Carrier 38MGQ Series Service Manual page 35

Remark
Use a multimeter to test the DC voltage between the L2 port and S
ports of the outdoor unit. (Fig. 31) The red pin of multimeter
connects with the L2 port while the black pin is for the S port.
When the AC is running normally, the voltage will move
alternatively between - -50V to 50V.
If the outdoor unit has a malfunction, the voltage moves
alternatively with a positive value.
If the indoor unit has a malfunction, the voltage will be a certain
value. Example: 10- -13VDC small fluctuating amounts indicates
indoor unit malfunction.
Fig. 31 – DC voltage test
Remark
Use a multimeter to test the resistance of the reactor which does not
connect with the capacitor (Fig. 32).
The normal values should be around zero ohm.
Otherwise, the reactor must have a malfunction and must be
replaced.
Fig. 32 – Reactor resistance test
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