I just bought a 90 f250 with a bad 7.3. I just happened to have a 7.3 in stock so I swapped the engine. Got it running good but the transmission won't shift apparently because the tachometer does not work. So I put a brand new tach sensor and still nothing. Checked all the wires from sensor to pcm and are good. Checked ground and found battery wasn't grounded to cab good. Put new ground wire on it and tach worked, drove it shifted great! So I thought I had it but a few miles down the road it stopped working again. Got some more wires and grounded to frame better, cab with two wires to each battery. Nothing. Tach doesn't work and it won't shift. Any other ideas?