JPI Probe connection question and my #6 EDM overheat

[Houman]

Hi,

 

As some might remember, I have been having some issues with cylinder #6 CHT being about  50 to 75 degrees hotter than the other 5 cylinders. Sometimes less on low cruise.

 

Today we were doing an oil change on the Rocket and I asked my mechanic to check it. The idea was to interchange #4 and #6 JPI probes so to see if it is a cylinder or probe problem.

 

When opening it up, we saw something completely unexpected. It seems that the EGT/CHT probe has 2 cables and there is the part that comes from the JPI EDM, then near the cylinders, it has another interconnect to the heat shielded cable to the CHT/EGT probes. The cables are composed of 1 yellow and other red for each cylinder. In the #4 and #2 cylinders, the yellow coming from JPI connects to yellow cable going to the cylinder and red to red.

 

On #6, the Yellow wire coming from JPI was connected to Red extension to the cylinder and Red from JPI to yellow going to cylinder. We found that extremely strange that someone would make such an obvious mistake, or was it a mistake.

 

Wondering if that was the problem I was having. My mechanic was telling me that these cables are not interchangeable, since even the length of the cables are important in the resistance and data provided to the JPI EDM.

 

We reconnected on #6 the Yellow on Yellow and Red on Red, but I have not flown it to see if that would fix the temp overheat on #6, how can I be sure if the temps given to me know is the right one, can I trust the EDM data displayed ?

 

Here are couple of pictures, wondering if it was normal to have Red on Yellow on #6 where on #2 and #4 we had yellow on yellow and red on red.

 

Thanks !

 

 

 

 

[KyleKJ90]

Just out of curiosity, did you turn on the JPI after you swapped probes and if so, what were the readings?

[Houman]

We did just to run the engine after the oil change, CHT and EGT seems near the other cylinders, but didn't run long enough to get a real idea... the weather was too hot to fly and I had other stuff to do..

[Guest]

Cable length makes no difference to the JPI, its in the install manual, however polarity sure does.

Clarence

[Houman]

Thanks Clarence, do you know the usual colors used for CHT and EGT probes, if red is usually EGT ?

 

One thing that was difficult to see was to see if the Red was always to EGT ( on the exhaust ) and yellow to CHT ( to the cylinder head ), since the colored cables were going into a heat protection to both of them and then into a kind of cable protection tissue so not easy to tell which went where...

[N201MKTurbo]

By doing that you are creating two more thermocouples of opposite polarity. You will get the sum of the thermocouple voltages of all three thermocouples. The bottom line is you won't get anything meaningful. They need to be wired red to red and yellow to yellow.

 

The CHT should get a red and yellow wire and the EGT should get a red and yellow wire. Standard colors for a K type thermocouple.

[Houman]

By doing that you are creating two more thermocouples of opposite polarity. You will get the sum of the thermocouple voltages of all three thermocouples. The bottom line is you won't get anything meaningful. They need to be wired red to red and yellow to yellow.

That is precisly what we fixed, we now have on all 3 even number cylinders yellow on yellow and red on red... We did'nt bother to check the other side ( odd number cylinders ), since we tought it would be too strange if the default would have been Yellow on Red and Red on Yellow.

So now that we have the same color from JPI to the Probes, hope the numbers will be more inline and real...

Wondering if there is an actual way to verify, like a manual tempeature gauge on the ground we can put against the cylinders and check...

How do they usually verifiy temp accuracy when installing these probes ???

[N201MKTurbo]

You can get kinda close with a handheld IR thermometer. The problem is the emissivity of the cylinders and exhaust and the temperature gradient on both. But it is probably the best you can do. Even with a handheld thermocouple meter with a K thermocouple you won't be able to get it at the same place that the probes are. 

 

One way that I've used to check the accuracy of the probes is to remove the probe and tie a thermocouple from a handheld meter to the probe with lock wire so they are thermally the same and then heat them up with a propane torch and see how they match. 

[carusoam]

Notes on thermocouples...

1) Two different wire materials are used specially for the purpose...

2) When the two different types of wire are in contact with each other, they give off a small voltage.

3) this voltage is temperature dependent, hotter it gets, more voltage it delivers.

4) the different types of wire at the junction give a specific output in voltage. This makes it useful as a simple low cost sensor.

5) mixing wires unknowing will give a random result. Each junction of different wire types can generate additional voltage differences.

6) reversing the wires probably gives a negative result, decreasing value with increase in temp.

7) thermocouple types are named by the wire choices that are used. J thermocouples are lower in cost and have different useable range than K thermocouples. K's cost more and have slightly better range and accuracy... They are not interchangeable.

8) the color of the insulation is usually a hint to the type of wire that is being used.

9) resistance of wire in ohms/ft is so small that it will not be very consequential to our application. Expect to see no differences with longer or shorter wire runs.

10) adding splices can be done following the like wire to like wire method. Don't mix wire types or use copper as a form of extension.

11) Oil temperature uses a different technology called a thermistor... The thermistor is a resistor that varies it's resistance in relation to temperature.

Really cool stuff from Polymer processing 102...

Best regards,

-a-

[Houman]

Thanks Carusoam, that means it was wrong to begin with having the yellow wire connected to red on #6, so by putting the yellow to yellow and red to red, we are now using same type of wire all the way from EDM to Cylinder.