The amazing cryogenic JPI EDM 930

mondtster said:
Did you or your mechanic have to do any harness building/pinning that could have gotten reversed? The JPI manual does a good job of not referencing positive or negative for the thermocouples but the red wire on traditional thermocouple wiring is the negative one and in some measurement systems when the positive and negative are reversed the indicated temperatures will go down instead of up.
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Only the OAT wires. We have checked them for correct continuity.
 
I don't want to reread the thread, but did you ever make sure the engine is properly grounded?
 
Pinecone said:
I don't want to reread the thread, but did you ever make sure the engine is properly grounded?
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I believe we did; the ground strap is properly installed and we have confirmed good ground continuity between engine and airframe with less than 0.5 ohm resistance between any points.
 
Katamarino said:
I believe we did; the ground strap is properly installed and we have confirmed good ground continuity between engine and airframe with less than 0.5 ohm resistance between any points.
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Might want to undo the strap and brighten up the contact surfaces on both ends. Can't hurt.
 
Half Fast said:
@Katamarino Any news?
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None yet really - I was traveling the last few days and JPI have not replied to me email asking for an update. I will try and call them tomorrow.

I did fly a lot over the weekend. The behaviour continued to be intermittent. On one of 6 flights, it worked perfectly. On two flights it barely worked at all. On the remaining three it worked properly over 50% of the time.
 
Pugs said:
You skipped pulling tubes and going down to the drug store to use the tube tester :D
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I had my own tube tester. I often fixed friends and neighbors TVs. First diagnostic was a slap. If that helped I assumed a tube was flaky and brought out the tube tester.
Sometimes while pulling tubes I would see a blown capacitor or resistor. If it wasn't tube or a visible issue, I gave up. But most of the time I was successful.

Often people would give us their old non-working TVs. My younger brother and I would take them apart, piece by piece and sort and store tubes, capacitors, resisters, power supplies etc. We took up most of the family room with our work bench. We often had a good part on hand to fix most tvs or radios. It was a fun way to pick up a little spending money.
 
Pugs said:
You skipped pulling tubes and going down to the drug store to use the tube tester :D
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And fiddling with the rabbit ears antenna. Aluminum foil sometimes helped, for mysterious reasons.
 
Reading through this makes it almost certain to be a grounding issue.
1) The system has worked as expected, so no wire polarities are reversed.
2) The broken times affects ALL probes at the same time and to the same degree. Only thing they should all have in common is the ground wire.

This is going to be an interrupted ground issue. It could be that the ground path is fine but that something is vibrating to be in contact with the ground path messing it up. So when you look at the ground wires and connectors they are fine, but it is really something nearby causing the issue.

As previously stated:
Salty said:
This is a fascinating and frustrating problem.
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Half Fast said:
@Katamarino

Ever solve this?
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JPI analysed the first defective unit, and found what they described as a "suspect" component. Based on that they suggested that the installation should be OK, and sent out a third unit, first testing it extensively over a few days.

Since installing the third unit I have flown about 40 hours and so far it is working as expected.
 
Signal vs power ground issues usually don’t change much over 4 hrs. So I’m skeptical about that being the cause. Does the unit have an internal backup battery? Because it’s a 4 hr problem, I’d look in that direction.

If you’re still wanting to go down the bad ground path …. with very small signals like you have, you almost always want the “return” for that signal to be isolated from power ground and run into the unit on a separate “signal ground”. This should not be connected to “power” ground aka the negative side of the battery. If it is, it’s called a “ground loop”.

I’m a retired FSE for a major avionics company so I sympathize with you. Fly safe.
 
guest user said:
Reading through this makes it almost certain to be a grounding issue.
1) The system has worked as expected, so no wire polarities are reversed.
2) The broken times affects ALL probes at the same time and to the same degree. Only thing they should all have in common is the ground wire.

This is going to be an interrupted ground issue. It could be that the ground path is fine but that something is vibrating to be in contact with the ground path messing it up. So when you look at the ground wires and connectors they are fine, but it is really something nearby causing the issue.

As previously stated:
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as for point 2-

All the signals could be biased by whatever method they're using to handle the cold junction compensation.


This to me is far more probable than a grounding issue. The TCs don't need to be grounded to generate their very tiny levels of emf. The amplifier or AD converters will need power, but they're not measuring a signal that is even referenced to ground

The datasheet has some PDF links in it that are interesting reading if a person is inclined
 
bentmettle said:
as for point 2-

All the signals could be biased by whatever method they're using to handle the cold junction compensation.

This to me is far more probable than a grounding issue. The TCs don't need to be grounded to generate their very tiny levels of emf. The amplifier or AD converters will need power, but they're not measuring a signal that is even referenced to ground
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My electronics knowledge is admittedly limited, especially in the actual finer points, so you very well could be right.

However, I point to this from the opening post:

Katamarino said:
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.
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The fact that it will stay mostly dead, but then occasionally return expected values suggest that some reference value is changing while in operation. Hence my opinion on an intermittent ground reference.

Now that we have this updated tidbit:
Katamarino said:
JPI analysed the first defective unit, and found what they described as a "suspect" component.
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It seems pretty likely the two installs were defective hardware, after all, and not aircraft related.

My mind now wonders how many defective JPI units they will be needing to be replaced, since its unlikely that the OP scored the only two defective units in a production run.
 
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