Carrier 69NT40-561-019 Operation And Service Manual page 80

a. Probe Diagnostic Logic
In the perishable mode of operation, both pairs of supply and return probes are monitored for probe disagreement.
Probe disagreement is considered a difference of 0.5C (0.9F) or greater between the supply air sensors and/or a
difference of 2.0C (3.6F) between the return air sensors. Probe disagreement found in either pair can trigger a
defrost probe check.
In the frozen mode of operation, only the controlling probes are considered. Disagreement of the controlling probes
can trigger a defrost probe check, which will occur when the difference between the sensors are greater than 2.0C
(3.6F). Normally, the controlling probes are the return probes but if both return probes are invalidated, the supply
probes are used for control purposes. Probe disagreement of the non-controlling probe pair will not trigger a defrost
probe check.
If the supply probes agree and return probes agree, all supply and return sensors are valid and the unit returns to
normal control.
If supply probes disagree and the return probes agree, then invalidate the worst supply probe. If the probe check is
run as part of Pre-trip P-5, an alarm will be triggered for the invalidated probe. If it is a run time defrost probe check,
the invalidated probe will be passed over and no alarm will be triggered. However, if the best supply probe is
greater than 1.2C (2.2F) difference with respect to its return probes, the best supply probe is also invalidated. If
the unit is in perishable operation, a probe alarm will be triggered for both supply probes.
If the supply probes agree and return probes disagree, invalidate the worst return probe. If the probe check is being
run as part of Pre-trip P-5, an alarm will be triggered for the invalidated probe. If it is a run time defrost probe check,
the invalidated probe will be passed over and no alarm will be necessary. If the best return probe is greater than
1.2°C (2.2F) difference with respect to its supply probes, then the best return probe is also invalidated. If unit is in
perishable operation, a probe alarm will be triggered for both return probes.
b. Probe Check Procedure
A probe check diagnostic procedure is executed during Pre-trip P-5. A defrost cycle probe check may be accom-
plished at the end of defrost by energizing the evaporator motors for eight minutes at the end of the normal defrost.
The defrost light will remain on during this period. If supply probes are within limits and return probes are within lim-
its, the unit will return to normal control.
T-359 5–6