Faulty Rpm Indicator

[Mark Hislop]

Jose:

Before you send out your instruments, I would check the wiring coming from the tach output on the mag, going to the tach. I have had a similar problem, right after maintenance. The pigtail coming out of the tach has a "quick disconnect" type of spade lug. On my aircraft, one of them was worn and loose, so that vibration (or big mechanics hands) could make it have intermittant contact. On the other engine, I had a broken wire inside the insulation, so it would work intermittantly. Also check inside the mag, where the tach contacts are (they are just a set of points, I think) and make sure they are clean.

Hope this helps, since it is a cheap fix!!

Mark