Hearthsone 8720 Pilot won't stay lit

[Tucson 20]

I'm working on a Hearthstone Tucson model 8720. The pilot light looks good, lights up fine and is hitting the thermopile and the thermocouple. I hold the pilot light button one for a full minute and a half. The pilot goes out as soon as I let off the button.
Following the troubleshooting guide for this model, I checked the voltage on the thermocouple: 23.6 mV. It only requires 15. I checked the thermopile voltage and got it up to 450 MV +. It says to check voltage between the valve body and solder connection on the rear of the valve, where the blue wire connects. It should be no less than 7 mV. I get 0 mV. Cause would be faulty connection. It attaches to a device, pictured in the second attachment (with two connections). The other wire was not plugged in to anything (third picture). I tried jumping the thermostat connections. I hear a click, but it does not change anything.

I changed the valve body out, and am getting the same results with all the tests. Reasonable to assume that the old one was fine. Next step it to change the thermocouple even though it tests out to spec. Would anyone have suggestions?

 

[DAKSY]

Change the thermocouple. Less than 27mV usually won’t hold the pilot electromagnets open. I’d be surprised if ANY unit worked at 15mV.

 

[Lennox65]

I'm working on a Hearthstone Tucson model 8720. The pilot light looks good, lights up fine and is hitting the thermopile and the thermocouple. I hold the pilot light button one for a full minute and a half. The pilot goes out as soon as I let off the button.
Following the troubleshooting guide for this model, I checked the voltage on the thermocouple: 23.6 mV. It only requires 15. I checked the thermopile voltage and got it up to 450 MV +. It says to check voltage between the valve body and solder connection on the rear of the valve, where the blue wire connects. It should be no less than 7 mV. I get 0 mV. Cause would be faulty connection. It attaches to a device, pictured in the second attachment (with two connections). The other wire was not plugged in to anything (third picture). I tried jumping the thermostat connections. I hear a click, but it does not change anything.

I changed the valve body out, and am getting the same results with all the tests. Reasonable to assume that the old one was fine. Next step it to change the thermocouple even though it tests out to spec. Would anyone have suggestions?

The third picture is the electrical connector that the thermocouple makes contact with. It should be slid into the place on the valve that the thermocouple threads into.

 

[Millbilly]

The third picture is the electrical connector that the thermocouple makes contact with. It should be slid into the place on the valve that the thermocouple threads into.

This is your problem.

 

[Tucson 20]

That disconnected wire was the cause. This isn't in the wiring diagram. The clue was in the new valve body I ordered. I noticed the blue wire that was soldered to it went to a spot that wasn't connected to anything, #4 in figure 9, labled as "Thermocouple connection". The old valve had the blue wire going to the device in picture 2 above (maybe a sensor?). And as I mentioned the black wire went nowhere, but I now know it went to the thermocouple before, plugged in at #13, "alternate TC connection". I tried it with the blue wire directly to the thermocouple, but the pilot would not stay lit, so that "sensor" is a necessary part of the circuit. I don't know what caused it to be disconnected. I placed it behind the thermocouple and the pilot will now stay lit, and the furnace operates normally. The new thermocouple has a collet that wouldn't thread into the hole like the old one, so it's held in with a collar. The furnace has a new thermocouple and a new valve. Thank you, all.

 

[Millbilly]

Interesting. I've never had a thermocouple not thread into a valve. Sometimes a pia

 

[Tucson 20]

It does thread into the valve, with the spade connection of the black wire behind it. I was referring to the other end, the pilot light assembly where it can be threaded in or pushed in. The thread of the nut that would hold it in didn't fit the thread of the hole in the assembly. Though it looks like it matches, it's slightly too large.