I lost multiple electric items today and assume they are related. The voltmeter dropped to battery voltage 12+/- volts, the ac stopped working and the tach died. The Check engine and Gen lites came on briefly several times. An external voltmeter shows 12.6 v. The 120a fuse is good as are other fuses. I suspect the alternator but am not sure. If it needs and alternator can I upgrade from a 120a to a 136a without changing other components?

Any help is greatly appreciated.

 

Sounds like a typical engine speed sensor, connector or it's wiring problem.
The ESS is located at the lower engine pulley, if it's not functioning correctly it will tell the computer the engine isn't running, nothing electronic will work.
You can get the ESS at most any auto parts although sometimes it's mistakenly called a crank position sensor.

 

On looking at the sensor I discovered two broken wires. The serpentine belt had started to fail and some filiments cought the sensor wires. A little solder and shrink tubing made all ok ( and a new belt).

Thanks for the help