MFI engine stalls at idle when RPM transducer is engaged
 

Hi all,

Here is the problem...

Car is a 1969 911E (2.0 liters with MFI) which was completely rebuild.

http://forums.pelicanparts.com/uploa...1694013175.JPG
http://forums.pelicanparts.com/uploa...1694013175.JPG

The only thing that was not working was the RPM Transducer (also called the Speed switch I guess).

So I bought a new one :

http://forums.pelicanparts.com/uploa...1694012940.JPG

When I test the transducer it's ok : I put 3000 rpm, press the microswitch and the rpm oscillate between 1300 and 1600.

When I stay at idle, after a few seconds 12V is intermittently sent to the shut off solenoid and the engine end up stalling (the solenoid should not be energized below 1300 rpm and my idle is a solid 900 rpm one).

Here is a video of what happens :

https://youtube.com/shorts/lgupWjk-Fgo

With some folks on the Early 911s registry forum we checked all the wiring and everything seems OK.

Any idea ?

Sounds like the "new old" is bad. What happened to the old one?

Old one was sent to a specialist for rebuild !
But I tested the new one on a very similar car and it worked well !!!

I am assuming you want it as was configured when new. Otherwise you delete those pieces. My experience is with the later pumps, so don;t know what issues you would have with the 69 pump, it was different. 70-73 pumps work well without the extra fluff. Bob

Pump is matching to the car and also completly rebuild.
Engine is running well with the transducer disconnected except for exhaust popping on decel.
And you are right, the idea is to have the original configuration !

Have your "specialist" test the "new" one.

If you checked the new rpm transducer in another car and it worked well then it’s not your issue! And I would definitely not delete fuel shutoff on overrun these mfi cars can have some huge 12ga backfires. The 70 - 73 cars used the same system. Your answer probably lies within the wiring diagram.

If your wiring is good (cdi box, coil, transducer, microswitch, etc. I would try a different cdi box.

The shut-off solenoid gets power from the microswitch. The microswitch gets power from the transducer. The transducer determines WHEN to send power to the microswitch based off RPM. The transducer gets its sense of RPM from terminal C of the cdi box.

It sounds like you either have a wiring issue (maybe crossed wires) or the transducer is receiving an incorrect “rpm” signal.

The cdi box gets its "rpm” signal straight from the points.

Does your tach work well?

Then the CDI is OK!

Yes, the tach needle may be set incorrectly.
Again, then RPM transducer should still be checked for the correct on/off RPM.

Yes the Tach works very well.

Here is the wiring I have :

http://forums.pelicanparts.com/uploa...1694064000.jpg

I checked the transducer on a sportomatic car which is way too rich at idle so maybe the problem was not "visible" on that car. So I still have a doubt on the transducer.

The micro switch on the left stack should be preventing any current from getting to the solenoid regardless of what the rpm transducer is putting out.

So as long as that is functioning and is adjust properly, then I suspect the issue is in the trigger circuit. (Points signal)
Start by checking points adjustment, dwell, properly grounded distributor.
You may try disconnecting the black and purple wire from your tach to eliminate that.

You found something !

If I disconnect the black/purple wire from the distributor the engine runs fine....so what do I have to do ?

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.

By disconnecting the bl/pr wire you are disabling the signal to the transducer if your car is wired just like your diagram. (I was looking at a different diagram SL33 which is for a 69E) it shows the signal coming straight from the cdi box to the transducer

http://forums.pelicanparts.com/uploa...1694090348.jpg
http://forums.pelicanparts.com/uploa...1694090412.jpg

#10 is the switching scr which sends the signals to the tach…do you have that? It sounds like your car is wired differently

No I don’t have the switching SCR and my Tach is a post 72.
This is why my wiring is also a post 72.

Very simple operation:

1. Switch closed and above ~1200 RPM - fuel shutoff - RPM drops (oscillations of RPM occurs with manual ops)
2. Switch closed and below ~ 1200 RPM - normal idle
3. Points dwell (duty cycle) has NO effect. Signal - +12 to grd

1/ OK
2/ KO…engine stall
3/ Dwell angle checked OK

Then your idle is too high (>1200), or your tach is incorrect, or the RPM unit is bad.
Adjust the idle very low, to about 500, and see if the engine still runs.
Again, dwell has no effect!

No my idle is very good, about 800 engine cold and it rises to 900/950 engine warmed up.
My tach is also very accurate.

The RPM unit is working well on a similar car.

Something is sending 12V to the stop solenoid at idle and I will change the microswitch to eliminate that part...

Just use a testlight to check the RPM box's output, with the shutoff wire disconnected
as the RPM changes while watching the tach. With any RPM below 1200, the light should be OFF!

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

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.

Ignition points bouncing?

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.

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...

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 ?


http://forums.pelicanparts.com/uploa...1694957586.jpg

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.

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.

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.

^ 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.

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

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..

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!

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

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.