With nothing to loose Nate and I played around some, not knowing much about these type thermostats or air handlers and heat pumps. We decided to remove the suspect controller page assembly which involved labeling about 12 wires and pulling a 10 wire edge connector. There is no schematic that I could find on the page assembly but at least many of the input output wires were labeled on the board which is a big help.
I brought the page assembly home and examined it with high power glasses and a magnifying glass. Wow, lots of corrosion. It must be getting some moisture from the heat pump indoor coils which are located above the board. Then the edge connector contacts on the board were extremely corroded.
So I pulled out the dremmel tool and polished all the edge connector contacts until they were bright and shinny. I noticed the 3rd lead over was labeled ‘fan’ and the lead was 90% gone, the worst of all the leads by far. And since it is the fan that did not start I found a place on the board to solder a wire to the same trace and knew I could bypass this connector with this hardwire. Also on the board the capacitors had a lot of corrosion on the ends also. The board was heavily shellacked and the shellac had turned very dark so I could not read all the numbers on the two IC chips. One chip started with 68HC which is likely a type of microprocessor I am familiar with. But microprocessors are hard to work with if you don’t have the code or at least the flow chart. Fortunately they seldom fail.
Well we reconnected the board and jumpered the ‘fan’ lead to the yellow wire on the thermostat cabling. Now for the first time in a while the fan started up on the air handler when the fan switch was energized on the thermostat. Unfortunately the 5 amp fuse would blow when either heat or cooling was turned on!
So I brought the board back home and replaced 3 corroded capacitors. On one I did not have a matching size so instead of a 33 microfarad 50 volt capacitor, I installed a 10 microfarad 50 volt in parallel with a 22 microfarad 35 volt. (With the board installed I measured the voltage across these capacitors at 34 volts which is awful close to the voltage rating.)
I further observed two places that had turned black indicating electrical arching on the board. These burnt spots were at the relay contact traces. The board traces left only 1/8 inch between traces and there is 240 vac on these tracts. I was able to remove all the blackened board using a dental pick. It amazes me if these were really caused by current leakage or arching that the board did not blow up and completely destroy itself. I also washed much of the contamination off the board using alcohol.
Eventually I realized the ‘fan’ lead on the edge connector was not used on this installation so I removed my clever jumper, replaced the fuse and walla all seems to work now. The fan comes on and heat and cool functions work. We have not run the heat pump for an extended time yet though.
It is interesting that when one turns up a thermostat It sometimes can take like 10 minutes for the outside unit to start the big fan or the compressor. Not knowing the micro-controller programming leaves one in the dark why this happens.
I ordered all new capacitors of the right farad and voltage rating from Digikey and should get them in maybe 3 days. They cost like 50 cents apiece. So if needed we can quickly install some new capacitors. The ones I put in were used and could easily be 20 years old.
Maybe we should consider putting a metal shield above this control board to divert any water droplets away from this board. The system is about 18 years old.
Bottom line - The main problem with the system was the corroded leads on the edge connector making no contact. After polishing they are likely almost as good as new.
The controller board
The edge connector is bottom left, there is no contact material on this side.
This is before cleaning.
This is the edge connector before cleaning.
The edge connector after cleaning and polishing.
Notice the 3rd tab from left is missing most of the conductive metal.
Some of the corrosion before cleaning.