Tach not working properly?

[frdtrckmnrich]

My truck original did not come with a tach. However i got a complete instrument cluster with tach. Put the sensor in the timing cover and plugged it into the factory wiring on the passenger inner fender. (green wire and a black wire) The truck is a 85 f350 w/6.9 it is a xl.
When i start the truck the tach does seem to go up to approx 300 rpm. Going down the road it floats all over.

Ideas???????

 

[Agnem]

Is the sensor new, or questionable?

 

[frdtrckmnrich]

It is used, i got it from darrin.

 

[Agnem]

I'd start with a new one just to be sure.

 

[Cat_Rebel]

Mine works when it's under throttle but when you come up to a stop sign (most of the time) it will fall down to 0. The OD light will start flashing too. Just go to the dealer it about a $30 part. That's what I'm going to do today.

 

[ttman4]

Grounds???

May not be your prob, but here's a thought.
Grounds drove me up the wall about my tac few year ago.
Erratic tac or no tac, or squirrelly instruments. Turn headlites on & tac drop 200 RPM, etc, etc.
Even let dealer talk me into replacing factory sending unit once when I had it in for other repairs.....even after I strongly suggested to them it sounded like bad/insufficient ground to me! Their factory trained, certified, many years experience under his belt, USDA Certified Choice Inspected mechanical genius shop Tec said definately "it's a shot sending unit, guaranteed! Be fixed tomorrow!".....

Next day it was bad instrument panel needed replacing.....for about a Gizillion $$$. I told 'em "No thanks, just to figure my bill up & I'll quit worrying bout it!"

I added grounds here & there from block to body & frame...helped.
Finally pulled instrument panel & soldered couple extra ground wires to back of panel....been fixed ever since.

& I got 2 good Tac sending units too.....

BTW, lots extra ground wires added round in engine compartment to my wifey's '89 Caddy miraculously solved the erratic & intermittent running. After I tried to tell 'em I thought it was ground problems, a shop changed the nearly $400 catalytic converter that they nearly guaranteed was the problem. Turned out to be intermittent ground was driving the puter brain bananas. (I got an extra catalytic converter too)

 

[frdtrckmnrich]

Thanks for the help guys. I found the problem, The sending unit was bad. But i greatly appreciate the ideas.