Thermocouple? like I've never seen.

Old 01-28-17, 03:06 PM
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Thermocouple? like I've never seen.

I'm fixing a Coleman furnace, circa 1970, model 4023 with precious little information available. Everything is clean and working, and it bench tests fine. However, this thermocouple isn't holding, and it's not like any I've ever seen in my (limited) experience.

First, the reading end of the thermocouple cannot be unscrewed - it's molded into the gas valve assembly. Second, the operation that holds the pilot valve open is *mechanical*. Meaning, once the thermocouple heats up, a little plunger expands and physically holds the red button/pilot valve open.

What is this type of assembly called? It will stay open and keep the pilot lit, but when the burner fires, I guess there's enough wind that the voltage drops a tiny bit and it lets go and shuts everything down.

I'm sure I just need to replace this ancient "themocouple" with something similar, but I have no idea what to search for since I don't know what this sort of mechanical setup is called. There don't seem to be any OEM parts available any more for this furnace.

I've seen similar things on autos, where heat expands wax and pushes a plunger. Seems like something similar.

Any ideas? Thanks!
Old 01-28-17, 03:22 PM
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I sure would like to see some pic of this.
Old 01-28-17, 04:02 PM
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Welcome to the forums.

A 1970's furnace. ..... and a Coleman at that. Hopefully you've had the unit professionally serviced as the heat exchangers in them don't last forever.

This is propane and in a motorhome.... correct ?
The numbers after the 4023-xxxx are very important.

The thermocouple on a 4023-5871 is replaceable and outputs 18-30 millivolts.
With that vintage there are several different pilot methods. You probably have the hydraulic mercury element.

click pic for large view.

Also check and see if this is your furnace instead.............
bryant rv/docs2/temp/coleman.pdf
Old 01-28-17, 07:53 PM
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Pete, Thanks! I had no idea it was a mercury element - hopefully that will help me track down a part or solution.

FWIW, it's a 4023-889, and thanks also for the PDF - that reads like a perfect match.

If I get a chance tomorrow I'll post a pic.

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