THSCC Forums

All of a sudden my EGT gauge quit working. It has a thermocouple in the number one exhaust header pipe that goes to the guage. It is not 12 volt powered, and has two wires ,so it must be some heat/energy thing in the probe. How can I tell if it's the probe or the guage? Charlie, just chime in anytime. Thanks,All
Chris

It may be a thermistor: resistance changes due to temperature. If two leads then maybe it's hooked up to something measuring the resultant change in current, sensed elsewhere and added into whatever supporting logic there is. I know jack about cars, though, so I don't know the intended use and may be off

Do you have an ohmeter? If so, one thing to try is to look for some resistance value across those two leads. I'd say it's a very small value if it's connected to current sense circuitry. If this part is bad it may read an open circuit.

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Beauty is in the eye of the beer holder.

It is probably a thermocouple. It is two dissimilar metals welded together.
When heat is applied a weak voltage is generated proportional to the difference in temp at the hot end - the cold end (the meter). It is probably a type K which I do not recall the two metals envolved. I used to make one by twisting the two leads of type K wire together and doing a crude hammer weld on the twisted end. This is likely to be encased in a metal sleeve packed with a ceramic material to protect the thermocouple from the corrosive environment in the gas stream.

If it is bad it will look like an open circuit at room temp. If it is good it should have a very low resistance.

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

I had to check to be sure, but in a nutshell, with a cool example, here's what a thermocouple is and does:
Thermocouples take advantage of an electrical effect that occurs at junctions between different metals. For example, take two iron wires and one copper wire. Twist one end of the copper wire and one end of one of the iron wires together. Do the same with the other end of the copper wire and the other iron wire. If you heat one of the twisted junctions (perhaps with a match) and attach the two free ends to a volt meter, you will be able to measure a voltage. Similarly, if you hook the two iron wires to a battery, one junction will get hot and the other will get cold!

Courtesy of "How Stuff Works" a Cary-based website.

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Brad

Brad,
Did you read my post? I did not mention the heating and cooling effect of the two joints. BTW that is how electric ice chests work. Except they are using semiconductors as the heat transfer medium.

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George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

Peltier coolers

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www.ncvw.org
1993 VW Corrado VR6 - DSP15 (RIP)
1986 MB 190E 2.3L 16v - STX (SOLD)
1997 BMW M3 3.2L - BS for now

thanks for the input. I turned out to be a chafed wire that I missed and the probe has been replaced. You guys are good!!
Chris