A couple of issues

[bab029]

Where does the tachometer in the dash get it's signal from? Occasionally I am getting the "lazy tach", where at an idle up to 1000 rpm or so the needle is at zero, but as soon as I speed the engine up, the tach jumps up to what I think is an accurate reading. The reverse is also true, when coasting to a stop the tach indicates correctly all the way down to 1000 or 1100 and then just falls down.

The kicker is that occasionally, the thing acts perfectly. Usually, after a long interstate cruise it works fine.

This is not a problem in and of itself. However this behavior affects the transmission in some pretty annoying ways. First, if you are going about 35, the trans will shift up-down-up-down, presumably because it sees a low rpm and shifts down to raise engine speed, which is too high in the lower gear, so it upshifts again and so on. Second, if you are easing around (like through a parking lot) under 1000 rpm, (the tach shows zero) and the computer decides to shift to second, the trans will throw a code, o/d light blinks, and you are in limp mode (presumably because the computer doesn't see a rpm reading) until you can take the battery off for a few minutes. That is really annoying.

I have cleaned and greased (dielectric) the connectors on the firewall and at the transmission, as well as the two-pin at the tach sensor. So the question is what is my problem? Do I need a new tach sensor?

 

[Desertrig]

I found mine had rubbed through the wire insulation on top of the alternator before it went into the main harness, although it might be a little different on a serpentine setup (my 92 is standard....). I've read several people reporting pulling and cleaning the tach sensor, some say all fixed others say well now I know I need a new one. They cost ~70 from fordparts, so being cheap, I'd try to clean the pickup first. I hate when ive got the special order part in hand and i yank the old part and say oh i could have fixed this without waiting on special part. I've also seen people mention bad instrument cluster grounds (normally on the old cab stuff), but that usually messes with more than one gauge. I've been going through some VSS pain on my 90 and it seems to be more about when to up shift, so it seems like your on the right track.

 

[gandalf]

The tach pick-up is a large, about 1 inch nut like thing on the timing gear tower at the front of the engine. It has two wires coming out of it. Find your oil fill and move right a bit, toward the driver's side.

 

[icanfixall]

The posting above really tells you al you want to know. Try removing the sender and clean off the magnetic tip of it. Thats usually what happens when "stuff" get stuck to the sender. Also look at the top of it. If you see oil creeping or leaking out between the bogy and the plastic piece then its leaking and time for a new one.

 

[bab029]

I know where the sender is, and have cleaned it, both the pickup and the connector. Looks like it's time for a new one.

 

[IDIBRONCO]

I'd say it's time for a replacement too. It can affect your shifting as well as the tach.