MFI engine hesitation when speed switch/RPM transducer connected

[SFE]

Hi all,

When the micro switch of my RPM transducer is engaged (therefore at idle or when the accelerator pedal is released while driving) the engine has some hesitations.

1/ IDLE : it is unstable and the engine ends up stalling after a while.
2/ While driving: when I press gently the accelerator after a deceleration there is light backfiring in the exhaust.

If I disconnect the relay the car runs fantastic : perfect idle at 900 rpm, no backfire (except a bit in deceleration time to time which is now normal...).

At first we thought of a defective speed switch relay but no because it works very well on another identical car (911E 2.0 liters).

Important difference: my tachometer is connected directly to the ignition distributor, so if I understood correctly as a 2.4 liter.

Here it is :





So in my config, if I connect the speed switch as it should be there is something that disturbs the engine RPM.
But the 2.4 engine also have this relay with a tachometer connected to the igniter distributor like me and of course everything works well !!!

Could it be my 3-pin BHKZ (below) that wouldn't work with my config ?




THANKS !

[edmayo]

69 and 70 cars used an "intermediate unit" before the tachometer, as I understand it to improve the signal to the tach to match the old style Kettering (amperage across the points) which is less amperage with the C.D. ignition. In 72 the tach circuity was changed to accept the lower amperage signal. So a pre 72 tach must have the intermediate unit and a 72 on tach does not need it. What year is your tach and if pre 72 are you running the intermediate unit. Or a "tach adapt" from Bob Ashlock solves everything.

[SFE]

69 and 70 cars used an "intermediate unit" before the tachometer, as I understand it to improve the signal to the tach to match the old style Kettering (amperage across the points) which is less amperage with the C.D. ignition. In 72 the tach circuity was changed to accept the lower amperage signal. So a pre 72 tach must have the intermediate unit and a 72 on tach does not need it. What year is your tach and if pre 72 are you running the intermediate unit. Or a "tach adapt" from Bob Ashlock solves everything.

Hi Ed,

Unfortunately I don't have that intermediate unit : Porsche Part No.: 901.602.601.00. Bosch Part No.: 0-227-990-001.

My Tach is a Pre 72 that's for sure and it is working well even without that intermediate unit but it is connected directly to the ignition distributor and it shouldn't be.
The problem is if I connect the RPM transducer the engine is fluctuating so I can't use the fuel shut off fonction upon deceleration as it should be.

[tom1394racing]

I had a very similar issue with my '70E restoration. I used a later model '71-'73 tach and bypassed the intermediate unit on the relay board to make the tach work properly. When first shaking down the MFI system, I had a problem with the idle being too high (around 1200 RPM). With the micro switch engaged at idle, the RPM transducer would "cycle" and the engine would hunt between low RPM and 1200. Since the engine ran fine, with no backfiring, I simply backed off on the micro switch to disable the RPM transducer and had the pump adjusted to reduce the idle. I drove the car for 1 1/2 years and put around 1500 miles on it with no issues.

[SFE]

I had a very similar issue with my '70E restoration. I used a later model '71-'73 tach and bypassed the intermediate unit on the relay board to make the tach work properly. When first shaking down the MFI system, I had a problem with the idle being too high (around 1200 RPM). With the micro switch engaged at idle, the RPM transducer would "cycle" and the engine would hunt between low RPM and 1200. Since the engine ran fine, with no backfiring, I simply backed off on the micro switch to disable the RPM transducer and had the pump adjusted to reduce the idle. I drove the car for 1 1/2 years and put around 1500 miles on it with no issues.

Hi Tom,
So the RPM transducer is not working at all on your car.
I guess your consumption is a little higher and you said no backfiring at all throttle off ???

[tom1394racing]

Gas mileage seemed fine and no backfiring. FYIW....My local experts said no need for the MFI transducer.

[gled49]

I don’t run the decel system either. My car backfires with it hooked up, so I back off the screw and all is fine. My mileage has never been good.

[SFE]

Do you think the "Tach Adapt" mentioned by Ed could solve this issue and allow me to use the RPM transducer ?

[tom1394racing]

I would think that if you bypassed the intermediate unit already, and your tach is working properly, the tach adapt will not help.

I would consult with Bob Ashlock and get his thoughts.

[SFE]

Hi Ed,

Unfortunately I don't have that intermediate unit : Porsche Part No.: 901.602.601.00. Bosch Part No.: 0-227-990-001.

My Tach is a Pre 72 that's for sure and it is working well even without that intermediate unit but it is connected directly to the ignition distributor and it shouldn't be.
The problem is if I connect the RPM transducer the engine is fluctuating so I can't use the fuel shut off fonction upon deceleration as it should be.

I have to correct this msg, my Tach is not a pre 72....it has a "TD" connection so must be a post 72 which explain why it is connected to the distributor I guess.