Why it took me and hour to change a thermocouple

**addamsmasher** · 03-21-2008, 11:50 PM

Ran into something new today. Got a call that a pilot wouldn't stay lit on a furnace. First thing I thought was thermocouple. I ended up being right but it wasn't as easy as I thought to get to that conclusion. The thermocouple didn't go back directly into the gas valve, instead it was intersected by this small rectanglular device that was wired in series to the limit switch I guess to prevent the pilot from lighting if the limit were to open. Once I figured this out I tested the limit to make sure its wasn't open before replacing the t.couple. Anyone know what that litte f***ing thing is called? Also can anyone explain why on some carrier furnaces (and maybe others) one of the wires coming from the limit has a resitor? I know its so you can't jump the limit out, I learned that by blowing a fuse on the board.

**Iceman75** · 03-21-2008, 11:58 PM

Its called a junction block, and it purpose is to extiguish the pilot flame in the event of a limit opening its contacts,most supply house stock them,havent seen one in a while,but been on a few calls where the junction block was bad and not the TC,hope it helps!

**addamsmasher** · 03-22-2008, 12:01 AM

thanks so whats inside that junction block just a set of NC contacts that simply opens w/limit? what about resistor on limit wire?

**mrteboe** · 03-22-2008, 12:02 AM

The box you are talking about is the pilot safty shut off if the pilot goes out it will shut off power to the gas valve this is not as we say 100% shut off system the pilot is usally feed right off gas line. I belive the resister u are talking about is the thermal safty shut off if the heat or flames come out the front of the furnace the thermal link will shut off power to the gas valve just like the limit would

**beenthere** · 03-22-2008, 12:06 AM

The junction block is just that, a junction. It doesn't have any moving, or temp sensitive parts in it.

The contacts are open. The wires going to the limit, is for ECO(Emergency Cut Off), incase the water temp starts to get too hot.

The resistor you speak of, is a fusable link. Blows if it gets too hot.

**addamsmasher** · 03-22-2008, 12:09 AM

how come then when I tied the two limit wires together and tried to start the furnace as soon as I guess 24v ran through the limit it blew the 5A fuse on the circuit board? I'm pretty sure that resistor prohibits you from jumping out the limit, maybe it also serves as a fusible link too