Power to Fuel Pump Fuse?

[72T]

87 911, replaced a dead battery, now it will turn over but seems to not be getting fuel. While I was changing the battery I found a recent mouse house so I've started tracking to see if the wiring to the front fuel pump may have been chewed down there.

Here's my question: With the key 'on', several fuses show powered, but not the #6 that goes to the fuel pump - I changed the fuse but no change. I am not hearing that pump whirr before starting and I think I have in the past. Should that fuse show powered before the car starts?

Thanks for any help, I'm not a mechanic or a real do-it-yourselfer, but want to take a run at this.

[gerakroo]

Your fuel pump is supplied from the DME harness. Its a red-green wire and that should be on the 3rd fuse of the 10 pole fusebox. This wire comes from terminal 87B on the DME relay under your chair. If you need further help please let me know !

[72T]

OK, thanks. I switched out the relay just in case but no difference. I'll try to chase that wire.

[gerakroo]

do you hear the relay "click" ?

[76FJ55]

The fuel pump fuse will not have power without the engine running of the key in the crank position. the DME computer sends ;;a signal to the DME relay to close the fuel pump contact when it sees engine rotation.

for testing purposes you can jump power from the neighboring fuse which is hot all the time to the fuel pump fuse to power the pump directly without the DME circuit (see excerpt from the service manual below). if the pump comes on then you n=know the path from the fuse to the pump and the pump itself are good. if it doesn't come on then you can start investigating that portion of the circuit and the pump.

[nospiners]

Interesting, subscribed !! Crank no start after replacing battery , what else are we goin to learn from
these cars ?

[72T]

OK, I hear the DME relay clicking so I guess I'm good to that point. Is my next step removing the metal panel and accessing the pump to see if power is getting that far? If so, will my simple lighted voltage detector?

[gerakroo]

Its better to first test if your fuel pump starts running if you bridge the fuses as described by 76FJ55

[72T]

OK, I'll do that first. Thanks

[gj22056]

I'm learning a lot from this thread

[72T]

Morning Update: I bridged the fuses last night and the pump started. I'm guessing that means the problem is between the DME relay and the pump?

[gerakroo]

I think there's something wrong with your crank sensor.

Your crank sensor gives no signal to DME (sensor defect)
Or the DME gives no output signal to the DME relay. (DME defect)

[76FJ55]

Have you verified that you have spark? If you do not have spark then you need to start there. as mentioned above, the DME controls the DME relay which powers the fuel pump. if there is no spark the DME will not close the second set of contacts in the DME relay to power the fuel pump.

[72T]

I'm reading that since the pump activates with the bypass at the fuse box, I can see if the car will start. If so, that tells me the problem is within the DME system. Sound reasonable?

[76FJ55]

yes, it is worth a shot. if it does, then the first place I'd look is the DME relay, even though you state that it has been recently replaced, it may be worth trying one from a running car if you have anyone near by that you can swap with to test.

[76FJ55]

If on the other hand, it doesn't start. Check for spark. I will assume you don't have spark if it doesn't start, so then I would proceed to compare the resistance values pin to pin on the speed and ref sensors. they are identically sensors, so the measured values should be very similar between like connections on each sensor.

If you need to replace the speed and ref sensors there is equivalent BMW part that is about half the cost of the Porsche part. the BMW sensor just has slightly longer leads.

[72T]

Update: No spark at the plugs. Do the sensors just die? It seems odd to me that the car was running well, battery dies, and that takes the sensors out? As mentioned, there was a mouse in the frunk, so I started to check the wire from the fuel pump to DME -I'm assuming there is one - and found this in the smuggler's box. It almost looks intentionally disconnected but I have no idea what it could be.

[nospiners]

Any updates ,did you fix the problem ?

[76FJ55]

Originally Posted by 72T

Update: No spark at the plugs. Do the sensors just die? It seems odd to me that the car was running well, battery dies, and that takes the sensors out? As mentioned, there was a mouse in the frunk, so I started to check the wire from the fuel pump to DME -I'm assuming there is one - and found this in the smuggler's box. It almost looks intentionally disconnected but I have no idea what it could be.

Yes, the sensors randomly die.

I'd look at the sensors as post previously.

that is not a wire per say. that is actually the temp sensor for the evaporator. it is used to control the A/C temp.

[blucille]

The stuff is the smugglers box is all related to AC. It won’t keep your car from running. Time to look elsewhere.

Crank sensors, cyl head temp sensors, dme relays all fail. Suddenly. If this is uncharted territory, they can all be untested with simple tools and with guidance by searching this forum but it takes some time to find the culprit.