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

[Literati914]

... I had suspected the VDO gauge but as it turned out the VDO gauge performed well when coupled, no pun intended, to the Dakota sender.

Wait are you saying that the analog VDO gauge reads correctly (it’s got a reputation for being off) when using the Dakota thermocoupling wire & ring? Meaning that the wire is actually the compensated part of the puzzle? Or are you just saying that it fires up and “works” in general? I was wondering this a few weeks ago when I purchased the complete dakota setup while really wanting the VDO to be a more viable solution. Sorry to be so dense, but can you confirm?


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[Superhawk996]

... I had suspected the VDO gauge but as it turned out the VDO gauge performed well when coupled, no pun intended, to the Dakota sender.

Wait are you saying that the analog VDO gauge reads correctly (it’s got a reputation for being off) when using the Dakota thermocoupling wire & ring? Meaning that the wire is actually the compensated part of the puzzle? Or are you just saying that it fires up and “works” in general? I was wondering this a few weeks ago when I purchased the complete dakota setup while really wanting the VDO to be a more viable solution. Sorry to be so dense, but can you confirm?


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Both use the same K-type thermocouple sensor.


VDO is an uncompensated cold junction - its just basically a milli-volt meter with a gauge face that is calibrated to cylinder head temperature. It assumes the cold junction ambient temp is 68F (maybe 70F??) if I recall. This is why VDO has more error.

Dakota Digital is compensated cold junction and is more accurate. Internally, it measures the temperature of the cold junction and applies a correction to the displayed value based on the ambient temperature at the gauge (i.e. the cold junction).