RPM Transducer/Speed switch options - absence led to small intake fire?

[sunlion]

Hello folks. I've been investigating the cause of my small intake fire on MY '72T and have realized that the RPM transducer, #5 in the diagram and RPM sensor #3 are both missing from my engine compartment fuse/relay board. The intake sounds like popcorn popping. I was advised to check the microswitch, and in doing so, found the RPM T missing which essentially means the microswitch is not functioning so the stop solenoid continues to deliver fuel at idle. Correct thinking? The engine runs fine above 2K rpm with the occasional pop or miss. It remains puzzling why these two relays would have been removed.

Is the part # 901 615 111 00 the only option or is there a generic relay which can function as the RPM T?


Cheers,

Ravi

[Flieger]

The engine idles (or should idle) below the fuel cutoff, so the relays don't effect the amount of fuel there for a fire at idle. They will effect the amount of engine braking and your fuel economy as well as the popping.

[beh911]

Do you have the cold start solenoid on top of the filter housing hooked up to the air box? If so is it operating properly?

[304065]

A few calibrations to your understanding of the MFI system:

The speed switch and the cold-start system are independent of each other.

The speed switch senses engine RPM from a black/violet wire that starts at the points and ends at the back of the tach.

The way the solenoid works is, when it's energized, it pushes the main rack in the pump to turn the fuel OFF.

The speed switch logic is:

Above 1500 (75hz input) - current flows to solenoid if microswitch is closed and fuel is cut off
below 1300 (65hz or lower)- current does not flow to solenoid, normal operation

So in your case, you don't have a speed switch, so the pump is on all the time. When you are operating at high RPM, and the throttle snaps closed (such as at the end of a straightaway), and the RPM is above 1500, the speed switch relay sends power to the microswitch, which being closed, sends it to the solenoid, which then moves the rack to cut off the fuel.

This helps with fuel economy as well as keeping the soot off the car behind you (no kidding, my race car, which didn't have a speed switch either, used to shoot 1' flames out of the tail pipes at the end of a straight).

However, it has nothing to do with cold-start, so the source of your fire is probably elsewhere.

See CHECK MEASURE ADJUST for more system logic about the speed switch and cold-start system.