Tach needle bouncing.

[Matt Frediani]

Hey folks,
I installed a tach (Tach conversion kit) in my 87 TBI 350 GMC V2500 truck. All the gauges work as they should but the tach needle bounces. At Idle its not bad but when driving the truck or giving the truck gas the needle bounces around. I grounded the tach to the firewall and was sure to scud the metal up good before I grounded it. Any ideas? I also used the hot for the ignition because the conversion kit wire had an end on it that plugged right in.

Thanks
Matt

 

[Vbb199]

EMI from the sparkplug wires or coil possibly.

I added a tach on my burb. It was fine until I added the spark controller and coil. Same symptoms you're saying.

I don't have that stuff on my truck now, tach reads fine and doesn't jump around.

 

[Matt Frediani]

UPDATE-
Even though the truck was running great I decided to replace the coil because it was original. As soon as I replaced the coil the Tach quit jumping. Problem solved

 

[Whiskey Junior]

I had exactly the same experience. I rebuilt my dash with a tach upgrade from LMC, and the tach would bounce when driving, but not at idle.

The old tach looks like the original, with at least 200k and 33years on the clock.

New tach and it works great!

 

[75gmck25]

The original factory tach had a noise filter in the wire that connected to the distributor. The OEM filters are no longer sold, and reproductions are pricey, but I’ve seen some diagrams that show how to make your own.
This is the type of filter they used.

https://www.topflightautomotive.com/products/corvette-america-1975-1991-chevrolet-corvette-tachometer-filter/?gclid=Cj0KCQjw4uaUBhC8ARIsANUuDjXMT5_4owLg8-2Fso2jUX45nqbPuTcfrae9t4adtz5KNBjquQgkeisaAvqSEALw_wcB