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Tom, interesting points. We did measure continuity between the probe wires (the little tails attached to the probes) and ground on the OAT and the Carb T. Both had continuity to ground which we found a bit odd - but we're not experts on thermocouples. The fact that they work normal some of...

Agreed. It's just odd that every probe is affected the same, from the OAT in the wing to the various probes on the engine. It's as if the temp circuit in the EDM itself is building up a slight charge. I will suggest that to my A&P. I'm sure we could attach the ground directly to a probe and try...

Correct. I specifically asked them if there was anything at all which we could have done on the install to cause this; they said they'd be asking the same thing, but that they could not think of anything.

Yeah - so far, I have been very happy with their response.

Indeed. JPI are sending a third unit, which they say they will stress test "extensively" first in the same configuration as my aircraft. They've also received the first unit back so I am eager to see the results of their inspection. I will discuss the harness option with them.

We could try running a ground directly to the battery negative terminal maybe (I'm not an electrical engineer so I don't know if this is a sensible idea)? JPI's installation instructions do stress very clearly the need to ground to the engine block and not the panel or airframe, but I'm open to...

Not quite. The first unit exhibited this behaviour from the beginning, before we'd even run the engine. After a few minutes being powered on from cold, the temps all started to drop. The replacement unit was fine for about 4 flying hours, and then started to do the same. Today we powered it...

Bell, that's correct. The disconnected probe goes to dashes, the others all still show the odd behaviour. Once the temp goes well beyond the normal range, the readings go to a red X. The problem seems to be identical - if not a bit more severe on the second unit. The first one, the temps only...

My friend recently installed one, and they're chasing a weird issue. Every time he transmits, no matter what frequency, it also transmits on 121.5. You might want to check that when you do your testing!

For the EGT and CHT, yes. We just removed the big plug connector from the back of the instrument. For the single other probes, we disconnected the wires where the probes connect to the harness; they use ring terminals and little nut/bolt combos for ease of swapping probes.

It's an interesting thread and I appreciate you pointing me to it. However, JPI provide the wiring harnesses with the unit, almost fully complete - we have not added any wiring ourselves. So I have trouble seeing how this could cause our issue, especially as unit two worked fine for 4 hours!

Correct. We have tried the unit with the EGT & CHT harness disconnected and get the same issue; although haven't done that from a "cold start" so that might be worth a try. The Oil T, Carb T and OAT are on the same plug as the power and ground so for them, we disconnected the probes one at a...

You said it..!

Should be; the wires used came from JPI already connected to the plugs, as most of a harness. All we did was cut them to length at the sensor end and connect them in line with the instructions from JPI.

You are a true gentleman.

Only a single ground for the whole unit, which runs to the engine block. MP, RPM, Oil-P, fuel flow, fuel levels all indicate normally and rock solid so the issue seems isolated to the temperature circuitry.

The serial numbers were 250 apart! We had the same thought. My concern is that some peculiarity of our install is killing EDM temperature circuitry, although JPI is adamant that can't happen.

I have been thinking about ground issues and charge build-up, too. However, the first unit exhibited the behaviour without the engine ever running. The second unit, after the initial 4 hours, also shows the problem without the engine being started. With battery on, we did measure voltage...

Might drop in then. @eman1200 can you add me to the chat?