Well, I did a thing, It was a thing I knew better then to do, too. (Thermocouple connecting) Options

[VaccaRabite]

Continuing to live my life as an object lesson for others...

I recently had to pull my engine for some repairs. For me, part of pulling the engine includes removing the thermocouple from under the #3 spark plug and fishing it out of the engine tins. This runs to the CHT gauge in the cabin of the car.

And we all know you are not supposed to cut the wire. Everybody knows that.

So I figured I'd cut the wire, add a nice weatherproof automotive plug, and see what happens. All in the name of convenience and curiosity.

I drove 300 miles last weekend. Including going fast up a really long couple of hills (like miles long hills) in 5th gear.
So the results.... At cold start, the gauge reads the same as ambient temps.
Under normal driving, the CHT gauge read slightly cooler then before I added the plug, but within 10 degrees of what I expected.

However, when pushing the car, the error rate grew exponentially. Where I would expect the car to be running 340-350 head temps, the gage only showed about 300. And pushing 90 up hill in 5th for over a mile I would expect the gage to read 360+ the gage only ever got to 325. I did not push past what I know this engine required to get to 375 (which is my back off point).

The result of this experiment is that if I did not know and expect bogus returns on the gage, I could easily roast an engine and start dropping valve seats, while a quality CHT gage was reporting I was WELL under the 400 degree danger zone.

I have another cable I can swap in. Kind of a pain, but not a huge deal. I'll add it o the maintenance schedule.

But, yeah. In this case the group think is accurate. The very slight resistance that the plug will add to the cable will be enough to throw off your CHT monitor.

Do not cut the line between the thermocouple and your CHT gage.

Zach

[Superhawk996]

Free white paper for the day:

Cylinder head temp gauges are just a thermocouple gauge designed to read a specific type thermocouple. There are many different types of thermocouples available depending on what type of wire is used in its construction and what temperature range is being measured.

Dakota Digital uses K-type based on images of their sender and extension wire that I see online.

In K Type Thermocouple positive leg is composed of 90% nickel, 10%chromium and a negative leg is composed of 95% nickel, 2% aluminum, 2% manganese and 1% silicon. These are the most common general purpose thermocouple with a sensitivity of approx 41µV/°C.
https://tempsens.com/blog/k-type-thermocouple

Each type of thermocouple can be identified by its wire colors.

There are two different types of K-type wire that can be used to route to the gauge.

1) Thermocouple Grade wire -- this can be used to make the sensing end and could be used all the way to the gauge -- but it is more expensive wire so it is rarely used all the way to the gauge.

Thermocouple grade wire has a brown exterior insulation and for K-type the internal wires will be red (actually the negative wire -- UGH!) and a yellow wire (the positive wire)

2) Thermocouple Extension Grade wire -- this cannot be used to make the sensing end of the cable. Extension Grade wire can only be used to extend the sensor to the gauge or data acquisition rig. It's wire is made of material that closely mimics the sensing wire but is not exactly the same. It is however, vastly different than copper wire.

K-type Extension Grade wire will have a yellow exterior insulation. The internal conductors will be red and yellow.

Note: There are exceptions (aren't there always). The thermocouple wire can be obtained as single strands - without an exterior insulation holding both together. And if I recall my old VDO - it had wires that were covered with a braided material and had a non-standard color code. Likewise there are huge color code variations between the US and Europe regarding color coding and identification of wire type. If you look online you can find identification charts that cover all the wire types, as well as the US and International color coding.

Thermocouple extensions can be spliced.

However, they are spliced with a connector that mimics the behavior of the thermocouple sensing end and the extension cable. The connectors are specific to the type of thermocouple wire that is used for the senor / extension cable.

Here is an example of a K-type connector
https://www.farnell.com/datasheets/2339540.pdf

There are several different styles and sizes of connectors but each has to be matched to the thermocouple wire type.


Moral of the story: If you replace your standard wiring connector (copper / tin terminals) with the proper K-type thermocouple connector, you'll be back to a fully operational system and won't have to rewire the whole thing all the way back into the cabin.

[stownsen914]

What type of connector did you use? Since thermocouples/CHT setups are sensitive to small changes in resistance, you might just need to use a high quality connector like a Deutsch that's intended to minimize such issues. One of my cars has an old school EGT which uses thermocouples, and it's set up with Omega connectors that are intended for thermocouples like these.