i installed a shift light, tachs been in a while but just recently wired it, and they are not working properly. shift light comes on way to early and tach reads high. ive switched the back of the tach to see if i had it on the wrong number of cylinders and ive tried different pills in the light and they all come on early. i know the light is coming on early cause with a 3000 chip it comes one not to far off idle and i know the tach reads high cause i set my idle with a timing light at about 750 and the tach reads about 1200 at idle. wondering if my distributor can be sending out a bad signal but the engine still run right. i did try disconnecting the tach to see if it was somehow interfering with the light but no change

 

Are you running an MSD box? If so the tach needs to be connected to the tach port on the box or the tach will never read correctly.

 

No box just a normal hei

 

You're using the tach output of the HEI, right?

Are the tach and light reading precisely double? Some timing lights also have tach displays - very useful for checking stuff like this.
If it's showing double, then that implies that the tach and light are set up for 2 cycle instead of 4 cycle. I wouldn't expect that on a shift light, but hey, stranger things happen.

Simplest thing is to double check wiring. Any breaking or fraying in the tach wire, or routing near other high(ish) frequency signals, or EMI-noisy spark plug wires can cause noise, and older/cheaper tach/light units don't deal with noise very well.

I do not know the HEI module internals, but I would expect it to be possible for the tach output to be subtly messed up while still being able to run the engine.

 

Yes I am using the tach output. Wiring is all good. Tach worked fine with same ignition before I decided to put it in dash and the light worked when it was in the car I pulled it out of.