Aftermarket tach issues

[doctor4766]

Hi guys.
I'm running an aftermarket tach in my Charger along side a MP electronic distributor and orange box.
The reading on the tach has never been instantaneous and doesn't show revs past around 2200rpm
The feed is taken from the coil negative and it's on the 8 cylinder setting
We've also tried another new aftermarket tach with the same results and also this tach on another car.
Any ideas where I'm going wrong?

[68 RT]

You should have 4 wires, red to 12v keyed, black to a ground, white for light, green to neg side of coil. What kind of tach?

[doctor4766]

Just an inexpensive auto parts Topex brand that suited the location I wanted to mount it.
The wiring seems to be correct.
I'm just not sure if the modern electronic tachs are suited to the orange box set up

[ODZKing]

Just an inexpensive auto parts Topex brand that suited the location I wanted to mount it.
The wiring seems to be correct.
I'm just not sure if the modern electronic tachs are suited to the orange box set up

Mine works fine with my orange box on my 73.
I uploaded a pdf of the instructions of my cheapie SunPro.
Download it here if you want to see them: http://www.retrorarities.com/images/SunProMiniTach.pdf

[doctor4766]

Ok, after reading a couple of things on various forums I decided to run the red wire directly to the battery instead of a switched supply from the ignition switch to see if it made any difference. It seems I was only getting 8v and not the full 12v for some reason. 
I now get a higher RPM reading which is great!
It still isn't as responsive as I would have expected but we're getting there.
I've also read that you can connect the signal feed to the grey wire on the orange box for a more accurate reading. I'll give that a go later today and report back.