Service Mnaual

18
6a) Defrost
Early DT's were designed with 8-hr. interval and 20-min. defrost cycle.
Later DT's were designed with 6-hr. interval and 25-min. defrost cycle.
Manual advance in and out of defrost by turning the defrost dial is quickest way to
troubleshoot components.
Defrost Initiation: DT terminates. DH energizes. If CTh closed, Comp, ConFM,
EvapFM, PH, and MH de-energize.
Defrost Termination: DT terminates. DH de-energizes. If CTh closed, Comp, ConFM,
EvapFM, PH, and MH energize.
Diagnosis: Defrost Initiation: Manually advance DT cam to terminate DT. Conrm DH
energizes and Comp, ConFM, EvapFM, PH, and MH de-energize. If not, replace
DT. If Comp, ConFM, EvapFM, PH, and MH de-energize, check DH. Conrm DST is
closed. If not, let DH cool, then recheck DST. If DST is still open, replace DST. Check
DH amp draw and DH continuity.
Defrost Termination: Advance DT cam to terminate DT. If CTh closed, conrm Comp,
ConFM, EvapFM, PH, and MH energize. Conrm DH de-energizes. If not, check DT
voltage terminal #2 (R) to a neutral (W). If 115VAC is present, replace DT.
7) PH and MH Diagnosis: Check that PH and MH energize. Check for 115VAC at PH and
MH. If 115VAC is not present, check power supply. If 115VAC is present, check amp
draw of PH and MH. If an amp reading is not present, check the continuity of PH and
MH. If defective, replace PH or MH.
Legend: Comp–compressor; ConFM–condenser fan motor; CTh–cabinet thermostat;
DH–defrostheater; DST–defrost safety thermostat; DT–defrost timer;
EvapFM–evaporator fan motor; HPS–high-pressure switch; MH–mullion heater;
PH–perimeterheater; TXV–thermostatic expansion valve