Help with Front drive shaft U Joints and Tach

[dsscrusher]

I have a 93 F350 4x4 with Dana 60's and 7.3 non turbo IDI. I have been to 2 different parts stores and got 2 different answers. The front drive shaft has 3 U joints, 2 of them in a CV type configuration. Are all 3 u joints the same type/ part #? One place I went to, said the single one at the Front diff is different than the other two which are at the transfer case. The other parts store, said they are all the same.

My next question is how do I test the factory tach. The guy I bought the truck from said the original tach didn't work, and he then changed the whole dash due to a lot of cracks. The thought changing the tach would fix the problem. Where does the signal come from, and how do I test the tach and wiring as well as the sending unit?

I bought the Haynes manuals for this truck and for the diesel...What a waste of money that was.