Holley old wall gas furnace

Twice in the last two weeks, the pilot has blown out in our wall furnace. We are having windy weather. When relighting pilot, I press the transfer button on the relay, normally, after 15 seconds, the gas valve relay will latch on allowing me to release the button and pilot remains lighted.
What is different now, AND NOT TYPICAL, is NOW, when I relight the pilot, and hold button in for 2 minutes or longer, releasing the button allows the pilot to turn-off, the gas valve relay did not latch-on. Thwe thermocouple junction is being lighted by the pilot flame and area is free of dust.
The only way to cause the relay to latch-on and keep pilot lighted, is to hit the valve-relay body several times with the handle of a plastic screw driver.Recently not-latching has happened twice.
What could be wrong? Is the thermopile [thermocouple] bad or is the gas valve-relay bad? Or is something else bad?
Thanks for your thoughts. Dave_s
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Dave_s wrote:

One or the other. Probably the thermocouple.
Richard Perry
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The general rule it to replace the easiest and cheapest part first. The thermocouple is it. Before you replace it you might try disconnecting and reconnecting it at the valve. This will sometimes clean a poor connection and solve the problem. Don Young

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Don Young wrote:

And that would be from the "Rules of Hackery" guide? :)

Maybe 1 out of 100,000 times loosening and re-tightening will correct the problem. In over 20 years I've never once encountered a system on which this would have accomplished anything. I have heard others say they had gotten a unit to work by "re-tightening", but invariably the customers called back. IOW, the pilot may have held on that try even if they hadn't touched the thermocouple. When the mV output is marginal this is a common mistake among technician and homeowners alike. It's just coincidence. The general rule is that if the connection was clean when the thermocouple was installed, which would be indicated by the thing actually working like it should afterward, then dirt can't accumulate between the metal faces of the junction because it has no way to get there. If on the other hand the connection is loose, then simply tightening it can sometimes be the cure, and this isn't the same as re-tightening. If it was already tight, then re-tightening isn't going to cure a thing, though you can fool yourself into believing that it did, that is, if you let yourself. Then again this is all immaterial, I prefer to simply check the mV output rather than play guessing games and/or engage in parts swapping.
The other day my seat belt light was irritating me, so I reached down and pressed the latch button. The light went off at the exact same time. I thought hey, you don't have to insert the clasp into the latch to turn off the warning light. Next time I tried this it didn't work. It was just a coincidence first time, but it was a very convincing illusion.
Richard Perry

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RP wrote:

SNIP...............
    Good thermocouples read how many millivolts [mV]?     MArginal thermocouples read how many millivolts [mV]? It's

...........SNIP..........
    I have good Digital Multimeters.     How many millivolts [mV] should a good working thermocouple develop?
    Dave_S
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On my old in_the_wall HOLLEY Gas Heater, how can I test if the thermocouple is OK? I have good multimeters. Do I just disconnect the thermocouple from the gas-valve and measure the voltage when the thermocouple is hot and also is cold? What should the voltage be when hot and what when cold? Thanks Dave_s
Dave_s wrote:

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