Test the timer for continuity using a multitester. Set the multitester to the ohms setting X1. The timer has four terminals. Locate the common terminal, it should be labeled "3" or "C". If the terminals are not labeled, determine which terminal coincides with the white wire in the connector plug.
Once you have located the common terminal, touch one probe to it. Touch the other probe to each of the three remaining terminals. The multitester should display a reading of zero or near to zero ohms (which indicates continuity) for one pair of the terminals and possibly two pairs. The third pair of terminals should show no continuity (infinity).
Locate the timer switch and turn it clockwise until you hear it click. Now retest the timer as you did above. One pair of terminals should indicate continuity (possibly two pairs). At least one pair should give a reading of infinity. Note however, one of the pairs that showed continuity in the first test should now read infinity and one pair that read infinity should now show continuity. If the defrost timer does not pass these tests, it is likely that it should be replaced.
* We have received two reports of cases in which only one pair of terminals shows continuity and when the timer is advanced, the same pair still shows continuity. For these isolated cases, this was reported to be the correct operation of the timer. At this time we have not been able verify this case. This may indicate a specific model or models that use a different wiring configuration than discussed in this article. Also, note that this article applies to mechanical defrost timer controls and not electronic or adaptive defrost controls.