MSD Polarity (Wiring)
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(all information below is a recommendation from Race-1 not necessarily from GM)
This is a very common problem. Below is copied from our 602/604 Timing Page in our FAQ Section. To view the page, click here »
The purple wire DOES NORMALLY change position between the harness and distributor—but a final check of correct polarity is done by checking rotor phase and should ALWAYS be confirmed. After the timing is set align the timing marks with the engine shut off—then remove the dist. cap and validate rotor phasing.
The rotor should be pointing directly at either #1 or #6.
If the rotor is pointing between terminals, —
The pick-up coil wires are reversed and must be corrected and then timing reset.
(The above procedure is for a distributor with a LOCKED advance mechanism.)