Troubleshooting a Tach Problem need Help

[JGross]

I’ve installed a tachometer I got from www.classicparts.com in my 65 C10 that running a small block V8 with a HEI distributor. The paperwork with the tach said it would work with the HEI system however the tach is operating very erratically, instead of it moving smoothly up and down the RPM scale when the engine is revved it will jump up to 1000 RPM then jump again up to about 1800 and will not go higher.

Here’s my question, has anyone installed this tach on a V8 with HEI and had it work? If so I know that I have a wiring problem or a bad tach.

Thanks

[HEI451]

One thing I have encountered is that some few HEI setups I do, and some of the older Pontiac HEI systems had issues with tach operation. Those few systems had erratic operation, and most stopped reading at about 2,000 to 2,500 rpms, just stopped reading any higher.

Pontiac used a simple tach filter, and I used to have a filter diagram, will try to find it and post here, but, I haven't had this issue in a long tme, might have to make a new diagram.

I do remember that some of my conversion customers made their own, after the Pontiac units, cost of parts was less than two bucks using Radio Shack parts. The filter is dirt simple to make.

You also need to check and recheck all connections, and make sure you have a good 12 volt feed, ground, with no voltage surges in the system.

These tach filters were used in Firebird cars, 1975 thru 1983 or so.