I am putting my 1997 Dodge 3500 Cummins back together after a wreck. I do not have ignition feed to the alt. and the tack does not work and no ground to the a/c compressor relay. Any ideas? 

 

The PCM gives the ground signal to the A/C compressor relay, as well as drive the tach. 

On the alternator, the PCM also drives this, but may also be caused by a faulty automatic shut-down relay in the PDC.