Truck Repair: Cummins M-11, drive pulley, wire weather

Question
I have 94 Volvo M-11 Celect.  Accesory pulley is leaking and discovered loose shaft is it hard to replace?  Second, the Tach quit working.  How can I check the sensor at the block and at the tach?

Answer
The sensor is under the accy drive.It has a 4 wire weather pak connector.It should ohmn out between 1000-2000 ohmns.There are 2 coils in it.I beleive it is the 2 outside wires is one & the 2 inside wires are the other.If the accy drive pulley is stampesd steel it will slide off after you take the big bolt out of it.