cameraboy75

I followed the "How To" on this site to install the Fuel Cut switch and it has me puzzled.

Stock 1987 Turbo II

I installed a SPST rocker switch that will kill the engine sometimes (rarely).
The rocker switch is installed on “B” position on the circuit opening relay which should have the same effect as the Air Flow Meter to open the circuit on the relay coil.

There are 3 scenarios that occur when I open my switch ...listing in order from frequent to rare :
1. Nothing – don’t hear the fuel pump circuit opening relay click.
2. I can hear the fuel pump circuit opening relay click and the engine keeps running
3. I can hear the fuel pump circuit opening relay click and the engine will stop in a couple of seconds.

Any ideas?

misterstyx69

iTrader: (142)

Personally I think a fuel cut switch is a lazy way of not getting your injectors cleaned and serviced professionally.
All that switch has to do is kill the pump on the positive wire..somewhere.
I don't know the write up you used so I won't comment any further as I would not put a band-aid on a bullet wound.

cameraboy75

I searched on "How To: Install a fuel pump kill switch" on this site and member sonicgroove posted a nice write up with photos which I followed to a T.

I also looked at the wiring schematics carefully in my factory service manual. This has me puzzled to the point that I want to understand what is happening.

satch

iTrader: (1)

The wire from the AFM is for when the engine rpm climbs above 2000. Before that occurs this particular wire does not prevent fuel from going to the engine w/key to start.

cameraboy75

I agree that the AFM (or my cut switch) will have no effect when the ignition switch is at the start position. The circuit opening relay has another coil that is always grounded during start.

According to the wiring diagram.... once the ignition switch returns to the on position the AFM switch is responsible for grounding the circuit that energizes the circuit opening relay coil that sends voltage to the fuel pump.

I believe that it's a safety feature to cut fuel if the engine isn't running. I can't find anything to support your claim about 2000 rpm though. And while I'm no expert, I don't think the engine could idle if this is true.

satch

iTrader: (1)

If you disconnect the ground from the AFM and turn the key to start the engine will receive fuel. If you want the fuel cut switch to work effectively then this side of the equation needs to be addressed. If you're waiting for the engine to start to give the fuel cut switch control over the supply of fuel then it is too late in the process. I believe the FSM has the info relating to 2000 rpm. If you're not relying on the fuel cut switch to prevent flooding on startup then please disregard these comments. The engine idles because at that time the car has started and the AFM door being open it uses the internal fuel switch to supply the ground to the Circuit Opening Relay. Many a time there will be a post where it is claimed the engine starts but dies after a couple of seconds. This is sometimes a function of the AFM's fuel switch being kaput or the door is stuck closed.


If the ground from the AFM is truly interrupted and the engine continues to run it could be caused by the relay being stuck or the relay is continuing to have the starter circuit wire still energized when it should not.

cameraboy75

Satch - Thanks for your time & input. I'm not trying to prevent startup but only allow control of engine shutdown by fuel starvation.

I guess I'm going to need to take some voltage measurements at the circuit opening relay to figure this out.

A sticking relay would certainly explain what is happening here. But sometimes I hear the relay click in response to me opening the circuit with my installed switch. I would interpret hearing a click with the relay opening (not sticking), but the engine seldom dies.

I would think that if there's an issue with the ignition switch providing voltage to the start coil in the relay, the starter motor would still be cranking???? when the ignition is at the on position.

This is driving me crazy, but it's been too hot outside for me to take any voltage readings.

satch

iTrader: (1)

The Brown wire from the AFM which supplies the relay w/the ground after startup should register 12 volts w/key to on but engine off. Are you using the Brown wire for your switch? The power to the relay provided by the Engine fuse (top row center of three wires) should disappear w/key to off. If it were backfeeding voltage from a buss fuse (constant power no key necessary) then it won't turn off. If the Brown wire had a constant ground (could be due to an accidentally grounded wire or the fuel check connector is jumpered)the engine should still turn off because the B/W wire would lose voltage.

cameraboy75

I'm not using the brown wire going to the AFM.

I'm using the B/W middle wire of the 3 on the relay that provides 12v to the coil as you stated.

As I stated in the original post of this thread, sometimes my SPST rocker works but most of the time it doesn't. Sometimes I actual hear the relay click and most of the time I don't.

It sounds like the other coil in the relay used to start the engine, is still energizing the coil. I didn't think that was possible unless the starter was cranking.

cameraboy75

I installed the rocker switch in the panel below the steering wheel that has the A/C vent in it for the lap cooler. I removed the panel to take it inside to cut a whole in it and installed the rocker switch.

When I reinstalled the panel, the back of the switch where the wires connect to it were forced against the flex hose air duct that is connected to the lap cooler vent. This caused the switch to function erratically. I removed the panel to troubleshoot the problem and take voltage readings. With the panel removed from the lower dash the switch worked 100% of the time. I simulated the pressure by hand that the switch was being subjected to when it was installed and I was able to reproduce the intermittent behavior.

I removed the flex hose that was connected to the lap vent and the rocker switch works as expected.