silent.

What (other than a bad ignition switch) would cause this?

Since this test does not involve cranking of the engine, there would be no tach bounce, etc.

Thoughts?

dr914

fuel pump for 944 models works in the crank position only 85,2 and up on fuse 34. early on fuse 2 of the aux fuse block

Dash01

Thanks.

To clarify, fuel pump does NOT come on and so no juice flowing until the engine starts to crank? I'd read that inserting the key and turning to ON (but not crank) would energize the circuit and pump, such that it is faintly audible while pressurizing the fuel lines so that when cranking starts, there's fuel flowing.

Further question: If the fuel pump is only activated when the key is turned to crank, then how does fuel flow when the engine starts and the key is returned to the ON position?

Seattle 993

Dash, I just went out to my 944 turbo, turned the ignition to on (not crank) and I do indeed hear a faint sound like an electrical motor toward the rear of the car. I assume it’s my fuel pump but didn’t crawl under the right rear of the car to see. My 944 starts right up.

i would go to Clark’s garage if you haven’t already and look under engine troubleshooting in the garage section. Clark did an amazing job of helping the entire 944 community before he passed a few years ago. So many people refer to his website. I’d follow that trouble shooting closely.

tempest411

The ECU (DME in Porsche-speak) will continue to deliver 12V to the fuel pump via the DME relay IF the starter is spinning up the engine fast enough AND it sees a signal from the speed and reference sensors.

Dan Martinic

Yep.. needs to be min. 200rpm I believe for Fuel Pump to operate (cranking or running). It's a safety feature: in case of accident and engine stops, fuel pump stops too.

Not sure what Seattle993 is hearing but if it's the FP, then perhaps he's got a solid state DME Relay with "pump prime" (using the other circuit in the DME Relay--the one for the ECU--to power on the FP at the same time)

Dash01

Thanks for checking that, which confirms that the fuel pump is energized by the key first in the pre-crank ON position, and of course continues running with the now-started engine after the key is released from START back to ON. This is how all my Bosch fuel injection systems on several BMW bikes and cars of common '80s vintage to our Porsche 951 cars, as the fuel systems must pressurize before start, to enable starting. Most or all have check valves to maintain pressure in the lines after shutdown, so you won't have to crank and crank at the next startup. OTOH, if line pressure from the last run is properly retained by check valves, the car may restart without much if any extra fuel pump input, and the pump comes on as needed while driving.

Unlike your running car, my '86 951 fuel pump is silent with the key ON before cranking, and a circuit tester inserted into fuse slot 34 (also , 19, and 15, fwiw) shows no juice flowing when the key is in the pre-crank ON position, so I suspect the ignition switch itself is the culprit in this crank-but-no-start situation.

Otherwise, I've swapped in known-good DMEs, fuel relays, cap&rotor, etc. from my spares collection, with no change. So, I'm wondering about the ignition switch, given the factors noted above.

Thoughts?

Dan Martinic

Dash01... read this https://philipraby.co.uk/porsche-dme-relays-explained