Evcon/Coleman furnace acting strange

[MichaelWR]

Hi all;
I have an Evcon (or whoever controls Evcon now) AH12X15BA electric furnace. The problem is the furnace cycles randomly. Here is some background of what I have done so far; but first a bit of my background, I have been an electronics bench tech for many, many years and am familiar with soldering and PC board work and electricity/electronics in general so I felt comfortable working with it.

Initially the problem was a broken solder joint at the furnace element spade connection to and on the circuit board (part number 3612-320). This was repaired and strengthened both mechanically and for heat dissipation. This fixed the original issue, one element not functioning, low heat output. However now, some time later, the fan cycles on and off randomly. When it runs, it runs great; good heat, good air flow. The green status LED is on steady, and my prior soldering is fine. I suspected the fan control relay so I replaced it. No effect, same problem. I didn't think the high temperature limit switches were a problem because if they failed the green LED would indicate this and the elements would not fire up. I have not tested the motor because it seems to be strong when it is running. Granted, this is no test, but a gut feeling. Everything points at the PCB, a very expensive part I might add. I should note that I have tested all the obvious issues such as primary voltage presents and others such as loose wires and other cold joints. What is the consensus here on the board? Short of more testing on the motor, should I replace the board? What else could be at fault? The overall system seems to be pretty straight forward and I cannot think of much left but the board. I would really appreciate any input!

ForWhatItsWorth - My understanding of the board function: To control the sequence of the element activation. It steps the elements on one at a time so there won't be a huge spike in current draw at turn on. It also seems to watch dog over the elements should the high temperature limit switches are activated. It also allows the furnace to be controlled by the low voltage thermostat on the wall. A mystery is why there is two high temperature limit switches on the board?
Okay, long post but thanks for reading,
~Michael

[Robert]

Hi,

I have limited info on your AH Furnace it seems. If you have a wiring diagram, can you take a pic and send it so I can see what the circuitry is like.?


Thanks,
Robert

[MichaelWR]

Robert, the furnace is in the attic, which is a REAL pain to reach. I fear I may have to replace the board and was wondering your thoughts about my problem and do you have the board in stock. I see many replacements for the 3612-320 board, most call for a 331-01993-000 as a replacement board. My main concern is, how close it the replacement??? Close enough? Please let me know if you have a replacement.
~Michael

[Robert]

Hi Michael,

With it not being in a mobile home and it not showing in the online store, Mark will most likely have to order it in.

To save you time, call Mark at 605-229-2627 M thru F from 9 to 4 . He also repairs boards, so may can do that.


Thanks,
Robert