Have 92 F150 xlt 5.0 auto and the stock tach just started holding at about 2800 rpm. It will go higher as you rev the motor, but will not go lower than 2800.

Turn the truck off and the needle goes to zero. Start it back up and same thing.

What do I need to check?

 

when did you notice it? did you hit a big bump or something?

 

No, just started doing it. Can't say it ever went back to normal for a time or not either. Just started doing it and stayed that way.

Sending unit??? How do these tachs work?

 

I think your tach is shot. My 1988 F-150 does the same thing almost exactly but mine won't drop below 2200 RPMs with the engine running. There used to be a mechanic that frequented this board and he claimed tachs like this were shot. Does your tach read the same RPMs that it used to at say 60 MPH when cruising in high gear? The tach in my '88 was this way when I got it over ten years ago, it reads 3000 RPM at 60 MPH with 3.08 differentials and a five speed manual transmission in fifth(overdrive) gear, I think it is showing too fast. Now that I think about it you may not reach 2800 RPMs at normal cruising speed. Oh well I typed it now I will post it.

 

if i remember correctly, your gauges have white grease in them and after it starts to get some age on them they can stick. same thing happened with my old gauge cluster on the speedo.

 

Don't think it is just a sticky needle or the like. It drops to 0 when you turn motor off. When cranking, the needle comes up a little and pulses as the motor is turning over until it starts then back to 2800. I don't think it is accurate when reving above 2800 either, just registers something.

 

Wonder, is this tach something to try to fix? Or not worry about it. I would like it to work.

 

i honestly wouldnt worry about it but then again i dont have a tach in my truck and it would be nice to know what my shift points are. if you want to fix it, i'd recommend you go to a junkyard and take one out because they can be expensive new. or you could wire in a gauge like sunpro or autometer or whatever and use that.

 

How do I determine if the tach is defective or something else?

 

i'd say the tach is no good because it stays right at 2800 until revved am i right? even when the key is off? if so then its no good.

 

With key off the needle is at 0. When cranking it pulses just off 0. When motor starts then it goes straight to 2800, rev motor then it will go up higher but not lower till motor is turned off then it goes back to 0.

Any way to test the system?

 

The wire from the tach goes to the tach side of the ignition coil with no devices in the wiring.
If it is not working right you have a bad tach.

 

Can a new replacement tach be bought? Price? How about a used tach? Worth the effort?

 

you can get a new one through lmc truck http://www.lmctruck.com/ICATALOG/FD/full.aspx?Page=43

 

Problem solved, sold my 92 and bought an 04 new body style.