digital meter

67Dart273 said

All the tach wire actually is on a "real" (GM) hei is a connection to the coil NEG terminal. I have no answer unless it was RF interference. Try twisting the test leads, position the meter away from the HEI, lay the leads down along the engine, and clip them to ground and the tach connection with as little "untwist" that you can manage.

Can/ have you tried it on another vehicle?

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RF interference sounds like it could be my problem. I do have a real HEI with a 65,000v coil. My tach lead feeds a capacitor/choke then through the fire wall to my dash tach. I have tried my meter lead on both sides of the capacitor. Also the meter works properly on my 70 Dart with 340 and orange box ignition. I may try wire extensions with shielded coax and set my meter on the work bench.
Thanks to everyone for the suggestions, MOPAR guys are cool beans. Jerry