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MSD CD Installation

Electrical

Vehicle Information: Year: 1977; Engine (Size, Modifications): 3.0; Total Mileage: 70000;

Mechanic installed a new MSD CD on my 77' 930. He also had the tach modified by another shop to accomodate the new cd. The orginal CD was found bad therefore the new mod. With the new installation the car starts and runs fine, but the tach is erratic. At start up, the tach may or may not operate. When it does operate, it is erratic and may quit operating (go to zero) after driving for awhile. It's indications are way too low at engine idle and when the engine is rev'd, the tach will move fast or slow, hesitate, or stay stuck at an rpm for a few secs. The wiring and hookups have been double checked at both the cd and tach with no errors noted. Any thoughts on this problem?

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