So my Tach works fine when I start the truck up and for about 10 minutes. After 10 minutes of driving the Tach drops out, presumably from increased engine heat? IDK
Truck still runs and drives normal.
Also, I have a Bluetooth OBDII scanner/reader I use with my phone. When the truck Tach dies I still get accurate RPM readings on the Torque app Tach.
Thoughts?

 

Would have to check the manual to be sure, but perhaps gems gets rpm signal from crank sensor instead of from the alternator like the gauge does?

 

Looks like from the GEMS manual there is no "input" for engine speed, only output, and the output is only used on Range Rover. So I'm guessing that it must use the input from another sensor, probably crank, for its engine speed. That would make me think that your alternator is bad, causing just the gauge to stop working.

 

My 97 gets the rpm from the small flying lead plugged into the rear of the alternator. It sends out an AC stream that varies in frequency by the rpm. Normally this stops when the brushes of the alternator stop touching, due to wear. Charging volts may drop at same time. Of course, there is always the problem of wiring harness, headers, etc.

 

I'll check the wires on the alternator.

 

It is also correct that the CKP sensor produces the rpm signal used by the ECU, and displayed on the OBDII port.