Northern Digital Tach, revisited, my experience

The hour meter resets each time I run the tractor, and it reads 2000-4000 rpm in an idle and fast idle. NO WAY, my idle is that fast.

• tach

Your comment about the ground and picking up all four spikes rang a bell in my head. Had a similar problem related to a bad ground in an old computer I was trouble shooting. The clock signal for the logic was distributed to several driver cards through out the chassis. These drivers had a low impedence so they could drive many cards at once. The power for the logic cards was jumpered from one card slot to the next all across the row of cards. Every so often the ground was also tied to the frame for added grounding. Right where the driver card was was one of these grounding links. It was loose and not making contact. The result was that the driver was "jerking" the ground up and down. So the adjecent cards had a moving reference for the voltage of the incoming signals. SO if the incoming signal was high AND at the same time the ground had been pulled high by the driver, the logic in the card thought the incoming signal was zero NOT one as it really was!

You might try this on your tach. Take a HEAVY gage wire from the battery ground (or at least the point where the ground lead is bolted to the chassis) to the closest point you can get on the tach ground lead to the tach case. That should eliminate any chance that variations in the groung potential is causing you trouble.

HTH
Larry

later,dave

later,dave