'91s or 91.5s do NOT have the plug under the dash for the factory (approved) tach.

Only when the voltage regulator was moved from the firewall, to the PCM, was the pre-wired tach connection installed in the wiring under the dash.

The crank sensor on the '92-'93 model trucks is where the tach gets it's signal from, but the crank sensor's original reason for being used was to sense motion at the crank, to tell the PCM when to initiate the charging circuit.

Dodge considered the tach an optional accessory in '92-'93, of course.

Mark.

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It may be Isspro, or possibly Autometer, that makes a sensor that gear-clamps to the case, then is calibrated using a formula that they provide.
The formula is also searchable on the internet, I think.

I believe that it reads the cuts in the armature.

No sense ripping the alternator apart.

Mark.