New Tach Install Question

[240ZRX]

I recently installed a 5" tach with rev lite in my 72Z SBC. I also have the stock tach working along with the new tach. problem is the new tach (APC make) is 500 rpm off at idle, and as much as 1000rpm at higher reps. While the stock tach runs well.

 

So, what is the problem. I have the tach lead wire from new tach attched to + side of coil.

 

If I wanted to get rid of the stock tach how would I hook up the new tach to the cars existing tach wires.

 

If I wanted to get rid of the stoc

[Georgia Flash]

The only wires that I used for my aftermarket tach was the solid green wire that went to the factory gauge's ignition connection point. I hooked that up to 12+ connection on my Autometer. I also used the factory (black) ground wire for my new gauge. For the signal wire I simply ran a wire from the back of the new tach through the firewall to the HEI distributor tach connection. Mine works fine. The factory tach is long gone!

 

By the way my car is a 280 instead of a 240 but I hope this helps you out.