The problem with our GEM 1200 has been solved! I delayed writing this until we
had enough "hours after tentative repair" to make a positive conclusion.

To recap: The GEM display would occasionally start dancing. There was no
apparent cause; the instrument was sent back to Insight and they returned it
saying all was OK with the unit. We tried a variety of suggested fixes
including: checking ground, adding star washers to the harness/probe
connections, reseating connections to the display, replacing the display with a
loaner unit .... to no avail. Finally, we got pestered Insight's customer
service enough that they put an engineer on the line. He gave us a new test to
try:

For each probe, measure the resistance between the leads and between the leads
and ground. The expectation was that resistance between probe leads in all
cases would be less than 2 ohms ....... and, at least for CHT probes, there
would be low resistance between probe leads and grounds.

Is it turns out, we had two probes that showed problems. One was the CHT probe
that was known to be misbehaving. It showed 240ohms between probe leads. The
other was an EGT probe that had 8 ohms lead to lead.

When test results were given to the Engineer he said that it was very likely
that the EGT probe was "opening up" some of the time under flight conditions.
And, when it did that, the leads back to the instrument acted as antennas
causing the dancing display. He also told us that we should not use the adapter
probes (they allow for the GEM and factory probes to "share" the use of the
cylinder screw in probe connection point). So ....

We replaced the EGT probe and switched to spark plug probes on the two cylinders
(one in each engine) that have the factory probes.

In roughly 50 hours of flying since the repair the dancing has not recurred.

Now if only the OAT gauge was accurate.....

Jim






Jim Kaufeld