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Hi,
I have aquired an XR7 Tach to put in my standard car. My car also has an MSD ignition and I have aquired P/9 8920, the MSD tach adapter. So where would the two wires on the back of the tach be hooked to. Where would they normally go without the MSD. If anyone knows what goes where with the 8920, evenbetter but I do have the schematic for that.
 

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I am no electrical maven, but this is how I think it works:

The tachometer on an XR7 originally is wired-in so that the current for the ignition coil runs through the tach and that is what it reads to display rpm. The wire that used to go to the coil in your MSD setup now tells the MSD box that the ignition is on, but the MSD box does not need to draw much current on that wire and the tach shows hardly any rpm. To solve this the PN 8920 is wired in with the white wire to the tach-outlet on the MSD-box, the red wire is spliced onto the original ignition wire, and the purple wire curled up and tugged away. Now the PN 8920 will create the current draw through the tach that the original Ford setup would have made and you are in business. (Once the XR7 tach is wired in with the ignition wire) I hope this may help.
 
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