weak tach signal

[travolta]

I have a 71, 351C with factory tach/funtioning gauges. I recently installed a mallory unilite 47 series distributior and petronix coil. I cannot seem to get the tachometer to accurately read the signal from the coil. The car idles at around 800-900RMP, but the tach reads at around 200RPM. When I rev the motor, the tach barely moves up to around 700RPM. I hooked my tach/dwell meter directly to the coil and it properly reads 800-900RPM at idle, so Im thinking the problem is somewhere between the coil and the tach. Any help or insight would be greatly appreciated.

 

[72HCODE]

that is because all the new ignition systems are not compatible with the older tachs, on the MSD there is a tach converter that changes the signal so the old tach reads correctly.

i'm not sure if mallory sells a similar converter.

they sell these

http://www.jegs.com/i/Mallory/650/29074/10002/-1?parentProductId=753957

http://www.jegs.com/i/Mallory/650/29078/10002/-1?parentProductId=753957

but not sure if that is the correct part

on the MSD its a tach adapter 8920 i think...

 

[travolta]

thanks for your help. i appreciate it. i was told by the tech guy at mallory that an adapter was not needed. i was also told that the tach wire goes to the positive side of the coil, but I thought the tach received it's signal from the negative side. can you confirm if the tach wire goes to the positive or negative side of the coil?

 

[72HCODE]

Tach is positive, the distributor goes to neg.

 

[goodnigh]

that is because all the new ignition systems are not compatible with the older tachs, on the MSD there is a tach converter that changes the signal so the old tach reads correctly.

We put a Pertronix coil and electronic "points" in my '71 M-code and the tach works fine.

mike