M20J cht and landing light wire

[Grant_Waite]

My Factory CHT gauge stopped working and I found the culprit today. The wire is frayed a bit where the connector is. I have a jpi830 so I don’t look at the factory one. I just want it to work is all. I got it working today when I moved the wire and put some Acf 50 on it. I would like to know what kind of wire I should get to fix it. What would be the best method of fixing it?  I also would like to replace my spade connectors on my landing light with one of these.  https://www.aircraftspruce.com/catalog/elpages/sealAllconnectors9.php?clickkey=42540 

I already have the connector but I believe my positive wire running into the firewall is too short. The ground looks long enough. The wire that run into the firewall are the fiberglass high heat wires. The other ones running along my cowl and to the light are just standard white one. I would like to know what size and type of wire I would need to extend the positive. My A&P put an amp meter on the light and it only drew 1.8 amps. So what size wire should I get? Preferably the same size as what I have. I’m not sure if it’s  18gauge wire or what. 

[Vance Harral]

The OEM CHT probe is a thermistor, not a thermocouple like modern engine monitors use.  The good news about this is a thermistor just needs conventional aircraft-grade Tefzel wire, not mono-strand thermocouple wire.  You can use the same type of wire for both the CHT probe and your landing light, though perhaps not the same gauge.  Something like this should be fine: https://www.steinair.com/product/18-ga-white-mil-spec-wire/

Regarding gauge, the critical factor is to ensure the current carrying capacity of the wire is greater than the circuit breaker which protects it.  The whole point of the circuit breaker is to ensure the wiring in the circuit doesn't overheat and catch fire.  Here's a current capacity chart for Tefzel wire: https://www.prowireusa.com/tefzel-amperage-chart

[PT20J]

The schematics in the service manual will show the correct wire gauge.

[Grant_Waite]

9 hours ago, PT20J said:

The schematics in the service manual will show the correct wire gauge.

I see sections talking about the cht and landing light wire, but no schematics are given for either of them

Edited May 31, 2024 by Grant_Waite

[Grant_Waite]

10 hours ago, Vance Harral said:

The OEM CHT probe is a thermistor, not a thermocouple like modern engine monitors use.  The good news about this is a thermistor just needs conventional aircraft-grade Tefzel wire, not mono-strand thermocouple wire.  You can use the same type of wire for both the CHT probe and your landing light, though perhaps not the same gauge.  Something like this should be fine: https://www.steinair.com/product/18-ga-white-mil-spec-wire/

Regarding gauge, the critical factor is to ensure the current carrying capacity of the wire is greater than the circuit breaker which protects it.  The whole point of the circuit breaker is to ensure the wiring in the circuit doesn't overheat and catch fire.  Here's a current capacity chart for Tefzel wire: https://www.prowireusa.com/tefzel-amperage-chart

Do you by chance know the part number or where I can get a new connector for the CHT wire? It appears to have seen better days. I’ve seen others talk about how they couldn’t believe, how small that nut is that holds it on. Would cutting, then soldering a new piece of wire on be an acceptable solution for both the CHT and landing light? It would be a real pia to replace the whole length of each wire. 

[PT20J]

6 minutes ago, Grant_Waite said:

I see sections talking about the cht and landing light wire, but no schematics are given for either of them

The service manual comprises two pdf volumes. The second volume has the schematics.

[Grant_Waite]

32 minutes ago, PT20J said:

The service manual comprises two pdf volumes. The second volume has the schematics.

Is this the information I’m looking for? Not sure how to decode the numbers. This is the first time I’ve seen these so it all looks like hieroglyphics to me.  

[PT20J]

The code is described in chapter 91 of vol 1 of the service manual. The last two digits are the wire size, so the landing light is 12 AWG. Be sure to use Tefzel insulated wire, and if you splice it use PIDG butt splices with the proper crimp tool or solder sleeves.

[LANCECASPER]

On 5/30/2024 at 5:18 PM, Grant_Waite said:

My Factory CHT gauge stopped working and I found the culprit today. The wire is frayed a bit where the connector is. I have a jpi830 so I don’t look at the factory one. I just want it to work is all.

Just remember that the JPI 830 is only for advisory, not for a "back-up" instrument. The ship's CHT gauge is the official number and the one that needs to be working to be legal for take-off.

[Vance Harral]

17 hours ago, Grant_Waite said:

Would cutting, then soldering a new piece of wire on be an acceptable solution for both the CHT and landing light? It would be a real pia to replace the whole length of each wire.

Splices are acceptable, but soldering them is contrary to the guidance in AC 43-13.  The theory is that solder joints are brittle and likely to break, crimp splices less so.  I'm not saying I necessarily subscribe to the theory, because I've had plenty of trouble with crimps.  But that's the conventional wisdom, and the reason @PT20J - who I greatly respect for his practical experience - is directing you to use aircraft-grade butt splices.

More importantly, you should generally be following the guidance in AC 43-13 as opposed to asking Some Guy On The Internet.  11-97 recommends replacing wire that has splices at less than 10-foot intervals unless for a specific reason, so yes it recommends replacing the whole wire even though it's a PIA and lots of mechanics ignore the advice in practice.  The whole of Section 13 discusses splicing itself, where acceptable.  It says there shouldn't be more than one splice in any single wire segment except in special circumstances.  Section 13 also has guidance on acceptable lugs, including the tiny ring terminal on the CHT probe, which really is designed in accordance with industry guidance.

FYI, other sections of AC 43-13 provide specific guidance to use "solderless" connectors, and discuss crimping extensively.  The only places in AC 43-13 that mention soldering technique are in the context of pins that are mechanically supported in connector assemblies, and - interestingly - the use of solder in mechanical work rather than electrical.