80SC fuel pump/air sensor switch

[CharlieB]

Just bought an 80SC. All stock and showing 118,00 miles. Replaced fuel pump and relay, fuel filter, accumulator,O2 sensor and relay. Fuel pump runs with ignition in ON position. Understand it should only run in START mode then air sensor switch takes over and provides current to the pump while engine is running. The old fuel pump relay has a small piece of cardboard wedged between the contacts. When I removed the piece of paper I put the ignition in ON and lifted up on air sensor lever. The pump didn't run. Checked behind the air sensor and the switch is connected to the sensor body. Had PO,s work orders and noted one that said "rewire fuel pump relay". I'm assuming that the sensor safety switch has been intentionally eliminated somewhere around the relay probably to take care of an SC hot start issue. With the current setup the car starts instantly hot or cold. Am I on the right track here? Any harm leaving well enough alone? I'll ask about engine "hunting rpm's" later, enough for now!!

[fred cook]

My '80 SC is wired the same way as yours. That is the pump runs with the ignition on even though not being cranked. And yes, my car starts instantly also. I have had my SC for about 7 years and there doesn't seem to be any problem with the way it is wired. Just be certain to pull the fp relay if you need to leave the ignition on but not running. Otherwise you will flood one or more cylinders and likely lock up a cylinder with compressed fuel. Of course, if you had a collision, the pump would continue to run causing a potential fire hazard.

Good luck!

[CharlieB]

Fred: Thanks for the reply. I just pulled the master fuse box and checked the wires going to the fuel pump relay connector. All the wires going into the rubber connection are accounted for except a small red wire that according to the diagram is from the rev limiter. I haven't pulled the connection apart yet to check to which pin each wire is connected. How do I get the rubber connection apart? I tried prying with a small screwdriver, got nervous and quit!!! I wonder if an airflow sensor contact switch can go bad like the rest of the CIS stuff. I hope I'm looking at the correct switch connection on the back(towards front of car) of the air flow sensor.

[cmonref]

This is a very curious statement:

"The old fuel pump relay has a small piece of cardboard wedged between the contacts."

The fuel pump relay is a sealed relay in the fuse box in the front trunk. If you have a relay with visible contacts, it is not stock. That means the PO's work may be difficult to trace and correct.

Fred's caution about leaving things alone is important. When the ignition is on, the pump is running and fuel it being injected into each of the cylinders. If a fuel line breaks and the engine quits, the fuel pump still runs. If you're upside down in a ditch and unconscious, the fuel pump will probably still be pumping fuel at 75 psi.

I think that returning the fuel pump power circuits to the stock setup is an important safety issue. That way, if the engine is NOT running, neither is the fuel pump (except when the ignition key is in START.) Any other way is dangerous.

Brian

[CharlieB]

Brian: The old fuel pump relay is a stock unit. Someone pried the plastic cover loose and you can see where it was glued to the relay itself. Fortunately they put the cover back in place. I put in a new relay, turned the ignition on and the pump didn't run. Went back and lifted the air sensor lever to activate the safety switch and the pump still didn't run. How do I get the rubber connection apart so I can check the wiring to the pins on the relay?? Also I just got back from a 30 minute run on the Interstate and now my idle is running back and forth between 900 and 1200 rpms and won't settle down to a steady idle. Wasn't quite this bad before putting all the new stuff on!!!! I think the bucket of worms has now been opened!!!!

[cmonref]

I've never seen a fuel pump relay with it's cover off. I had always figured the unit was "hermetically" sealed.

Is the air sensor switch connected? The external connector is on the passenger side (front side in direction of travel) of the air sensor unit.

If NOT connected, check to see if the wiring's two contacts have been jumpered by the PO -- that would fool the relay into thinking that the engine is running.

If CONNECTED, then disconnect and check continuity across the air sensor switch -- open with air sensor at rest, closed with air sensor lifted.

If the pump still does not run as it should, the PO's work may have been behind the fuse panel in the front trunk. Two or three screws and it moves neatly away from the sidewall (be gentle and proceed with care), allowing access to wiring behind. Before you do this, have the wiring diagram by your side to check wires.

The idle problem should not be related to the fuel pump problem. If the pump and engine are running, you can tune out the idle problem. But that is a different ballgame.

