1999 Peterbilt 379 2ws cat Speedo and tachometer not working

i would scan parameters. speedo sensor may be on the end of the trans.

 

Cam sensor is the primary engine speed sensor, crank is secondary. the engine will run fine without seeing the crank sensor but will not do the fine crank to cam timing adjustments without it.

Both of your gauges get their signals from the engine ECM, 2 wires for each gauge come from the ECM.
Pins 36 and 37 at the ECM are for speedo, and pins 38 and 39 at ECM are for tach. check continuity from ECM to gauges.

You say to have power at the gauges, do you have a good ground to the gauges?
I see it a lot with these older peterbilts where the cabs and dash no longer have good grounding circuits.

 

I have a 99 379 with a 2WS. My tach gets kind of funky. I think maybe its a ground as mentioned. It acts up more during the winter or when its cold out. It will stop working randomly. Sometimes if you tap on it, it will come back to life. Usually goes away when the cab heats. Ive had it apart a few times but never found anything. Maybe another sign its time to move south.