Tach trouble

[bowhawk]

'62 GT Hawk has original equipment tach. About a week ago it quit, dead, zero with the engine running. Later it began indicating 3000 with the key on.
I bought a new sending unit, read the directions and wired it in using great caution not to "burn it up".
Now the appears to read correctly at idle more or less but never gets over 2000 even when I know its turning 3500 or so.
The only other mod is a Mallory distributor.
Any thoughts anyone.
Thanks in advance.

[GTtim]

This problem is almost certainly due to a lack of a good ground between the tach case and the car body. Look at the back of the tach, there should be a bolt or screw or simply use an extra nut on the mounting bolts run the wire directly to the metal bar behind the dash and I think your problems will disappear. Happy trails.

[bowhawk]

The ground I installed was to "the bar" when I installed the new sending unit. I have since run a ground wire from the negative post on the battery to the ground wire on the new sending unit while the engine was running with no results.
Understand that the new sending unit is nothing like the old one and is wired completely differently.
The tach operates inconsistently. It seems to have a 1000 RPM operating range. It might idle at 800 or 1400 but the engine is running at about 800 using a dwell tach.
Thanks Tim for trying

[GTtim]

I'm still not clear if you ran a wire from the tach itself to ground. The sending unit should be grounded automatically by its attachment. The tach case won't ground itself because of the fiberglass dash.

[bowhawk]

Sorry. Yes, I installed a ground from the steel terminal on the tach to the bar where the sending unit is mounted. What I was trying to explain above was that while trying to diagnose the problem I even ran a wire all the way to the neg. terminal on the battery.
I have followed the attached directions to a "T". If you click on it I think it will open large enough to read. As you can see they are pretty clear about your chances of warantee work.

[bowhawk]

I have tightened up the ground wires and I believe it is fixed. Thanks to all.