Location of airflow detect switch

Can anybody tell me where I can find the airflow detect switch on a '79 3.0 SC? It's the switch that keeps the fuel pump from running unless the engine is turning. My fuel pump is on whenever the ignition is turned to ON which shouldn't happen unless that switch is either defective or has been bypassed.

Bill:

Unless I'm missing the question in your posting entirely, the fuel pump SHOULD be coming on when you turn on the ignition. The purpose of that is to make certain that the fuel system is pressurized, even though there should be a correctly operating check valve which maintains pressure when you shut off.

However, if you mean that the fuel pump is coming on when you switch on the first detent, which is the accessories detent, then you MAY have a problem. But the fuel pump definitely ought to come on with the ignition, it stays energized during the starting motor function, and when you release the starter, the fuel pump keeps running.

There is no separate switch for the fuel pumps on any SC that I know......Andras

In the schematic in my Haynes manual, there is an airflow detect switch. My understanding is that it keeps the fuel pump from running except while the engine is turning (either being started or running on it's own).

On the Pelican Parts electrical diagram http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part1-2.jpg
just below the fuel pump relay.
It is called the airflow contact sensor. It grounds the coil side of the fuel pump relay. I think a mechanic working on my car, bypassed this switch when trying to fix a cold start problem. I'd like to remove the bypass and get it working the way it should work.

Wckrause,
Reach around to the left of the air cleaner and around the back of the air box. I don't think you can see the connection, but feel around for it. Once you know where it is, it will be easyer to find next time. The hard part will be finding the wire and plug. Hopefully it will be nearby.
As you know, the fuel pump does not come on only with the ignition switch, but comes on with air flow from a running engine, when the air flow acuates this switch. I don't know if there is enough air flow at cranking to turn the pumps on through the air flow switch.
Bob w.

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Thanks Bob, I'll look for it. Is that why my mechanic may have bypassed the switch, to insure fuel pressure at start up? I don't think it made any difference. Is the reason the switch is there for a safety cut-off of the fuel pump, if the fuel line gets ruptured? If so, it makes sense to get it working properly again.

This air flow switch will turn the pump off if you get in an accident and the engine stops. With out it the pump runs and contineously feeds fuel to the engine even if it is not running. Some cars have an "impact" switch that will shut their fuel pump off in an accident. My Ford Tarus has a resettable impact switch in the trunk that turns off the fuel pump upon impact.(of the car) I have seen a replacement "impact" switch with bracket that can be used on a porsche to replace the air flow switch. With this replacement switch you can have the pump running with just the ignition switch on and still be protected in an accident. I see this conversion only useful if you are having some pressure leakdown problems and find this switch conversion an easyer (or cheaper) solution than finding the pressure leak.
Your mechanic probably did pull the connector off to get the pump running with just the ignition switch on.
Bob w.

I'm not having any luck locating either the switch or the wires going to it. I tried looking for the wires coming from the drivers side of the engine bay (brown/black, brown)but didn't have any success. Anybody got any helpfull advice?

I am not that familiar with your engine but your injection system (mechanical) is the same as some mid eighties Volkswagen, and i know them. The switch you are looking for for should be (is) in the airflow meter. Your problem could be that but also could be that a wrong fuel pump relay was installed or bypassed. The relay should turn the pump on for 7 to 10 seconds and shut it off until you crank the engine. I would check this first.
Mike

Mike,

The relay is the correct one because I've just replaced it. I've looked to see if it's been bypassed some where up near the front fuse box, but didn't see any wires that shouldn't be there. I'm guessing that the previous mechanic bypassed the air flow switch at the connector to the switch, but maybe there is another place where he could have done that.

WCKRAUSE,
I went out and looked at or rather felt the location of the air flow switch on my wife's '78SC. I had always unpluged the air flow switch from my 930 from the left side of the engine, but the '78SC can be reached to be unpluged from either side. It's about the level of the fuel distributor, about an inch behind and about 3 or 4 inches to the left. Reach around behind the fuel distributor. The factory manual and the bosch fuel injection manual both show it's location.
Try using a mirror to see the connection.
Bob W.

The air flow switch is not for safety in the event of an accident. It is there to insure that there is air flowing thru the induction before fuel is introduced thru the CIS system. You don't want the fuel pump running for several seconds after you turn the ignition on, then swat that mosquito with your left hand, then reach back and turn the key to START. By that time there will be enough gas in the system to produce a strong likelihood of a backfire.

