mach1drumstar69

I am trying to hook up the factory tach in my 69 mustang and hit a snag. i have the two wires coming out of the tach and hooked up in series correctly, but when i switch the ingnition to the on position, the tach buries itself all the way up. im thinking that the resistior wire is not making the correct amount of resistance, does anyone have any idea how much resistance is needed between the tach and the coil? or any other reason this would happen? i switched the wires to test if that was the problem, and the needle just went backwards instead and popped a fuse so thats not the problem. any help would be greatly appreciated!!!

urban_cowboy

Are you running a stock ignition or have you upgraded? What is the voltage at the coil when you are not cranking/starting? Which post on the coil are you connecting the tach to?

Tachs are either voltage triggered or current triggered. I believe the one in the 69 is current triggered and should have the tach trigger wire connected to the positive terminal of the coil via the pink wire under the dash which is the resistance voltage wire. If the resistor has been removed, that is going to cause you problems with the tach. The voltage at the coil should be about 9v with the ignition on. If it is 12-14v anytime other than when you are cranking (the solenoid may supply full voltage to the coil while cranking), the resistance wire has been removed.

palerider

Not sure it will help but here is some info on wiring. I just had mine rebuilt by Tachman and have not hooked back up yet. I know PO bypassed balast resistor so I need to figure out how to run wires on mine - I know there is currently some resistance since I get 9volts with igntion switch on. My guess is that is not your problem - do you know if your tach was good before. Also did your car run before you tried hooking up tach. In 1969 and 1970 tach is part of the igntion circuit if tach was bad car would not start. If it ran before without tach my guess is someone rigged wires up to bypass it. Also from I understand if you are running an electronic MSD with there is an adpater you need to gdt for tach to work correctly. Be interested in hearing what you figure out - keep us posted.

http://1970mgr.org/_TachBypass.htm