Dometic/Atwood Furnace AFMD30... grrr cntl board

Check the style of high voltage lead. That is main difference. Next is a resistor that is cut in some application that delays ignition. For testing, you would have to wait for 20-30 seconds before ignition sequence.

Yes. The one in the water heater was harvested from my last Suburban furnace.
I had to cut that resistor lead when i installed it in the water heater.

If you are talking about the 2 red wires with spades on the board? 1 Wire is smaller gauge than the other?. That is the connection for the Fan motor. It does not really supply voltage to activate the board and LED. The side board quick connect is what powers the board. The BLUE wire should have 12 volts when the Tstat closes. Check for that and then see if the LED light blinks or comes on. Doug

Dougrainer:
Ok will look at that.

Also I called Dinosar. He also said the12v on the separate wire is to power the fan.
He agreed that the PC board drawing down the 12v without some part on the board shows high resistance in that line. I will also check the 12v fuse box for corrosion.

SOLVED:

It was the thermostat.
Now.. one of the first things I did was bypass the Tstat by shorting those two wires together. So..??

But here's what pointed me back at the thermostat.

I drove the truck around next to the furnace. Got out the jumper cables.
I injected my own fake 12v call signal to the control board.
Saw a spark and the fan popped on.
I removed the fake 12v. Furnace left blower on for about 1 minute and back off.

So I pulled the Tstat (2-wire plus some anticipator thing that I didn't think I used.) and started trying to figure out this chinese puzzle of little contacts and plates.

Turns out the antipator is in the circuit.
The anticipator looks like a little round circle of (thermal?) wire with a little arm that you can put at any position around the circle to create different ... anticipation times.

This little arm needs to be ON the wire circle. NOT in the parked neutral position.

Somehow the little arm ... maybe vibration?...had gotten to the parked position.

The continuity back to the furnace seems to go through the anticipator.

And how this Tstat problem decided to give me BS EXACTLY at the same time I was cleaning lint out of the air slats.. IDK

groundhogy wrote:
And how this Tstat problem decided to give me BS EXACTLY at the same time I was cleaning lint out of the air slats.. IDK

What usually happens is you overload or short the tstat 2 wires to that metal tstat. It BURNS out the anticipator and then the tstat does not work. Did you also blow out the tstat itself and that could have moved the tab for the anticipator? Doug