no power to FP - 74 CIS

[speedster911]

Need help from the experts:

74 911, CIS everything stock. no power to the fuel pump

FP good, tested by connecting directly to battery (only long enough to feel it working)
Relay good, switched with horn relay and also switched with other relay from a different running 74 911
All fuses good

Everything was working fine, one day I accidentally left the key in the Accessory position for about a week and believe after that is when I heard the fuel pump stop working. Car has been on blocks for 8 months so I haven't been driving it.

On a 74 , I believe the power for FP comes directly from ignition switch? is 96461301200 the problem? I am not the best when it comes to tracking down electrical problems. can I jump it to diagnose- if so, what pins need to be jumped??Thanks for the guidance

[Walt Fricke]

The '74 fuel pump is, indeed, powered directly from the ignition switch. It is not powered in the off (obviously) position, or the accessory position. It is powered in the run position, and in the start position. There is no relay shown in the factory wiring diagram!

There is also no fuse in this circuit. The red and black wire from the ignition switch leads to the top of Fuse S12. Inserted into that same top connection is the red/green wire which runs to a two wire connector back in the engine bay, from which a blue wire goes to the fuel pump. A black wire goes from the fuel pump back to the same 2 wire connector, where it connects to a brown wire which connects to a ground.

At least this is what the factory manual shows for the 1974. Same with the 1975.

Do you have a volt meter? If not, get one. <$20 will do the job. You want to turn the ignition to run, and check all these points for 12-13 volts. Does your starter work? That means power is getting to the ignition switch. So turn it to run, and check the top of fuse 12 in the fuse box. If power there, check at the connector in the engine compartment.

I wonder if this is a common issue with the 911s which didn't use a fuel pump relay? All that current running through the ignition switch - do the switch contacts fail eventually? My '77 had headlights without a relay, and in 1985 it quit working. I took the switch apart - it is nicely constructed, but an internal contact had melted. The same might be the case with the ignition switch, though it is constructed differently.

What makes you think there is a fuel pump relay? Did someone add one? Sounds like a good idea, but if so there might be some mischief there for you to check out. And also maybe a fuse? By 1977 (probably 1976, when the pump was moved back to the front where it belongs) Porsche had added a relay and a fuse.

[speedster911]

^^^ - Thank you.

Yes motor turns over, all lights work,elec windows etc.

Regarding relay - thought I read on fuse box cover that the first black relay was for FP- I may be incorrect on that. But I did switch with good working relay.

Wires and colors are as you describe and I have attempted to make a few voltage tests to the connectors in the back - tests indicate no power.

Will make some additional tests targeting fuse 12 this weekend.

[speedster911]

Solved my problem, thought I was going crazy!

PO had a hidden switch cutting off power to FP. I like what they did now that I know its there. I am such a dummy, took me two months to figure this out.

So how do I prime a 74 CIS after its been sitting for eight months, FP removed etc??
Thanks everyone for your guidance

[Walt Fricke]

Since once the key is in RUN, your (on this model) fuel pump will run, you could crack a line - like the inlet to the WUR - and run the pump until you see fuel. Or something similar.

Me, I've never bothered with prime - 4-5 bar fuel pressure will fill things pretty quickly. Just expect the engine to turn over more than usual before that first start.

I don't think not finding an added anti-theft switch makes you a dummy, though. Other things, well maybe. Not that one.

[Charles Freeborn]

Originally Posted by speedster911

Solved my problem, thought I was going crazy!

PO had a hidden switch cutting off power to FP. I like what they did now that I know its there. I am such a dummy, took me two months to figure this out.

So how do I prime a 74 CIS after its been sitting for eight months, FP removed etc??
Thanks everyone for your guidance

No priming necessary. It's a "circulation type" fuel delivery (pump runs and circulates fuel from and back to the tank continuously, so if the pump is running it's got fuel (providing there are no other issues such as blocked filter, etc)

The CIS system has a "7th injector" which is a cold start enrichment that acts as a choke. If the WUR is in good nick charge the battery up and let it spin.