Was pulling my 3500# boat up the launch ramp. In 2wd it spun the wheels so I shifted to 4wd high. Engine reved up pyro came up to 900 but truck wouldn't move. Only way I could move it was by shifting to 4wd low. Meanwhile gauges were going nuts on and off and the tach completely stopped registering anything. Back on level ground I went back to 2wd high and pulled the boat home like normal except for the instruments and check engine light continuing to go nuts, still no tach. Disconnected batteries pulled and cleaned plugs on trans control unit reconnected everything. Truck seems to drive normal (haven't tried to pull the boat yet). When I start it the tach comes up but immediately drops back to zero (all other instrument work). Ideas?

 

Sounds like the crank position sensor on the front of the engine to me.
Without the signal the tach, charging, AC will not work.

 

OK thanks. Is there a way to test it? Do I need a code reader?

 

I believe you nailed it. No AC, charging or tach. I'll readjust and if that doesn't fix it, install new. Thanks!