I’ve got a weird one for the group. A few weeks ago my A&P and I completed installation of an EDM930 bought new from JPI. Initial power-up was normal – but after a few power cycles and leaving it on for longer, all temperatures start dropping from ambient rapidly and in unison. EGTs, CHTs, OAT, Oil T, Carb T. See the video.
JPI said that the temperature control circuit must have malfunctioned and shipped a new unit, which we installed. It worked completely normally for about 4 flying hours. Then, it started behaving like the old one did.
In flight, it is as if the unit is applying an ~360 degree negative offset to all temps. CHT and EGT read about 360 lower than normal, but with the same relative offsets between cylinders. OAT/Oil-T/Carb-T all go so low that the unit Xes them out, and EGT/CHT X out on the ground when the engine is off. Occasionally in flight the temps will all revert quickly to normal for a minute or less, and then drop again. See video, of this behaviour:
Once home, my A&P and I replaced the ground connection to a different part of the engine case. We confirmed good continuity of ground all the way from the pin on the connector at the unit, to the terminal on the engine, and then through to the engine itself (e.g. cylinder barrel). There was less than 0.5 ohm resistance across any of these points.
We then unplugged the EGT/CHT connector from the unit; the same behaviour continued on the other temps when powered back on.
We ran the unit with every other circuit breaker on the aircraft pulled; the behaviour was unchanged. See video:
We disconnected the OAT, Carb and Oil-T probes one at a time and powered on the unit. Temps were still displaying “off the scale” low. Visual inspection of all wiring in the harness that we could see revealed nothing abnormal.
We checked voltage differences between instrument case and panel ground, airframe ground, and engine ground when powered on. All were 0V.
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Some further really odd behaviour; if the unit has been turned off for a while, then at power-on the temps read normal. After some time, they start to drop in synchrony. The longer that the unit has been turned off, roughly the longer it takes before the temps start to drop after power-on.
It almost feels like some kind of capacitance is building up a voltage offset that affects the temp unit, slowly dissipating when you turn the unit off. I have no idea if that’s even possible or how it would occur, though.
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JPI don’t know what’s going on, but are working on it. The first unit arrived back to them today and I’ll be interested to see the bench test results. The avionics installer who sold me the unit is very experienced and has never seen anything like it. Does anyone have any idea what could be causing this? It seems unique.
Sure is an interesting one… 🧐
Reading all this, and despite you guys having checked some of the grounds, be sure that the ground wire (yes, the fatty one), or wires, between the engine and the fuselage, has proper continuity.
Next would be to check all connections of the master relay as well as the bus the EDM is connected to.
I guess the better part of the EuroGA brainforce will be in action soon 😉
Looks like a poor AD converter to me, or very noisy ignition. Hate problems like that 
Are all shielded wires shielded properly? Just guessing here.
We checked for good ground continuity between engine and airframe, as well as checking for any voltage difference between the instrument body, and the engine/panel/airframe. All zero.
Dan, can you elaborate on what checks you’d mean on the master relay and main bus?
LeSving, what does AD stand for? For shielded wires, do you mean those that are part of the JPI harness, or those running parallel to the JPI wires?
It’s particularly confusing that the replacement unit worked fine for 4 hours flight before the issue returned.
Yes, re the master relay I’d check no buzzing, and the connections. On the bus I’d also check the connections. A bad contact between a wire and a terminal, badly crimped or corroded, already led to mysterious stuff.
LeSving surely means Analog-Digital converter, and that could be a culprit too.
Katamarino wrote:
It’s particularly confusing that the replacement unit worked fine for 4 hours flight before the issue returned
Yep, sure is weird. Might be temperature related somehow…
Both failures occurred after operating the engine for a little while, right? I would try reconnecting everything, starting the engine, and checking all the leads from sensors to the EDM with an oscilloscope for possible surges (probably from ignition) that could have fried the circuitry. Protect the oscilloscope with something like a Zener diode to avoid frying it as well.
The first failure occurred before the engine had run. I think we had been on battery only; but may have used ground power at some point.
RPM, MP, Oil-P, fuel flow, fuel quantities, Amps and Volts all read entirely normal; the weird behaviour is unique to the temperatures.
FWIW, one of our club planes has a 930 and we had (and to some extent still have) endless issues with various sensors. Nobody ever managed to get to the bottom of all of them. That said, the critical ones (temp, pressure) always worked. Frankly, IMHO there’s something wrong with these units or the sensors that are used.
Yet another strange thing is that this unit was installed to replace a JPI EDM830 – which was rock solid for the entire 5+ years I’ve owned the aircraft.
These instruments highly likely log a load of data into FLASH. This can be extracted and examined. But looking at some other US products, it is likely this needs the instrument to go back to the US as there is no local engineering support.
