Engine/Transmission (1998.5 - 2002) Cold Junction

[plongson]

Now here is a topic that hasn't been discussed. Just installed the FP, boost and pyro gauges... The instructions for the pyro talk about "Cold Junction" and went on to describe this condition (error) as normal on self powered thermocouple devices. This is all well and good if the connections can be made inside the cab, where it is cool, otherwise it seem to me that the ambient tempreature in the engine compartment will give a (seriously) false reading. My connections were about 8" short of being inside the cab. They state that absloute thermocouple temps will be off by 1*for every degree above the calibration temp of 75*... making the gauge lower than the actual temp. (confused yet???) SO... at 100* at the connection and 1000* at the thermocouple, the gauge reads 975*. Here in Southern Nevada the summer temps can be 120* and under the hood I would guess 200*. That is 125* error... making an indicated 1250 at the gauge really 1375 at the pre-turbo thermocouple. This has me worried... any thoughts...



:--) :--)

 

[mgoncalves]

I'll take a shot at this one. I think how a k-type thermocouple works is it reads the difference between the probe in your exhaust and a separate thermocouple in the gauge itself. What they are assuming is that the gauge will be in a controlled environment, say 70 degrees. If the temp at the gauge is different than that, say it's actually 100 degrees where the gauge is then your gauge will read 30 degrees cooler than what the exhaust temp really is.



I don't think the term "cold junction" has anything to do with the connections of your wires. I believe to have a true cold junction setup you have to have one thermocouple in the exhaust and one at a constant 0 degrees. Then all the gauge does is read the difference, just like a typical setup except you would have a controlled reading instead of the temp wherever your gauge is located.



For all I know I am completely wrong



Matt

 

[plongson]

AH-HA... . What you say makes great sense... The connection point has nothing to do with it... as I thought about it , it seemed strange that the connections COULD have any referance to the indicated temp. If the other half of the device is in a controlled enviroment (the gauge) then it all makes sense... . Thanks for helping me follow this through.....