Furnace- Suburban NT-30k disassembly

groundhogy wrote:
Ok, so just to tie up the end of this thread for those who search them...

To open this unit up, there is only one tiny unremarkable sheet metal screw. The only one
that penetrates the enclosure. Remove. Push hard on the inside guts from the back toward
the front while holding still the enclosure.

Testing the gas relay. I wired it up separately and touched the wires to my car battery
to hear the relay click. Then I blew on it with my mouth and observed that the
airflow was cut off and on. On mine, the brown was +12, yellow was ground.

Testing the main control board. Looking at the schematic... you will need totally to
download the manual for this unit. Its on line and in pdf format. Schematic in there.
Put +12 on the red wire, gnd on the yellow. These are the wires that hang out of the
furnace. this gives it power supply. You will hear nothing. then cross or short the
blue wires together. this simulates the call from the thermostat to give heat. I pulled
one set of the brown and yellow wires from the gas relay and put the voltmeter on
there to watch for the call from the control board to the gas relay to turn on.
Nothing happened. (saw some high voltage activity on dvm when sparker came
on). should see a 0 to +12v transition.

sail switch was good. limit switch (goes OC if total furnace gets too hot) was good.

will change out control board.

while you have it apart and before you order the board make sure you've fully eliminated the sail switch. I know I'm a broke record but on both of the switches I had to replace it wasn't because the switch itself was bad. It was because the "sail" wasn't closing and closing the circuit. The arms had slop in them and on this last one it was letting it drag on the side of the box and keeping it from closing. Like I say....not one to beat a dead horse but I would manually close that switch and make sure that's not what your issue is.