GEM 1200 Problem

Kyler Laird wrote:

Jim Kaufeld writes:

Anyway, the GEM has an intermittently flaky display. Sometimes it goes crazy
and the bar graphs and digital temp readouts start fluctuating wildly. It
isn't just one engine and it isn't just one cylinder. Its all of them.

Does this look familiar?
http://lairds.org/Kyler/photos/disk0024/mvi_1757.avi
(It got much worse than that.)

Yes ... that looks familiar!

First verify that your engines are sufficiently grounded. Then
verify it again. Check your alternators while you're there.

What, exactly, should I check? I would have assumed that since the starters
work on both sides, the engines are sufficiently grounded. Is there typically
a separate ground that goes to the engine? Should I add one?

Next see if you can isolate a bad probe (or probes) by flying with
some disconnected.

One thing I've noticed is that when the GEM is working the CHT on the RE, #2
varies. I figure that either something is wrong with that probe or the
connections to the probe. Can just one badly connected probe -- or maybe a bad
probe -- make the whole instrument dance?


Thanks for the help.

Good luck.

--kyler

Jim Kaufeld

Jim Kaufeld writes:

First verify that your engines are sufficiently grounded. Then
verify it again. Check your alternators while you're there.

What, exactly, should I check?

I'm not sure of the proper way to test, but there should be (next
to) no resistance between the engine and the airframe.

I would have assumed that since the starters
work on both sides, the engines are sufficiently grounded.

It seems reasonable to assume that but it is definitely not
correct. (I was a counterexample.)

(Incidentally, not long ago I really toasted some wires on a
grain truck upon starting it because its ground cable was loose.
It had been starting hard for awhile, but when the cable really
went, the starting current took another path, through a small
auxiliary cable. *Poof*)

Is there typically
a separate ground that goes to the engine? Should I add one?

There should be a big strap from the engine block to the engine
mount. The rubber cushions of the engine mount electrically
isolate the engine but there are probably other things that
provide a path to ground. Even a ground strap can become
ineffective (as in my case - corrosion, I recall).

Next see if you can isolate a bad probe (or probes) by flying with
some disconnected.

One thing I've noticed is that when the GEM is working the CHT on the RE, #2
varies. I figure that either something is wrong with that probe or the
connections to the probe. Can just one badly connected probe -- or maybe a bad
probe -- make the whole instrument dance?

Yes. It happened to me. (I had multiple problems last time.
That made the diagnosis much harder.)

The probes seem to die much more frequently than I'd like. I
recommend getting some extras to carry.

Thanks for the help.

You are welcome. I hope you solve it.

--kyler

Kyler Laird wrote in message ...
Jim Kaufeld writes:

First verify that your engines are sufficiently grounded. Then
verify it again. Check your alternators while you're there.

Kyler gives good advice.

The common mode rejection on most things that measure thermocouple
voltages isn't real high. So anything that can affect engine
grounds, etc can be a problem.

The GEM should be grounded to one of the engines. Doesn't the
1200 have 2 grounds?

Making sure that terminal is staked well would be a start.

Bill Hale A&P

(Bill Hale) wrote:

Kyler Laird wrote in message ...
Jim Kaufeld writes:

First verify that your engines are sufficiently grounded. Then
verify it again. Check your alternators while you're there.

There is a copper cable going from the engine to the airframe. It was
recently replaced (on both engines) as part of the STC to install copper cables
to replace the original aluminum cables. BTW, that replacement made a huge
difference starting; before the engines would barely turn over, now they spin
fast!

The ground cable on the engine goes to a ground bus on the firewall; that cable
is copper. A cable runs from the battery to that firewall ground bus. An
aluminum cable goes from there to the insrument panel ground bus.


Kyler gives good advice.

The common mode rejection on most things that measure thermocouple
voltages isn't real high. So anything that can affect engine
grounds, etc can be a problem.

The GEM should be grounded to one of the engines. Doesn't the
1200 have 2 grounds?

The GEM has one harness for each engine. The harness is continuous, that is,
it goes from screw connectors at the probes to the "plug" at the back of the
instrument. The ground and power are at the same point for both harnesses --
the instrument panel circuit breaker and the instrument panel ground bus.

Do you suppose that aluminum cable from the firewall ground bus to the
instrument panel ground bus could be the problem? There is no STC for that to
be replaced by copper ........


Making sure that terminal is staked well would be a start.


We're going to do that tomorrow (Friday), I think. Insight is sending us
another instrument and we're going to bring the airplane in and swap it as soon
as that instrument arrives. It might get delayed until Monday because one of
my partners is going to Sarasota tomorrow, early afternoon and returning on
Sunday.

Bill Hale A&P

Thanks again for the advice. Hopefully we'll track this gremlin down soon.

Jim Kaufeld
Jim Kaufeld

FWIW, the JPI instruments get grounded to the engine instead of to the instrument panel to avoid
this type of problem. Could it be that the GEM should be wired the same way?

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

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