IO-360 EGT/CHT probe wire routing

[PT20J]

One of my JPI EGT probes is getting wonky and the wiring and connections check good. These probes have over 1500 hours on them (JPI claims 600-1000 hour life expectancy) so I'm going to replace them all. The original installer bundled the EGT/CHT wiring with the ignition wires which is not recommended by JPI, although I've never noticed a problem. But, since I am replacing them, I'd like to clean up the installation. 

Anyone have pictures of how your IO-360 EGT/CHT wires are routed to the cylinders?

[EricJ]

This is about as good as I've got right now.    I'll probably be at the hangar tomorrow to finish my engine inspection, so I can take more pics then if there's anything specific you want to see.   This is for a JPI900.

[PT20J]

1 hour ago, EricJ said:

This is about as good as I've got right now.    I'll probably be at the hangar tomorrow to finish my engine inspection, so I can take more pics then if there's anything specific you want to see.   This is for a JPI900.

Thanks, Eric. Is there a clamp or just a ty-wrap hiding behind the #4 lower spark plug wire -- I can't quite make it out from the picture?

Another interesting thing about my installation is that JPI suggests mounting the EGT probes 2-4" below the cylinder and mine are mounted 1-1/2" which I would think would be hotter and shorten their life but they soldiered on for 1500+ hours. Peak EGTs at about 65% are around 1450 deg F.

[EricJ]

53 minutes ago, PT20J said:

Thanks, Eric. Is there a clamp or just a ty-wrap hiding behind the #4 lower spark plug wire -- I can't quite make it out from the picture?

Black zip-tie pair holding the bundle to the fuel line.

53 minutes ago, PT20J said:

Another interesting thing about my installation is that JPI suggests mounting the EGT probes 2-4" below the cylinder and mine are mounted 1-1/2" which I would think would be hotter and shorten their life but they soldiered on for 1500+ hours. Peak EGTs at about 65% are around 1450 deg F.

My peak EGTs typically show around 1520F or so.   They started out lower when the probes were new, but have settled around there for quite a while now.

[mhrivnak]

Mine are routed similarly to Eric's, and they're closer to 4" from the cylinder than 2. So far one of them failed with less than 400 hours on it. I typically see peaks around 1480 at 65% power.

[Shadrach]

9 hours ago, EricJ said:

This is about as good as I've got right now.    I'll probably be at the hangar tomorrow to finish my engine inspection, so I can take more pics then if there's anything specific you want to see.   This is for a JPI900.

Maybe I’m not seeing what I think I’m seeing, but it looks like there are exposed transducer wires within an inch or so of that exhaust tube?

[EricJ]

7 hours ago, Shadrach said:

Maybe I’m not seeing what I think I’m seeing, but it looks like there are exposed transducer wires within an inch or so of that exhaust tube?

I don't think so, I think it has a gap on the inboard side, but I'll double check before I put it back together.   This is an old pic, but it happens to be opened up at the moment.

[Vance Harral]

Here's the EGT routing on our IO-360, as of a few years ago.  You'll note that the port side routing avoids the ignition wires, while the starboard side routing for the forward cylinder routes alongside the ignition wire and uses the same clamps.  Like you, I never noticed any particular problem with data from the cylinder that ran along the ignition wire, and concluded it didn't really matter in practice.  That said, the last time I had to replace a probe, I changed the starboard side routing to look like the port side routing and avoid the ignition wire.

I don't really like the way the thermocouple wires just sorta hang loose in the breeze, but the wire is monostrand, stiff, and supports itself to some degree.  I do think this "hanging in the breeze" arrangement likely contributed to a bunch of problems we had the first few years with the wires fatiguing at the crimps of the blade connectors that joined the pigtail from the probe to the connecting wire that goes to the engine monitor.  I complained to E.I. about this and they claimed no one else had the same complaint as me.  But then then they changed the type of connector they were using from a spade style to the "OLC" barrel connector, and our problems with wire fatigue have mostly gone away with those connectors.

We've replaced several probes over the last ~16 years since engine monitor installation.  They do have finite lifetimes.  I think the "fast response" (smaller) probes don't last as long as the massives.  They theoretically give better response, though.