With the 3.0 CIS, if you service the fuel system and release the residual pressure, you must refill the gas in the system. The manual describes it well. Take off the air cleaner, turn the ignition switch ON, then push up on the sir flow sensor until you hear the fuel pump run (the flow switch just activated the pump!). After a couple of seconds, turn everything off. Then start the engine normally.

All things said, it would be very risky to bypass the air flow switch.

Bob -- Thanks to your precise directions, and my wifes compac mirror I was able to find the connector, and the end of the cable that plugs into it. With one hand around one side of the air box and the other had around the other I was able to get it re-connected. I was expecting to see a jumper wire or something to bypass the switch, but there wasn't anything, and now everything works as designed. There is a 1/2 second lag from when the engine starts turning to when it catches. Before there was no lag, (but there was also an occasional backfire!). Maybe this solves that problem too. Whether it's a safety feature, or a backfire prevention feature, it's now fully functional. Thanks again Bob.

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Bill Krause
'79 911SC Euro

The air-flow switch has only one function, it turns on the fuel pump as long as the ignition is on and there is air flow through the air horn either from cranking the engine or from a running engine. As to why this switch is necessary, the Bosch fuel injection manual states, QUOTE: "The pump continues to run when the engine has started. A safety circuit is incorporated to stop the pump running and fuel being delivered if the ignition is switched on but the engine has stopped turning (for instance in the case of an accident)."
(me again)If this air-flow switch is unpluged, the fuel pump will then be on when ever the ignition switch is on. This is not a problem to the the fuel system and won't cause fuel to be injected. Once fuel system pressure has reached its set limit then excess fuel and pressure is returned to the fuel tank. When the fuel pump is shut off (ignition off or engine stop),system pressure is then maintained by the fuel accumulator. Again, this system pressure does not cause fuel to be injected.
Fuel is only injected when the air-flow sensor plate is deflected.(the big round plate in the air horn;not the air flow switch that turns on the fuel pump just mentioned) Deflecting the air-flow sensor plate causes fuel to be metered from the system pressure side to the fuel distribuiton side of the system. Deflecting the air flow sensor plate will inject fuel as long as the system has enough pressure to over come the injectors opening pressure.
The injectors are fitted with a valve needle that vibrates (chatters) at a high frequency when fuel is injected. This is what you hear when you deflect the sensor plate. These two systems (fuel pressure/fuel pump and fuel metering) act independent of each other. Moving the sensor plate will inject fuel whether or not the fuel pump is on. Movement of the sensor plate does not cause the fuel pump to turn on but only causes fuel to be allocated to the injectors as long as there is system pressure. Whether the system pressure is from residual pressure(pump off) or from pressure maintained by the fuel pump(pump on), it makes no difference-fuel will be injected.
There is no timed pre-running and then shut down of the fuel pump on this system. The pump will not run with just the ignition on(for any length of time).
Running the fuel pump alone will purge the system high pressure side. Deflecting the sensor plate will allocate fuel through the fuel distributor and purge the fuel injector lines. This information is in the fuel injection manuals.
Bob W.

wckrause,
Glad I was of help in locating the switch and connector in your SC. Hope everything is ok now.
Bob W.

OK, so how bad is it to have this switch dis connected? I was having trouble a while back starting my car (I had to crank and crank to get it to start). My mechanic said that it was this switch you all are talking about and that sometimes they go bad. As a quick fix he just disconnected it. After it was disconnected it started up fine. The only drawback was the fact the with the key in the on position the fuel pump runs. (you can't leave the key on to listen to the radio) Should I replace the switch?



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Leland Pate

___79 SC Targa

I think another problem with disconnecting the switch is that the warm up regulator, the temperature valve, and the supplementary air valve get power when the key is in the on position rather than when the engine starts. This may effect how the engine cold starts. This could be bad if you hesitate long enough between turning to ON and to START. I think IMHO, as I have read in other posts, it's better to have it working as it was originally designed (either by Porsche or by Bosche).

Leland,
I always considered disconnecting the switch only a safety issue, as having the pump running won't hurt any thing. However, I had forgotten about the radio. And the noise! Who wants to listen to the pump with the engine off? I would replace the switch to not have to hear the pump run (and to be safe in an accident). But if you don't, again it won't hurt any thing.
Bob W.