Analog CHT gauge inaccurate

[dcastor]

The analog CHT gauge in my M20J is reading well below (about 75 deg) what I see on my JPI engine monitor.  Anyone know of a way to calibrate this instrument?  The lead to the probe was shortened a little bit when the terminal at the end corroded, but it was probably a half inch, I can't imagine that would cause such an inaccurate reading.

thanks in advance,

Dave C

[DXB]

What type of CHT probe do you have from the JPI to the cylinder with the factory probe?  The spark plug gasket probes  read too high, by roughly 75 degrees, suggesting your factory probe is actually the correct one.   The ring probes that go under the factory probes instead read too low- I've heard by about 30-40 degrees, but my old one was off by more than that.

[dcastor]

I think I've got the ring probes that go under the spark plugs...ill see what I can do to confirm.  That's surprising because it would mean my CHTs seem below normal range that I've heard from others.  How did you confirm that the factory probes were accurate?

 

 

[DXB]

I think it's typically just the one cylinder monitored by the factory probe that has that instead has a ring probe input for a JPI that is not primary?  The other 3 cylinders have bayonet probes in the factory slot.  

In my case,  my old factory probe ran hotter than a ring probe  piggybacked on the factory probe that was read by an old EI monitor.  When I replaced both with a JPI 900 as primary (bayonets on all cylinders) the JPI probe behaved the same as the removed factory probe used to.  

[carusoam]

A few things to keep in mind...

0) The four channel JPI wants to use four wells, one in each cylinder. One of those wells is already occupied by the ship's CHT.  The usual complaint is typically one of the CHTs on the JPI is reading low.  A hangar fairy may have swapped the ship's instrument with the JPI instrument. 

1) Location of probes is key.  Style of probe is next important. 

2) Knowing that the temperature sensitive part of the probe is the point where two dissimilar metals are contacting each other.

3) Shortening the wires shouldn't do anything unless there was more to it...

4) The bayonet probe is no more accurate than a ring probe.  But, the ring probe doesn't fit down the hole that the bayonet is in.

5) the ring probe on the spark plug has the most variation.  It is furthest from the CHT probe hole.

6) the ring probe on the CHT bayonet is closer, but still not in the hole.

Also keep in mind, I am only a PP, not a mechanic.

Hoping this is helpful information. Let me know if I have missed something.

Best regards,

-a-

 

[DonMuncy]

  On 12/29/2015 at 5:03 AM, carusoam said:

 

 The four channel JPI wants to use four wells, one in each cylinder. One of those wells is already occupied by the ship's CHT.  The usual complaint is typically one of the CHTs on the JPI is reading low.  A hangar fairy may have swapped the ship's instrument with the JPI instrument. 

Expand  

In case you (or your hangar elf) decide to swap probes around, remember that the original cabin gauge uses a "thermistor" probe. It reads a variable resistance depending on temperature. And it will only fit in the well on the underside of the cylinder.

The JPI uses a thermocouple probe. They can be obtained with the bayonet type end to go in the well, or in various ring sizes which can be used under either spark plug, or under the original probe. As long as you have a "required" cabin gauge, I think you would have to use all spark plug rings for a JPI or live with one cylinder reading a little different due to using one spark plug type probe.  

[N201MKTurbo]

I agree with the consensus, it is possible that all your probes are accurate, they are just measuring different places on the cylinder head. When you are talking about the normal operating range of the head, I would have to assume it is referring to the temperature at the bottom of the thermowell.