Garwin Cluster Cylinder Temp Zero

[Mellow_Mooney]

Hey guys,

Got an interesting issue here. My garwin instrument cluster cylinder temp indicator was working and all fine, until I swapped the probe on it. Using a JPI or Insight Probe, the cylinder temp idles at zero until I turn my master on. Once master is on, the dial shifts BELOW zero… almost like there is a wire on backwards (it would normally give a reading). 
 

Any idea what could be causing this? 
 

I thought it might have been a bad instrument, so swapped it for another one I found on eBay, and same thing. So I’m assuming the instrument itself is fine, and something is just going wrong with the wiring. 
 

I would throw the old probe back on to test it, but the shop threw it out. 
 

Any ideas appreciated. Thanks! 
 

-Mellow

[1967 427]

Your new thermal couple is it a type K or J?

[Yetti]

Check the polarity of the thermal couple wiring.    Make sure you have proper k or j

[47U]

  On 3/23/2025 at 2:22 PM, Mellow_Mooney said:

So I’m assuming the instrument itself is fine, and something is just going wrong with the wiring. 

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The part number for the correct CHT probe will be found in your wiring schematic.    

[Mellow_Mooney]

  On 3/23/2025 at 4:46 PM, 1967 427 said:

Your new thermal couple is it a type K or J?

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We've tried both the K type (JPI) and J type probes (Insight) and neither of them worked. Both had the exact same result.

Right now we've left the J type in there. Is that the correct one to use?

[MB65E]

I was helping Mellow with this. Both the spark plug style probes have 2 wires.

Original probe was only a single wire using the probe itself as the ground.

So we’d need to find the single wire coming out of the Garwin cluster. I don’t know why the shop switched to the spark plug style probe. 

JPI also no longer makes the piggy back style probe for their units. 

Just some additional notes while we throw darts. 
 

-Matt
 

 

[Mellow_Mooney]

  On 3/23/2025 at 6:11 PM, 47U said:

The part number for the correct CHT probe will be found in your wiring schematic.    

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I'll take a look at my manual tomorrow. It's a 67 M20F so those manuals don't really tell you much... I think mine is only 30 pages and I've gone through it multiple times. I don't remember a wiring schematic. 

[Mellow_Mooney]

  On 3/23/2025 at 11:03 PM, MB65E said:

I was helping Mellow with this. Both the spark plug style probes have 2 wires.

Original probe was only a single wire using the probe itself as the ground.

So we’d need to find the single wire coming out of the Garwin cluster. I don’t know why the shop switched to the spark plug style probe. 

JPI also no longer makes the piggy back style probe for their units. 

Just some additional notes while we throw darts. 
 

-Matt
 

 

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Thanks Matt!

[1967 427]

  On 3/23/2025 at 10:52 PM, Mellow_Mooney said:

We've tried both the K type (JPI) and J type probes (Insight) and neither of them worked. Both had the exact same result.

Right now we've left the J type in there. Is that the correct one to use?

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Don’t have manual for 67F, but according to my manual the correct pn for a 67E is AN5546-1

[1967 427]

Just found the page with 67F schematic and it’s the same pn.

[Fly Boomer]

  On 3/23/2025 at 11:13 PM, Mellow_Mooney said:

I'll take a look at my manual tomorrow. It's a 67 M20F so those manuals don't really tell you much... I think mine is only 30 pages and I've gone through it multiple times. I don't remember a wiring schematic. 

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This is all I have.  I didn't look at it.

IPC 67 F Schematic.pdfFetching info...

 

[PT20J]

  On 3/23/2025 at 11:03 PM, MB65E said:

JPI also no longer makes the piggy back style probe for their units. 

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Is this the one you mean? It's still on the JPI website. https://www.jpinstruments.com/shop/m113-38-cht-gasket-probe/

[Vance Harral]

The OEM CHT probe on that vintage of Mooney is not a thermocouple.  It's a thermistor, which works on a different principle, and uses the engine itself as the ground connection (hence why there is only one wire to the probe).  CHT probes from JPI and Insight are 2-wire thermocouples, they simply don't work with the original CHT gauge in the Garwin cluster.  That's the whole reason "piggyback" thermocouple probes are used when a non-primary engine monitor is installed in an aircraft which needs to retain the OEM CHT probe.  It allows the original thermistor to stay installed to drive the original gauge.

This is the CHT probe that fits our 1976 M20F: https://www.aircraftspruce.com/catalog/inpages/rochcht.php.  Older M20F models may use the same one, but double-check your parts manual

[MB65E]

  On 3/24/2025 at 12:00 AM, PT20J said:

Is this the one you mean? It's still on the JPI website. https://www.jpinstruments.com/shop/m113-38-cht-gasket-probe/

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No that’s the spark plug probe. For the piggyback probe to work, it’s installed into the cylinder probe location to run the JPI, then the Factor probe slips into the piggy back fitting. 
My understanding is the shop put the EDM-700 in and kiboshed the Garwin/Rochester probe. I just need to trace and find the single wire that was omitted. At least that’s how my airplane is wired. 

Learning things,

Matt

[PT20J]

  On 3/24/2025 at 4:00 AM, MB65E said:

No that’s the spark plug probe. For the piggyback probe to work, it’s installed into the cylinder probe location to run the JPI, then the Factor probe slips into the piggy back fitting. 
My understanding is the shop put the EDM-700 in and kiboshed the Garwin/Rochester probe. I just need to trace and find the single wire that was omitted. At least that’s how my airplane is wired. 

Learning things,

Matt

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That's the 3/8" piggy back probe. It goes under the factory probe.

This is the spark plug probe. https://www.jpinstruments.com/shop/cht-gasket-probe/

[PT20J]

My experience on a Lycoming IO-360 is that compare to a bayonet probe, the spark plug gasket probes read about 20 deg F high (because the spark plug is the hottest part of the head) and the piggy back probes read about 20 deg low (because they are reading surface temperature rather than the temperature in the well).

[MB65E]

Hmm, I think the pictures don’t match or something. JPI said they no longer make their adaptor type CHT fitting for the cylinders. The wires would get into the threads when the Rochester probe was screwed in.
Thinking the Insight 2856 probe with the Rochester 3080-38 should work to get the Garwin working again. Just need to find the single wire. From the Garwin cluster. 

Any other thoughts?
-Matt

[47U]

  On 3/23/2025 at 11:13 PM, Mellow_Mooney said:

I'll take a look at my manual tomorrow. It's a 67 M20F so those manuals don't really tell you much... I think mine is only 30 pages and I've gone through it multiple times. I don't remember a wiring schematic. 

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@1967 427 called it.  Manual 106 lists the CHT probe as pn AN5546-1.  This is the mx manual, not the IPC.  @Vance Harral is correct, this probe is a thermistor, but the ‘67F uses a different probe than his ‘76F.

The AN5546-1 probe is secure into a bayonet fitting that is screwed into the cylinder head.  If a JPI or E.I. engine analyzer is installed that is not certified as primary, then a ring CHT probe is used in conjunction with the bayonet fitting and the factory CHT probe is retained to be legal.    

This is a picture of an Alcor CHT probe, equivalent to the AN5546-1, from Aircraft Spruce.  It’s a two-wire probe.

This is a picture of an E.I. ring CHT probe installed under the factory bayonet CHT probe fitting.  

On my ‘63C, I wrapped the factory bayonet and E.I. ring probe together in the fire sleeve and think I get a much more accurate reading.

 

[Mellow_Mooney]

Great information here guys, thank you.

 

I'll get with Matt soon and see if we can get it solved. Will report back.