RPM Transducer - MFI Backfire Fix

As my '72T Targa restoration progressed to the drivable stage, I found that the shut-off solenoid on my MFI pump was not functioning. The symptom was mainly backfire while deceleration.

Deciphering the schematics in the Haynes manual showed that the solenoid was connected to the RPM transducer. So, I checked the transducer.

Sure enough, as I pulled it from the fuse panel, it rattled. I opened it up and found a loose transistor and two dangling ones.

A trip to Dow/Marvac and $5.40 later, I soldered in the replacements and reinstalled the box. Now, I have a functioning shut-off solenoid.

Brendan      <bimtron@earthlink.net>


RPM Transducer - What Is It?

The rpm transducer is used to activate the shut-off solenoid on the MFI pump.  It prevents the pump from continuing to pump fuel into the engine when coasting at high speed, which will result in backfiring and other unpleasantness.

The rpm transducer puts out 12v above about 1600 rpm, and 0v below it. It's connected to the MFI shut-off solenoid through a microswitch that's activated by the accelerator linkage--the one on the left side of the engine, attached to the input stacks.  With your foot off the accelerator, the switch is closed, and if the engine speed is above 1600 rpm, i.e., you're coasting at speed, then  12v is applied to the relay and it shuts off the pump.  If you're below 1600 rpm, even though the switch is closed, i.e., you're idling, the solenoid is not activated because the sensor is putting out 0v, and fuel is pumped to the engine.

The transistor leads corrode and break over time.  Virtually any generic NPN, like a 2N2222, will work.

Bob Spindel
72 911T
73 911T (project)


Radio - no AM signal

> I just purchased a 1987 Carerra Cab with 23,000 miles. It has the radio antenna 
> built in to the windshield.  I can't pick up AM stations at all. Is there a signal booster > that I can use without installing an external antenna?

Sounds to me like you have lost the 12V power to the signal booster that you already have.  Small black wire running from a small black box in line with your antenna lead.  Its an easy error during a radio swap not to connect the booster.

Namderd@AustinTX.net (Dean Redman)