Defrost cycle


Old 11-20-02, 07:49 AM
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Question Defrost cycle

My 10 year old Amana 3 ton heat pump has a problem that appears to be in the defrost cycle. The unit works/heats well until the defrost cycle is initiated/activated. The original setting for the defrost timer/cycle was at 90 minutes. The problem begins once the defrost cycle is initiated. Within the span of a couple seconds from when the defrost relay switches, the reversing valve clicks and the strain on the compressor is obvious, however the outside fan does not shut off, and the defrost relay switches back and this cycle is repeated – permanently. The only way to end this constant switching of the relay (at ~ 2 second intervals) is to pull the “selector wire” (I’ve called it the selector line, but I doubt that is the correct term. I’m referring to the line that lets you choose between 30, 60, and 90 minute defrost cycle settings.) Pulling this ‘selector’ seems to reset it, and the heat pump will work normal until the defrost cycle is initiated again. That may be hard to follow so let me try explaining it this way:

1) Normal heat pump operation
2) Defrost initiated, defrost relay switches
3) Reversing valve activates (to what position I’m not sure), split second compressor strain
4) Defrost aborted (after only a couple seconds), defrost relay switches back
5) Reversing valve activates (returns to heating position)
6) Back to 2

This problem occurred while I was away. My wife told me that the central unit was acting funny – funny was explained by her to mean the green light on the thermostat was blinking (at the ~ 2 second intervals) and the lights ‘blink’ at the same time. However the house stayed warm as normal.

The green light is the ‘auxiliary heat’ indicator. And the lights blink is due to the compressor strain. Note that since the compressor is already in operation the strain is, I guess, due to the switching of the reversing valve and for an instance the compressed fluid has nowhere to go. As I understand it, the auxiliary heat indicator comes on each time the heating strips are activated and that this occurs each time the defrost cycle is activated. Assuming the heat strips are being activated, this would help cause the lights to ‘blink’, but my ‘calibrated ear’ tells me the ‘blink’ in the lights corresponds to the strain on the compressor.

In my diagnostic attempts, I used the ‘test’ setting on the electronics card for the defrost cycle. By shorting the test pins as described in the owners manual, the defrost cycle is reduced by a factor of 256 (60 minutes becomes a little over 14 seconds). It is hard to tell much though, because the defrost cycle appears to be shortened by the same factor. Even though the defrost cycle is initiated correctly, it is terminated so quickly that I was able to gain nothing about it’s operation.

I’m sure the defrost relay (Aromat HL2-HTM-24VAC) has seen more than the expected lifetime of operation and I will replace it, but I don’t believe it is the problem. I don’t believe the reversing valve is the problem, it has a distinct or marked sound/click. Although I’m still leaning towards the problem being the electronics card, I don’t see anything obviously wrong/damaged on the electronics card (such as a burned component). It sounds suspiciously like a bad capacitor in some timing circuit on the electronics card, but it’s certainly plausible that the electronics card is doing exactly what it is supposed to do – that is initiating the defrost cycle and waiting on a reset from some other source before starting the cycle over. If so, from where would a reset come? And the defrost ‘test’ circuit appears fine (unless the defrost cycle is supposed to complete while in the ‘test’ mode). Any ideas/advice?
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Old 11-20-02, 12:27 PM
GregH's Avatar
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You really need to have the refrigeration system diagnosed before you can diagnose any electrical problem.

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