Factory Tach question

[First c10]

Hello guys, i have been searching and found answers to if the factory tach works or what you have to get it to work with a filter.
my problem is
I have a new HEI Distributor and installed a factory tach but when at 2000 RPM's it reads about 300 or so?
Any idea what I need to do?

[Tdrcreyn4]

Have you verified you have the right tach. Are you running a V8/V6?

[First c10]

Sorry yes it is a V8 tach cluster out of a c50. It has the red v8 description on the back. The problem I am having is that it moves when you rev it up but only slightly.

[davepl]

I don't know about the factory originals but all of the aftermarket ones I've seen disclaim "NOT compatible with HEI" for some reason. I do not know the reason - though mine works with Pertronix ignition, I can't swear as to how accurate it is.

Worst case Dakota Digital (and others too no doubt) make a cheap tach adapter you can buy that will take most signals and filter+adapt them and probably be able to drive the tach.

[1969guy]

Unless it is damaged it should work just fine with an hei just it may bounce a little, the filter I believe will help curb that.

Now from my own experience with a factory tach, mine worked just fine with an original, obviously newer, hei but only after I realized that I had it hooked up backwards.

Check and make sure you have the wires running to the correct place, should be labeled on the back of the tach, and go from there.

When mine was hooked up backwards, it did just about like you said yours does, stays low rpms and barely moves.

[Rich69shortfleet]

I've read on here somewhere that the module in the HEI can make a difference. The recommendation was to only us a GM (AC/Delco) module to get the tachs to read right. I have a non-AC/Delco module and my reproduction tach reads just fine, but you said yours is a factory one so I could be all wet. If you are interested, my module is Standard brand.

[First c10]

Thanks guys for the information. I am going to try and switch the wires and see if that works. That will be the first and easiest step. I will report back. But any other idea please post away. M

[Stocker]

Originally Posted by davepl

I don't know about the factory originals but all of the aftermarket ones I've seen disclaim "NOT compatible with HEI" for some reason. I do not know the reason - though mine works with Pertronix ignition, I can't swear as to how accurate it is.

I had my old Sun tach (Super II, iirc) since before I converted to HEI back in the Eighties. No problems before the switch.... or after. Pretty sure it has the original AC/Delco module if that matters.