MFI engine stalls at idle when RPM transducer is engaged

[SFE]

Did it and at 900 thé Light is flashing that’s the problem !

[Tyson Schmidt]

Originally Posted by Zuffenwerker

This is totally false because the microswitch is closed at idle which would let current pass right through it!!

If the transducer thinks your above a certain rpm with the switch closed (throttle shut) it will send current through the switch to the solenoid.

Ah, you're right. I wasn't thinking about it stalling at idle with the throttle closed.

But on the tach being disconnected, the transducer still gets the tach signal. I was suggesting to disconnect the tach because I've seen them cause stalling, and no starts.
The coasting shutoff will still work with the black/purple wire disconnected at the back of the tach.

[mysocal911]

Originally Posted by SFE

Did it and at 900 thé Light is flashing that’s the problem !

Ignition points bouncing?

[SFE]

So we checked all the wiring for continuity or good ground.
The wiring is correct at the rear fuse box (compliant to the 69 design) and correct between the transducer and the Tach (compliant to 72 design).
I check a new switch and same problem.


I checked the behavior :

1/ Results at 2000 RPM :
Switch disabled: 0V --> normal operation.
Switch activated: 12V --> normal operation.

2/Results at idle (900 RPM) :
Switch disabled: 0V --> normal operation.
Switch activated: 0V for a few seconds, then a few very rapid 12V peaks (engine variation almost indistinguishable) then 12V becomes permanent and the car stalls. --> abnormal operation

Well for me there is 2 possibilities : my new transducer is bad (I'm waiting to my rebuild original transducer hopefully this week) or the problem is coming from the mixed wiring between 69 and 72 design.

[SFE]

Did another test today...if I disconnect the black/violet wire at the distributor the transducer don't work at all, which is normal I guess.
Then I have disconnected the same wire behind the Tach (PIA for the access...) and same behavior : engine stall at idle when the switch is depressed.
So I would say the Tach is not faulty...

[SFE]

Check my car with another CDI Unit today and same behavior...

I've checked all wiring again and everything is as per the 72 design.
Don't know where the 2 brown wires on pin 3 of the transducer are going but for sure they are grounded.

Here is my exact wiring, anything wrong with it ?

[mysocal911]

Originally Posted by SFE

Check my car with another CDI Unit today and same behavior...

I've checked all wiring again and everything is as per the 72 design.
Don't know where the 2 brown wires on pin 3 of the transducer are going but for sure they are grounded.

Here is my exact wiring, anything wrong with it ?

1. The speed relay should only have 12V on pin 2 when the idle (micro) switch closes,
and not always connected to 12V like the CDI.
2. The speed relay output (pin 1) goes to the fuel shutoff valve.
3. Pin 3 is ground.

[SFE]

The wire connected to pin 2 has 12V when ignition is ON.
Therefore I suppose the electronic inside the relay is authorizing 12V to the microswitch and then to the shut off solenoid depending on RPM and switch closed or not ; the problem is I have 12V at idle that energized the solenoid and it should not be like that !

The red wire on the CDI (pin B) is connected on the same fuse than the pin 2 of the transducer, so it has 12V also only when the ignition is ON.

Do you think the CDI red wire should be on one of the 2 other fuses which have a permanent 12V ?

[mysocal911]

Originally Posted by SFE

The wire connected to pin 2 has 12V when ignition is ON.
Therefore I suppose the electronic inside the relay is authorizing 12V to the microswitch and then to the shut off solenoid depending on RPM and switch closed or not ; the problem is I have 12V at idle that energized the solenoid and it should not be like that !

The red wire on the CDI (pin B) is connected on the same fuse than the pin 2 of the transducer, so it has 12V also only when the ignition is ON.

Do you think the CDI red wire should be on one of the 2 other fuses which have a permanent 12V ?

Read my last post. You have the relay wired incorrectly. Please correct the wiring.

[SFE]

Originally Posted by mysocal911

Read my last post. You have the relay wired incorrectly. Please correct the wiring.

I’m afraid I don’t understand !
Could you precise what I should change in my wiring ?

[mysocal911]

Originally Posted by SFE

I’m afraid I don’t understand !
Could you precise what I should change in my wiring ?

1. The speed relay should only have 12V on pin 2 when the idle (micro) switch closes,
and not always connected to 12V like the CDI, which may cause noise affecting the
relay's operation.
2. The speed relay output (pin 1) goes to the fuel shutoff valve.
3. Pin 3 is ground.

Your present wiring does not agree with the above. Please find someone who understands a wiring diagram.

[Jonny H]

^ I disagree, pin 2 is connected directly to fuse 2 in the engine bay so is always 12V.

Have you tried disconnecting the black/purple at the tach? I've seen tachs appear to work properly but drag the RPM signal down.


There's a circuit diagram of the speed switch here:

https://www.safetycolours.com/porsche-911s-1970-restoration/2018/7/13/mfi-speed-switch-control-from-cdi

[mysocal911]

Originally Posted by Jonny H

^ I disagree, pin 2 is connected directly to fuse 2 in the engine bay so is always 12V.

Have you tried disconnecting the black/purple at the tach? I've seen tachs appear to work properly but drag the RPM signal down.


There's a circuit diagram of the speed switch here:

https://www.safetycolours.com/porsche-911s-1970-restoration/2018/7/13/mfi-speed-switch-control-from-cdi

Then the speed relay would never function, i.e. NO fuel cutoff. Either the relay is bad or noise is entering the relay.

[SFE]

Originally Posted by Jonny H

^ I disagree, pin 2 is connected directly to fuse 2 in the engine bay so is always 12V.

Have you tried disconnecting the black/purple at the tach? I've seen tachs appear to work properly but drag the RPM signal down.


There's a circuit diagram of the speed switch here:

https://www.safetycolours.com/porsche-911s-1970-restoration/2018/7/13/mfi-speed-switch-control-from-cdi

Yes if I disconnect the black/purple wire just behind the tach I have the same problem.

[SFE]

BREAKING NEWS !

I tested the relay on a 2.4S and another 2.0E....same problem, the new transducer is definitely faulty.

Porsche will change the transducer to verify if it is an isolated case or if they have a production problem.

Meantime I'm still waiting for my original rebuild transducer..

[SFE]

Well I tested another new relay from Porsche and same problem...

The relay they sell at Porsche does not work at idle on our cars, it is part 901.615.111.01

My original relay (still waiting for it) has the ref. 901.615.113.00.....just hope that it will work once rebuild !

[mysocal911]

Originally Posted by SFE

Well I tested another new relay from Porsche and same problem...

The relay they sell at Porsche does not work at idle on our cars, it is part 901.615.111.01

My original relay (still waiting for it) has the ref. 901.615.113.00.....just hope that it will work once rebuild !

Why the long wait? It only takes about 15 minutes to repair one!

[SFE]

Well, I think the specialist who's repairing my transducer is overworked....

[SFE]

just receive my rebuild original relay (transducer) and no more problem at idle !!!

Porsche transducer 901.615.111.01 is definitely not ok for 2.0 up to 2.4 MFI cars.