Well I am on my second tach in 3 weeks...

1999 7.4MPI with Merc. gages.

When the first one went, I thought, it's a few years old, things go bad. So I bought a new one. Now the second one is going bad.

First they read high and then a little later they just get pinned at 6000 when the engine is turning maybe 3000.

I am not sure how a tach. really works other than getting a signal from the distributor.

Does the MPI brain give the signal?
What could be making them go bad?
Too much pounding in the waves?
Too much voltage or current?

bad ground!

Pat,
I agree...
Bad grounds will cause all kinds of problems..

I had a similar one with myself

Rob

Check your ignition secondary. Bad cap, wires, plugs will cause high readings.
Gary

Thanks for the help. Must be a bad connection. I messed around with the main harness plug before I went out tonight, and everything seems to be working.