Brian

[Gunter]

Originally posted by CharlieB
Brian: The old fuel pump relay is a stock unit. Someone pried the plastic cover loose and you can see where it was glued to the relay itself. Fortunately they put the cover back in place. I put in a new relay, turned the ignition on and the pump didn't run. Went back and lifted the air sensor lever to activate the safety switch and the pump still didn't run. How do I get the rubber connection apart so I can check the wiring to the pins on the relay?? Also I just got back from a 30 minute run on the Interstate and now my idle is running back and forth between 900 and 1200 rpms and won't settle down to a steady idle. Wasn't quite this bad before putting all the new stuff on!!!! I think the bucket of worms has now been opened!!!!

Air sensor contact is behind the metering housing.
The plugs are color-coded; make sure that the green plug is in the green socket.
Blue plug (CSV) in the blue socket, etc.
People have mixed up the plugs because they all fit, that's why the different colors.

I would really try to wire it back to OEM to control the pump.
Have a look at the main harness near the Bosch CDI module:
Do you see a small white 2-pin plug coming out of the 14-pin bundle?
With ignition ON, it should have 12V and Ground on the other pin.
If it is plugged in, it provides power to #15 for the module and functions in conjunction with the ASC.

The eratic idle is mixture related and is called "Hunting"
Usually too rich; adjust with the 3mm LONG Allen key after making sure that cap, rotor, wires, plugs etc are good and the timing is correct.
I set to maximum advance; go here:
Maximum advance for SC 3.0 or ss 3.2

[CharlieB]

Guys I really appreciate the input and advice. I hope I'm looking at the correct switch on the air sensor. Not a lot of room to look around. I do have the heater blower out so that helps. There are a couple of photos in the Bentley book that show the front side of the air sensor and the connector that I'm calling the sensor switch. I'll try all your suggestions. How do I get the fuel pump relay female connector on the front fuze block apart so I can check wiring to the relay pins???

[cmonref]

The fuse block is held to the fender well by 4 screws. Remove them and "fold" the block away from the fender. Carefully and not very far --you'll be able to see the wiring behind the relay socket. I doubt you can take the socket apart to check the wires.

[CharlieB]

So the fuel pump relay socket is basically prewired into a molded rubber unit? I'm thinking that perhaps the PO may have simply rewired the yellow hot wire that comes from the ignition switch "start" pole to the "run" pole so that power goes to the relay when the ignition is in "run" instead of "start". Of course if the air sensor switch isn't working and the relay is wired to "start" there won't be any power to the fuel pump once you release the key from "start"! I'll pull the ignition and check it. Found a photo in the Porsche Companion page 115 that shows what I think is the air sensor switch on the front of the air sensor. Can you confirm this??

[cmonref]

I don't have a copy of the Porsche Companion so can't confirm. You may have to remove the air horn to feel around on the "front" side of the sensor mount.

The switch looks identical to the one on the WUR and the AAR -- two prong. Note Gunter's description above and his caution about colors of the connectors.

Keep in mind that the simplest way to defeat the sensor switch is to jumper the wires in the connector -- or cut off the connector and wire together.

Brian

[CharlieB]

I think we're talking about the same switch. I can reach around on the heater blower side and feel the connection on the air sensor. The wires are intact and plugged into the connection and sensor. I'll get a light and mirror and check the wire colors tonite. Thanks again!!

[Gunter]

Charlie:
Buy the Bentley SC Repair Manual, you'll love it.
Forget the inside of the fuse sockets; check the wire colors going to the sockets by carefully folding back the fuse panel as suggested above.
Get the right electrical diagram and use the wire colors to see what kind of wiring was done by PO.

[CharlieB]

Gunter: I've got the Bentley book and have been studying the diagrams. All the correct color wires seem to be going to the pump relay socket. The socket has a black wire (to fuel pump), red/white (from WUR), red (to fuse #16), yellow (to starter switch), brn/red x 2 (from air sensor switch) and a red jumper wire. The only wire missing is a red wire from the rpm limiter according to the diagram. Everything else checks out per the diagram!

[Gunter]

Sounds like someone disabled the Rev Limiter. No problem.
The objective is to stop the pump from running with ignition ON.
Back to the Air Sensor Contact which controls the pump relay when OEM wired and working correctly.
Can you post a picture of the area where the 14-pin plug comes out?
Take the cover off the rear fuse panel.
Out of the bundle come 2 wires into a 2-pin plug, then continous to the CDI module.
I would like to see a picture of that area if possible.
The 2-pin plug has 12V to pin 15 on the module and the other one is ground.

[CharlieB]

I'll check it out this evening when I get home and the engine cools down from the trip home!!