Our probes are further down the exhaust stack than any other installation I've ever seen.  We just re-used the holes from an old analog 4-cylinder probe installation.  They're further away than the installation manual for our engine monitor recommends, but having been in a bunch of different airplanes with different installations, I've come to the conclusion it just doesn't make any difference where the EGT probes are installed from a day-to-day operational perspective.  Since absolute EGTs don't matter, and the numbers vary from airplane to airplane, it's not like you're trying to target any specific range of temps that make location critical.  Hard to say what effect probe distance has on longevity - again we had poor luck with this before switching to the barrel style connectors, but that's a problem with the wiring, not the probes themselves.  The conventional wisdom is that the closer they are to the cylinder, the higher the indicated temperature, and therefore the shorter the life.  But I can't offer even anecdotal evidence that our probes are lasting longer than others' because they're further down.

[MikeOH]

Well, speaking from both actual experience and as an EE, I don't think it matters much.  Mine were bundled with the ignition wires and I never noticed any issue. Then last year at my Top Gun annual they moved the wires as they said it was bad to have them that way.  Haven't noticed any difference since

To go all nerdy, this doesn't surprise me.  While the thermocouple voltage is pretty small (30-50 milli-Volts) BOTH wires run close together and parallel to each other.  The electronic circuit that is 'processing' this signal is looking ONLY at the difference between the voltage on the two wires and IGNORES any voltage common to both wires.  Given the physical proximity of the thermocouple wires any voltage induced on one wire will have nearly the IDENTICAL voltage induce on the other.  The circuit will reject that common voltage and the signal (temperature) will be unaffected.  Further, while the ignition lead is high voltage it is pretty well shielded (if not, you'd have major radio interference), so I suspect the induced common voltage is actually relatively small.  Finally, the thermocouple circuit is pretty low resistance (maybe 100 ohms max) which means quite a bit of current would have to be induced and the ignition system is not really high current, plus the thermocouple wires are close together which minimizes the loop area making the circuit less susceptible to current induced errors.

Oh, and modern electronics likely have a pretty decent filter circuit to reject high-frequency noise.

[EricJ]

6 minutes ago, MikeOH said:

Well, speaking from both actual experience and as an EE, I don't think it matters much.  Mine were bundled with the ignition wires and I never noticed any issue. Then last year at my Top Gun annual they moved the wires as they said it was bad to have them that way.  Haven't noticed any difference since

To go all nerdy, this doesn't surprise me.  While the thermocouple voltage is pretty small (30-50 milli-Volts) BOTH wires run close together and parallel to each other.  The electronic circuit that is 'processing' this signal is looking ONLY at the difference between the voltage on the two wires and IGNORES any voltage common to both wires.  Given the physical proximity of the thermocouple wires any voltage induced on one wire will have nearly the IDENTICAL voltage induce on the other.  The circuit will reject that common voltage and the signal (temperature) will be unaffected.  Further, while the ignition lead is high voltage it is pretty well shielded (if not, you'd have major radio interference), so I suspect the induced common voltage is actually relatively small.  Finally, the thermocouple circuit is pretty low resistance (maybe 100 ohms max) which means quite a bit of current would have to be induced and the ignition system is not really high current, plus the thermocouple wires are close together which minimizes the loop area making the circuit less susceptible to current induced errors.

Oh, and modern electronics likely have a pretty decent filter circuit to reject high-frequency noise.

Plus the probes and modern ignition harnesses have better shielding than similar systems in the past.   It seems like ignition interference happens a lot less than it used to.   The JPI probes systems seem to be very well engineered and shielded, so that seems to help, too.

[Shadrach]

My CHT and EGT wires are bundled with the spark plug leads; it’s not an issue.

I am more concerned with keeping the EGT and CHT wires away rom heat.

[Shadrach]

my set up is perhaps not as clean in appearance as others, but It does ensure that all uninsulated wires are protected from excessive heat. Creating a cleaner set up is on my list of things to do, but then so are about 150 other things.

[Yetti]

I don't have pictures, but drilled the holes and routed both EGT outside the exhaust tubes.   Used cordage to tie them together.   The biggest thing is to see if you can get them all to the center of the engine when you go to the firewall.   Less movement that way.

It's pretty low voltages so keeping them away from the spark plug wires would seem to keep the transients away.