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

[nditiz1]

I'm guessing you didn't have a setup similar to the dakota digital? That thermocoupler has disconnects already built in. With that in mind I'm surprised your setup would be much different. I don't know much about the inner workings of thermo dynamics just that heat is transferred via these two wires to the gauge. I guess if the metals were dissimilar in your connection points that would cause a discrepancy. Good to know though about other types of CHTs.

[VaccaRabite]

I'm guessing you didn't have a setup similar to the dakota digital? That thermocoupler has disconnects already built in. With that in mind I'm surprised your setup would be much different. I don't know much about the inner workings of thermo dynamics just that heat is transferred via these two wires to the gauge. I guess if the metals were dissimilar in your connection points that would cause a discrepancy. Good to know though about other types of CHTs.

It is a Dakota Digital actually. But the screws are under the tins. In the past I've always just fished everything out after pulling the #3 spark plug.

The thermocouples only produce millivolts of voltage.

You should be able to put a connector in series with the thermocouple as long as the connector doesn't add any resistance. It's only the tip of the thermocouple which produces the voltage. If you jiggle (official EE term) the connector while running does the gauge move around?

I'll have to see. Easy enough test to do tonight after work. Driving over bumpy roads though did not seem to have an effect.

Zach

[worn]

Getting the spark plug and ring out has always seemed such a major PITA. If you come up with a disconnect solution it will be a service to us all.

[stownsen914]

Well that makes the Omega plugs looks like a better option (IMG:style_emoticons/default/smile.gif)

One other thing - I have a VDO CHT that's missing the harness and thermocouple. It has what appear to be standard male blade connectors at the gauge. I'm not seeing chromel or alumel female connectors. I guess using copper ones would create an uncompensated junction (if that's the correct term)?

This is making the $60 VDO thermocouple and wiring kit more appealing haha

[Superhawk996]

Well that makes the Omega plugs looks like a better option (IMG:style_emoticons/default/smile.gif)

One other thing - I have a VDO CHT that's missing the harness and thermocouple. It has what appear to be standard male blade connectors at the gauge. I'm not seeing chromel or alumel female connectors. I guess using copper ones would create an uncompensated junction (if that's the correct term)?

This is making the $60 VDO thermocouple and wiring kit more appealing haha

If I recall the VDO did use plain copper female spade at the gauge. Yes connection to the back of the gauge that is considered the uncompensated cold junction for VDO so that transition from Chromel / Alumel to copper right at the gauge “should” be part of their gauge face calibration.

It’s been a long time since I had a VDO gauge in my hand but in all honesty - it serves a purpose which is to get general trends in cyclinder head temp without fretting over 20F one way or the other due to gauge error.

You’re right though - $60 for an extension, sensing end, and connectors isn’t outrageous in this day vs. sourcing all the bits yourself.