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#1
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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 |
#2
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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. |
#3
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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 |
#4
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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 |
#5
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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. |
#6
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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. |
#7
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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 |
#8
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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 |
#9
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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 |
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