Mark

I have used a Megger on engine loops, but I'm not a fan of using one.

A Systron loop is pressure filled, so a Megger shouldn't hurt it. The Kidde???? I would stay away from a Megger.

With this problem, after we used the decade box for substitution, the loop read 30MΩ (that was just from removing the one terminal lug).

We smacked the white doughnut pretty hard and the 4.5KΩ returned. It didn't fail after the beating.

I use a regular VOM on most engines loops. A LCR Meter can also be used.

A LCR Meter should "always" be used on body and wing overheat loops.