When does the fuel pump start?

[psychoideas]

Hello all.

It's a bog standard 1976 911S with 2.7 and CIS.

When I turn the key on and I get ignition lights, should the fuel pump run?

Currently it does not.

Or does it only run when cranking the engine? (When the sensor plate is lifted)

Which it does.


Thanks...

[Showdown]

On the mid year cars the fuel pump only runs when the key is in the “start” position. It won’t run in the “on” position like the SCs.

[psychoideas]

Originally Posted by Showdown

On the mid year cars the fuel pump only runs when the key is in the “start” position. It won’t run in the “on” position like the SCs.

Thank you for the information. It's a big help.

Frank

[timmy2]

Not supposed to run before start on the SCs either. Same basic wiring for the fuel pump relay from 76 to 83.
75 and earlier will run all the time with ignition on.

[psychoideas]

Originally Posted by timmy2

Not supposed to run before start on the SCs either. Same basic wiring for the fuel pump relay from 76 to 83.
75 and earlier will run all the time with ignition on.

So, on cranking, the sensor plate lifts up off the switch, starts the pump, gets to pressure and fuel comes out the injectors?
As well as squirting the cold start as required?

I'm having cold start troubles. It fires on the key but won't keep going. I'm about to look into the operation of the thermo-time switch and the cold start injector.

Thank you.

[Showdown]

Get yourself some CIS gauges and spend a lot of time reading:

https://jimsbasementworkshop.com/CIS/index.html

And

CIS Troubleshooting for Dummies

[timmy2]

Yes, to your questions about the sensor plate.

Double yes to Showdown’s post about getting gauges and reading up on the system. Fuel pressures are vitally important.

It is likely you are only running at crank because the TTS and Cold start injector are working correctly. They drop out when not cranking or the engine is warm.

[boyt911sc]

Originally Posted by Showdown

On the mid year cars the fuel pump only runs when the key is in the “start” position. It won’t run in the “on” position like the SCs.

Define mid year cars (73.5~’75)? Or up to ‘76-‘77? The FP system from ‘76 to ‘83 operate on the same principle. You got them backward. Where did you get this information?

Tony

[47silver]

My 75 pump starts with the key turned to run. The wiring diagrams do not slow any other switch and there is not any other switch that controls the pump.

[psychoideas]

Originally Posted by 47silver

My 75 pump starts with the key turned to run. The wiring diagrams do not slow any other switch and there is not any other switch that controls the pump.

There is a switch in the sensor plate/fuel distributor housing. If the engine stops, the sensor plate falls onto the switch contact and the fuel pump stops. The red relay for the fuel pump has a diode in it that plays a part in this equation. 😉

[boyt911sc]

Originally Posted by psychoideas

There is a switch in the sensor plate/fuel distributor housing. If the engine stops, the sensor plate falls onto the switch contact and the fuel pump stops. The red relay for the fuel pump has a diode in it that plays a part in this equation. 😉

Psycho,

Your above post is half correct and half false. The CIS 911 cars from 73.5 to ‘75 do not have air flow sensor switch and fuel pump relay. The fuel pump runs as you turned the ignition switch to RUN position.

However, your above post is correct for ‘76~’83 CIS cars but not for the early models. Where did you get this information?

Tony

[psychoideas]

Originally Posted by boyt911sc

Psycho,

Your above post is half correct and half false. The CIS 911 cars from 73.5 to ‘75 do not have air flow sensor switch and fuel pump relay. The fuel pump runs as you turned the ignition switch to RUN position.

However, your above post is correct for ‘76~’83 CIS cars but not for the early models. Where did you get this information?

Tony

Hi Tony,
I guess it was introduced as vehicles became safer.

I had a broken diode in my relay. And I could see the sensor plate switch spark when I let it back down (long before I got the engine to run)
So I checked out the current flow diagram and traced it back to the fuel pump relay.
I cut the relay open and found the faulty diode.

So I quest the second part of my information I got from experience 😂

Cheers,

Frank

[Walt Fricke]

The diode in this (and other relays which have one) is supposed to deal with "back EMF" (if I remember the terminology - maybe that is something different). A relay has a coil. When the relay is activated, current flows through the coil. When the relay is turned off (coil power or coil ground removed), the collapse of the electromagnetic field produces a voltage. Same as a spark coil. This stray voltage can harm electronic components.

I don't think it necessarily always does harm (many have substituted a standard relay for the fuel pump relay, especially if in a pinch, like a problem out on the road, without damage).

The sensor plate switch operates counterintuitively - when no air flows/plate in rest position, the switch is closed, completing a circuit. This disables the fuel pump relay coil, and the FP doesn't get current. When you open the switch, the FP relay can operate as normal. I can never quite remember how this does its magic. Relays can be used as logic gates, and Porsche has set this relay up as one. I've been able to figure out how this magic works, but then I forget, but I don't think it depends on the one way aspect of a diode.

Having a diode across a relay's coil can never hurt, though.