Fuel Pump circuit?

Tom '74 911 · 03-24-2007, 10:17 AM

My '74 911 (with '77 2.7 engine) wouldn't start yesterday evening. Fortunately, it's in my garage and not stranded in some far-away place! I had driven it to work, and home and was about to drive it to dinner when it wouldn't start. Typically, when I turn the key to start it, I can hear the fuel pump humm/buzz before the final twist of the key to engage the starter. Last night, it started to humm/buzz as usual, but then stopped - and the engine would turn over, but not start. I'm assuming because it wasn't getting fuel. This morning, still no humm/buzz from the pump.

I have read that some people have fuel pumps that humm/buzz and some do not. I've also read that some think the humm/buzz is a sign of imminent failure. The fuel pump on my car has always made noise - that's all I know about that.

The fuse in the front compartment is good, I have a spare relay that I swapped and still no luck. I have a circuit tester light that I placed on both sides of the fuse and it all seems good. So, I assumed it's the pump itself that's failed.

Now the strange part. I jack it up, pull the front pan (I should mention that although my car's a '74, the fuel pump has been relocated from the rear, to the front) and I turn the key on, which should make the circuit hot, and try the test light on the wires at the pump itself, and the light doesn't light up.

Am I testing the circuit correctly? Fuel pumps are expensive! I don't want to buy a new one and then find out it's a wiring issue. Can I hot wire the pump directly from the battery to see if it will run? I'm an electrical systems novice for sure and I don't want to burn myself or my car up!

len911 · 03-24-2007, 10:33 AM

Tom, if you car has CIS, you need to remove the air cleaner and push up on the air metering plate. The FP has a safety switch that makes it work only when there is air flowing into the engine. Lou

alan911 · 03-24-2007, 10:50 AM

On a stock '77 engine with all it's parts, len is correct, unless you push up on the metering plate with the key turned on, you won't get a complete circuit at the pump. If everything is working, with the key "on" pushing up on the plate will give you the pump buz and complete the circuit.

The "safety Switch" I forget what it's really called, is located behind the the throttle body and tough to get to. You have to use the brail system to get to it. It doesn't matter if it is connected or not. The car should start anyway.

Connected= no buzz when the key is turned on.
Dis-connected=buzz when key is turned on.

Rebuilt fuel pumps about $110. worked for me.

Tom '74 911 · 03-24-2007, 11:08 AM

Thanks for the tip on the air metering plate. Can one of you guys point it out in the diagram? Is this the right diagram?

Thanks,
Tom

alan911 · 03-24-2007, 11:57 AM

Not quite the right dia. but you can barely see it inside #1.

Just remove your air filter and you can see it clearly.

I think that's a '75 diagram. It should be fairly close but it's missing some '77 parts.

dm36415 · dm36415's Garage

A quick test is to remove your relay, jump pins 30 and 87a and listen to hear if the fuel pump whirs.

I had a similar problem a while back, turns out I had a vacuum leak on the back of my CIS boot, thus not enough vacuum was present to raise the sensor plate when I tried turning the car over. Long thread, but lots of good testing advice can be found at

1977 911 more fuel problems

Walter_Middie · Walter_Middie's Garage

The air metering plate is 5b.

teenerted1 · 03-24-2007, 04:37 PM

are you sure the FUSE is ok?

you checked the relay and its ok. and you checked both side of the fuse, but wouldnt that complete the circut?

Tom '74 911 · 03-24-2007, 05:00 PM

Quote:

Originally posted by teenerted1
are you sure the FUSE is ok?

you checked the relay and its ok. and you checked both side of the fuse, but wouldnt that complete the circut?

I'm confused too! After my last post, I went back out to verify everything once again. I did some jiggling and testing and more jiggling and wouldn't ya know . . . the pump fired up. I'm thinking that there may have been just a very slight amout of corrosion on the fuse. . . and that maybe I was testing the wrong one. . . I did admit to my electrical systems ignorance in my first post!

Anyway, for the moment, it's working again. Thanks for all the tips. I'll have more things to troubleshoot next time.

Thanks,
Tom