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Recently reconfigured power to coil to get full 12V for Pertronix unit. As a result, no longer have lead from my tach connected at coil "+", so tach does not currently function. I did not want to reconnect it because I was unsure of consequences of connecting an 8V source on top of my new 12V source to the coil.

My question: a mechanic told me I could run the tach lead to the
"-" post and still pick up tach signal ok. Anyone know if that is true? I'm nervous about connecting an 8V source to the negative coil post - not interested in frying something.
 

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If its a real Ford Rally Pac (RP) it will NOT work unless the power to the coil runs through the RP. They work on current flowing THROUGH the RP input transformer, not by a voltage pickup at the coil like most other tachs. That's why an RP failure can make the car not run. The RP requires a wire INTO it (power) and also a connection going OUT to the coil. Repros RPs are wired differently.

The original wiring used the pink ballast resistor wire between the Rally Pac and the coil. I do not know whether you will damage anything in the RP by shorting out the ballast resistor but its possible.

If its not a Ford Rally Pac the coil signal pick up will probably work, who knows.
 
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