GEM 1200 Problem

Our Seneca has a GEM 1200 - it was installed during an avionics upgrade about 2
years ago. Up until now it has worked fine BUT .....

I'm hoping someone out there has fixed a similar problem or has some insight (no
pun intended - into what the problem might be.

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.

So far I've not been able to associate any particular activity with the problem
except perhaps "heat".

Observations:
- It never shows a problem on the ground, even during runups to takeoff power.
- Usually it works fine for about 20 to 30 minutes on the first flight of the
day. However, I was just in Florida where I made three flights where the OAT
was over 80F. In all three flights the gauge was fluctuating within 60s after
takeoff. But then today I departed Sanford with the OAT at 62F and it worked
fine for about 20 minutes and then sporadically for the next 5 hours..... In New
Jersey (home base) the temperature has been 40F or below since this problem
started...
- On some flights the gauge is totally unuseable. That is, the fluctuations
are continuous. On other flights the fluctuations occur sporadically at
seemingly random intervals.
- I've tried turning on/off: mags, alternators (even both), heater, fans, all
other avionics, prop ice, overhead lights, dimmers, etc,. Nothing makes a
difference or seems to affect the flucatuations. Haven't tried shutting down an
engine.
- Pulling the breaker on the GEM and resetting it doesn't seem to help. I've
left it off for an hour, in flight, turned it on and had it "go crazy"
immediately.
- On advice from a mechanic, I tried pushing on the display to see if that
would stop the "jitters". No dice.

The avionics are cooled by a fan -- but there is no cooling directly on the GEM.
It gets warm to the touch. But then its been that way for two years and
certaily it is much cooler now than on a typical summer day.

It went back to Insight. They say they bench tested it and its OK. But I
wonder how thoroughly that was done?

My personal guess is that it is heat related but there isn't really that strong
a corelation.

We've also tried relocating the wire harness thinking that it was picking
something up -- like an antenna. No noticeable difference.

The only recent work on the airplane was that we replaced the aluminum cables to
the starters with copper cables. But then, they don't carry any current in
flight. The only aluminum cable replaced that carries current all the time is
the piece from the battery to the bus -- from the bus it goes to the starter
solenoids and to the avionics bus.

The wiring harness grounds have been checked. The wiring harness has been
inspected (both sides).

Jim Kaufeld

Jim Kaufeld

Loose connector on the back of the GEM.
A friend of mines Bonanza had the same problem.
He would be flying along, and the GEM would go dead.

Jay Smith writes:

Loose connector on the back of the GEM.
A friend of mines Bonanza had the same problem.
He would be flying along, and the GEM would go dead.

I've had that too. Reaching behind the GEM verified it. (Zip ties
are sure handy to have on long trips.)

The symptoms in this situation were different for me than what the OP
described. The GEM would usually go dead then, if it returned, would
take a moment to boot before doing anything useful. The "dancing
temperatures" is not consistent with that.

--kyler

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.)

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

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

Good luck.

--kyler

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.


I have the EI US8A. When this happened on mine the first time I assumed
it was a bad probe, so I replaced it. Then after about 10 hours it
started doing it again, so clearly it wasn't the probe. Turns out it
was the electrical connection that is midway between the probe and the
firewall. If you simply took the connections apart and reconnected them
the unit would work fine for a few hours then go back to the same
problem. Making sure the connectors have a good electrical contact
solved the problem. I have had my EGT/CHT for about 5 years now and
still have not had a probe go bad.

The best method I have found with milliampere instruments connections is to
place a stainless star washer with twisted teeth, between the two stake-on
connectors and which actually cuts into the metal both sides of the
connection when the screw is tightened, breaking through any oxide layers...
denny
"Newps" wrote in message
...



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.


I have the EI US8A. When this happened on mine the first time I assumed
it was a bad probe, so I replaced it. Then after about 10 hours it
started doing it again, so clearly it wasn't the probe. Turns out it
was the electrical connection that is midway between the probe and the
firewall. If you simply took the connections apart and reconnected them
the unit would work fine for a few hours then go back to the same
problem. Making sure the connectors have a good electrical contact
solved the problem. I have had my EGT/CHT for about 5 years now and
still have not had a probe go bad.

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