I'm starting to get around to fixing the small problems that my 84' T/A has so the other day I was trying to trace down why the tach doesnt work. What I found is that the signal wire to the HEI was not connected. When I connected it and started the engine the tach read backwards. Mabey I should just replace the tach or does this stem from the dizzy?



Brian

 

Brian,

The system is actually fairly simple. The distributor reluctor signal creates a trigger signal that causes the HEI module to switch current to primary winding of the ignition coil. The coil primary voltage switches from effectively zero to 12VDC when charging, then to a brief spike of about 250VDC when the module switches off, and back to zero. The tachometer simply counts the transitions from zero to 12V over a time base. There is a filter installed either between the coil and tach (on older models) or included in the tach electronics (on newer models) to clip the 250V peaks and smooth out the transitions so the tach only measures a signal at a more reasonable level. You might have a problem with the filter, or the tach itself. If the filter is somehow creating a neagative going voltage from the ignition coil signal, it could plausibly create the condition you describe.

Chances are, however, the tach itself is the problem.