The tube is filled with a fluid. Some say ether, others say alcohol. Whichever it is, the increase of temperature within the sealed tube causes the liquid to expand. This expansion drives a bellows inside the gauge changing the reading. Much as a coiled spring thermometer operates.

There are a couple of write-ups on the forum regarding repairing them but I've never known anyone personally who had any luck fixing them.

I might be wrong but I believe all of the W114's/W115's used this style. Probably all the '60's-'72 Benz. W107, -8, -9, etc.

The only difference I have seen was in the length of the tubes.

The one in my W114 came from a '67 W115.

Drop Ron (Melted Panda) a line. He's probably got a couple taking up space in his shop.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now