On the earlier years, I think that the TDC sensor is used, in conjunction with the "tach amp" which plugs into the socket on the driver's side of the engine bay.

I think the '85 federal (and the '84 version in California) still have the TDC sensor, but have no tach amp in the socket - the socket's there solely for diagnostic purposes (external tachometer, injection timing by the RIV method, etc.) There's another sensor on these "which is screwed in on intermediate flange to automatic transmission," according to my FSM. In the picture, it looks to me as though it's on the driver's side (I think I see the oil filter housing), but I'm just looking at a picture; no such animal on my car. I think this sensor is "read" by the electronic control unit (located in the passenger side kick panel) for the EGR system. This unit is protected by the overvoltage protection module (behind the glove box, with a 10A fuse), so that's another thing that one can check if the tach is dead. If you've been working under the hood, though, I agree that I'd find the sensor and make sure that it hasn't become unplugged first.

If I've got anything wrong here, someone please step in and correct me. I'm working more from what I've read, here and in the FSM, than from personal experience.


-- eskimo