My factory tach was converted to be HEI compatible but did not work yesterday when I tried it. An additional circuit was added and the tach was calibrated by the technician, I have the paper work. When I installed it in the dash I got no reading. I removed it and connected it under the hood, no reading! Before I send it back I want to make sure its getting the right signals. Here is how I was instructed to connect it. "tach" lead on HEI to marked stud #1 12 V switched to marked stud #2 (did not have this with points set-up) Ground to tach housing Lights plugged in. The lights work, but no needle movement during engine start or when engine is running. I checked the voltage coming from the "tach" lead of the HEI. No reading at the "off" or "on" key positions (expected). When the engine is running I get ~14V reading from this lead, if I rev the motor the reading says about the same. Does this sound right? What else should I check? The stud marked by the technician for HEI is not connected to the new wire that supplies 12V to the fuse box, its the lead beside it marked "tach" (on the HEI distributor), I think this is correct. I also checked the continuity between each stud to the tach housing (un-installed) and it does not seem to be shorting to ground, it also checks open between the two studs. I don't want to have to send this tach back AGAIN! :af: ou: Help!
Well I figured out my tach problem! The studs were insulated from the circuit board (they are suppose to be isolated from the housing only). Thanks Ken! Just for your information, here is the calibration results of the "HEI configured" `72 in-dash tach. Calibrator / Tach 1000 = 900 (rpm) 2000 = 2000 3000 = 3000 4000 = 4100 5000 = 5250 I assume this is typical of stock tach accuracy.
