fuel pump relay

[sean996]

Hey all,

I have a '73 911s that had a 3.0L put it in while still utilizing the stock mechanical fuel injection (old school . . . not Kjet or any derrivative). When I turn the key on the electric fuel pump that feeds the MFI runs whether or not the engine is running. I thought this was a by product of the engine conversion, but when I was looking at a schematic in a Haynes manual it seems this is the way it was from the factory. There's a "speed switch" in there and I expected to see that some how attached to the pump but seems like that's more associated with the governor.

If anyone is aware of the factory relay/sensor configuration that should turn the pump on and off (if there was one) please share. There's a Bosch relay that's triggered by ignition pulses that I can make work if there was no provision to begin with or other suggestions? Don't think it's a good thing the pump runs when the engine's not running, especially since she's a track car.

Thanks,

Sean

[Steve Grosekemper]

Sean
A 73 has the pump running all the time and is not relay activated.
The mechanical injection or carburated engine must be this way as you need the pump to run before the engine cranks to have the cold start system operating correctly.

The K-Jet that was originally on your engine had a series of relays and switches to control the fuel pump, but that is now gone. 3.2L and later use the computer to do this.

You could wire in a relay but it would have to be done in a specific manor and your cold start manors may suffer if the car has been sitting for a long period of time.

[sean996]

Sean
A 73 has the pump running all the time and is not relay activated.
The mechanical injection or carburated engine must be this way as you need the pump to run before the engine cranks to have the cold start system operating correctly.

The K-Jet that was originally on your engine had a series of relays and switches to control the fuel pump, but that is now gone. 3.2L and later use the computer to do this.

You could wire in a relay but it would have to be done in a specific manor and your cold start manors may suffer if the car has been sitting for a long period of time.

thanks for the info Steve

[sean996]

Any thoughts on how I could use the alternator/charge light on a relay and toggle swith to it once I fire 'er up? I think the light goes out 'cause the ground goes away? I've wanted to do this before to hook up an hour/hobbs meter but didn't seem so obvious how to get 12V when the car is running and nothing when it's not.

Thanks,

Sean

[Steve Grosekemper]

Any thoughts on how I could use the alternator/charge light on a relay and toggle swith to it once I fire 'er up? I think the light goes out 'cause the ground goes away? I've wanted to do this before to hook up an hour/hobbs meter but didn't seem so obvious how to get 12V when the car is running and nothing when it's not.

Thanks,

Sean

Very Good Sean.
You are on the right track. Porsche used this wiring in the 73-74 CIS injected 911's.
Here's the wiring for the relay using a standard 30a Bosch relay 0332019150
#30-power
#87-to the item you are powering up.
#85-ground
#86-Hook this up to the blue wire that comes from the back of the alternator to the dash charging system light (#61)

When the alternator starts charging you will get battery voltage out of #61 and the relay will turn on.

Hope this helps,

[sean996]

thanks, yeah that helps and I'll toggle between that circuit so I can prime it properly.