MFI engine hesitation when speed switch/RPM transducer connected

[edmayo]

I would also do two more tests. First I'd remove the wires from the micro switch, make a jumper wire with two alligator clips, cut the wire in the middle and wire in a door bell switch, then attach one end each to the removed micro switch wires. Now you can simulate the micro switch. Second I'd remove the wire to the shut off solenoid, and attach it to a standard test light. Now the solenoid won't work, but you'll know when it gets power. Not sure what you'll find, never know, might lead to something.

[SFE]

Ok, will do that this WE thanks !

[frederik]

To be clearer...I would pull the RPM transducer out of the relay panel and wire some direct connections to it. This would include all (4) wires to the RPM tranducer. A good known ground, a wire from the micoswitch, a wire to stop solenoid and the wire from the distributor. This, way you bypass any potential problem in the relay panel. Then try your tests again to see if anything changes.

With the risk of saying something stupid again... this is not how the transducer is wired up from the diagram. It gets a ground and 12v, input from the distributor, and outputs 12v if the RPM is above 1300. The microswitch lets this go through to the solenoid if the throttle linkage is at idle.

So: throttle linkage not at idle? Connection to the solenoid is broken by the microswitch, so fuel flow is never stopped.
Throttle linkage at idle? If RPM is above 1300, the solenoid engages and stops fuel flow. Below 1300, the transducer should output 0v so the solenoid disengages and fuel flow is restored, so the engine won't cut out.

The problem looks to be that the transducer sometimes outputs 12v even if the RPM is below 1300. Maybe it's faulty, or it could be picking up an unreliable RPM signal somehow. It would be really great if you could hook up a scope to the RPM input.

[SFE]

With the risk of saying something stupid again... this is not how the transducer is wired up from the diagram. It gets a ground and 12v, input from the distributor, and outputs 12v if the RPM is above 1300. The microswitch lets this go through to the solenoid if the throttle linkage is at idle.

So: throttle linkage not at idle? Connection to the solenoid is broken by the microswitch, so fuel flow is never stopped.
Throttle linkage at idle? If RPM is above 1300, the solenoid engages and stops fuel flow. Below 1300, the transducer should output 0v so the solenoid disengages and fuel flow is restored, so the engine won't cut out.

The problem looks to be that the transducer sometimes outputs 12v even if the RPM is below 1300. Maybe it's faulty, or it could be picking up an unreliable RPM signal somehow. It would be really great if you could hook up a scope to the RPM input.

Agree with you !
The wire coming from the transducer and going to the shut off solenoid must go through the microswitch.
There is no direct connection between the transducer and the solenoid.

[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...

[frederik]

Awesome, glad the riddle is finally solved!

[edmayo]

Back in post 60 you said it worked fine in another car? I'm still hopeful the transducer is bad however.

[tom1394racing]

Back in post 60 you said it worked fine in another car? I'm still hopeful the transducer is bad however.

I am guessing that the transducer worked fine in the other car because it did not warm up and the idle speed was too high during the test.

[SFE]

Back in post 60 you said it worked fine in another car? I'm still hopeful the transducer is bad however.

This is what made me lose 1 month of troubleshooting on my car...

First, that car was not warmed up and also known to be very very rich at idle, and second we did not check it with a lamp test....bad idea !

What happened is my mechanic called me to buy my transducer for a 2.4S he was supposed to deliver this week and the transducer was dead on that car.
So we tested it, this time with the lamp test and we saw the lamp light up at idle..., no engine stall but inconsistent idle.

Then we tested it on another 2.0E and again 12V was arriving at idle...here's the film :

https://youtube.com/shorts/exE8z6TnOUc

Now the interesting question is : is it an isolated bad transducer or do Posche has a production problem on this particular part ?

Answer by the end of this week !

[SFE]

I am guessing that the transducer worked fine in the other car because it did not warm up and the idle speed was too high during the test.

And we did not use a lamp test !