MFI engine stalls at idle when RPM transducer is engaged

[SFE]

Hi all,

Here is the problem...

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




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

So I bought a new one :



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 ?

[mysocal911]

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

[SFE]

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

[r lane]

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

[SFE]

Originally Posted by r lane

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 !

[mysocal911]

Originally Posted by SFE

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

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

[Zuffenwerker]

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?

[mysocal911]

Originally Posted by Zuffenwerker

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.

Then the CDI is OK!

Originally Posted by Zuffenwerker

Does your tach work well?

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

[SFE]

Originally Posted by Zuffenwerker

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?

Yes the Tach works very well.

Here is the wiring I have :



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.

[Tyson Schmidt]

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.

[SFE]

Originally Posted by Tyson Schmidt

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 ?

[Zuffenwerker]

Originally Posted by Tyson Schmidt

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.

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.

[Zuffenwerker]

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

[Zuffenwerker]

Originally Posted by Zuffenwerker

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

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

[SFE]

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.

[mysocal911]

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

[SFE]

Originally Posted by mysocal911

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

[mysocal911]

Originally Posted by SFE

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!

[SFE]

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...

[mysocal911]

Originally Posted by SFE

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!