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Registered
Join Date: Nov 2020
Location: Meredith NH
Posts: 75
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Fuel Pump control plug??
Hi 911ers,
Always get great info here... So, this plug, directly behind the center of the fuel distributor boot, has been unplugged on my car for as long as I've owned the car. I tried plugging it in once but, don't remember the symptoms, but I think the fuel pump didn't come on. Unplugged the fuel pump runs constantly and the car has always ran fine.. What's this plug for and how is it supposed to work properly. My guess is when the air input flap is open it turns on the fuel pump. Why does my car run better with it unplugged. 
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03-28-2025, 04:18 AM
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Registered
Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 4,184
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Quote:
Also often used to troubleshoot fuel pump/relay issues - eg unplugging the switch to make the pump run can be more convenient than jumping the relay. When working correctly, the switch completes the control circuit for the fuel pump relay - so any fuel pump(s) run when the air plate is deflected off the throttle plate stop by manifold vacuum - and so cuts fuel delivery when the motor stops cranking or running. On a 930 (which has a downdraft AFM arrangement), loss of residual fuel pressure may cause the piston in the fuel distributor to allow the air plate to droop slightly - some report this causes a momentary priming action after simply turning ignition on, and before cranking, until the piston raises again under pressure, switching the pump(s) off. As I understand it, SC has an updraft AFM - so gravity won't move the plate, only manifold vacuum. There's a number of reasons it might not work correctly. The switch itself may have failed, wiring may be broken/incorrect. Or the air plate may be binding/incorrectly adjusted - although this would likely lead to the pump running all the time, rather than not running. The air plate should rest on the stop with motor not running and switch should not complete the circuit. A very small deflection (think there's a spec in the FSM) off the stop and the switch should allow the pump to run. It's not right, but other than the safety consideration, it's not really a problem. Are you 100% sure that's the correct connector? On a CIS 930, there's a CSV (cold start valve) that uses an identical connector very close on the harness/manifold, and it's quite possible to connect them incorrectly. Helpfully, these are color-coded so you don't mix them up - I can't help but notice that your male/female connectors have quite different colors....
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'77 S with '78 930 power and a few other things. |
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03-28-2025, 08:00 AM
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Registered
Join Date: Nov 2020
Location: Meredith NH
Posts: 75
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Thanks Spuggy,
I was wondering that myself if it could be the wrong connector.. The other connector on a wire, which is also Blue, is plugged on to which I think is the cold start injector and that is also blue. It is a bit confusing, The Back of the fuel distributor is Blue, The cold start Injector is Blue. The plug on one wire is GREEN, AS PICTURED, And the plug on the other wire, plugged into the cold start, is Blue. The car always starts fine so I have to believe the cold start is working? |
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03-29-2025, 04:51 AM
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Correct Plug Check……….
Quote:
Targabill, The plug color for the AFM switch has been green or blue that adds confusion when installing the engine harness back after an engine rebuild. To check whether you have connected the correct plug to the AFM switch is to verify the WIRE COLOR. The AFM plug has two (2) wires both BROWN and the CSV has the yellow wires and brown (or red/black) wire. Tony |
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03-29-2025, 05:48 AM
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Registered
Join Date: Nov 2020
Location: Meredith NH
Posts: 75
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Thanks Tony, I will double check that.. The engine is now back in the car so have to get out my trusty inspection camera.. I feel like a Dr. using that thing..
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03-29-2025, 09:07 AM
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PCA Member since 1988
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Tony is right about how to check, but if the car starts fine cold, you almost certainly have the CSV connected to the right wires. I wouldn't worry about that part.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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03-29-2025, 10:09 AM
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Correct……….
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If the plugs were incorrectly installed, you would have some cold start problem and would have burnt your wire harness at AFM section by now. Tony |
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03-29-2025, 10:29 AM
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Registered
Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 4,184
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Quote:
Gotta love how people always fixate on the CSV... It's right there in the name, right? In reality, although it can assist by initially wetting the manifold & providing vapor-laden air for the initial few charges when cranking, I think it probably has less effect than many think. My CIS 930, left outside overnight, would still fire and run immediately with significant ice on the windshield. Even with the chaincase TTS flag terminal disconnected - because it refused to stay on no matter how much/often I tightened it, so I just stopped bothering. Seems likely to me that my CSV system didn't actually work at all - given that it made no observable difference, connected or not, whether ambient temps were below freezing or 100F. A few guys even report their SCs struggle to start from cold with the CSV enabled - but start fine with it disabled. So check/measure/test/verify is likely the best approach. There's no substitute for understanding what's going on.
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'77 S with '78 930 power and a few other things. |
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03-30-2025, 08:23 AM
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