The tach signal comes from the ICM, the schematic are below. My best guess is that the ground signal that charges the primary side of the coil is oscillating rapidly (rather than going to ground once for each plug firing event).
Normally, the PIP signal from the hall effect switch in the distributor goes to the PCM, the PCM generates the SPOUT signal which in conjunction with the PIP commands the ICM to send the COIL signal (pin 5) to ground. (SPOUT is to modify the timing of the COIL signal.)
The tach signal (going to page 60-5, w/pk wire) is decoded by a solid state circuit in the tachometer into RPMs.
So, lots of tach signals at 2000 RPM, should be a lot less at cranking speed (about 250 RPM).
So, a failing hall effect (PIP) circuit making the PCM think the engine is turning 2000 RPM could be the reason, or the PCM misreading the PIP signal and sending the coil signals at a 2000 RPM rate (and not synchronized with the engine).
You could pull the SPOUT plug and see if the PCM is confusing the ICM. After that, fixing the PIP generator in the distributor if it is broke, or replacing the ICM with the correct (black
) ICM. Pull the PCM and look for leaking capacitors, you could be seeing an early PCM failure that will get worse.
1996 Ford Bronco '94 EVTM picture | SuperMotors.net
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