Manitowoc MD0420A, Technician'S Handbook

Page: 92 / 132

92 Part Number 040005626 5/17
Compressor Electrical Diagnostics
The compressor does not start or will trip repeatedly
on overload.
Check Resistance (Ohm) Values
NOTE: Compressor windings can have very low ohm
values. Use a properly calibrated meter.
Perform the resistance test after the compressor
cools. The compressor dome should be cool enough
to touch (below 120°F/49°C) to ensure that the
overload is closed and the resistance readings will be
accurate.
Single Phase Compressors
1. Disconnect power and remove the wires from the
compressor terminals.
2. The resistance values between C and S and
between C and R, when added together should
equal the resistance value between S and R.
3. If the overload is open, there will be a resistance
reading between S and R, and open readings
between C and S and between C and R. Allow the
compressor to cool, then check the readings
again.
Check Motor Windings to Ground
Check continuity between all three terminals and the
compressor shell or copper refrigeration line. Scrape
metal surface to get good contact. If continuity is
present, the compressor windings are grounded and
the compressor should be replaced.
To determine if the compressor is seized check the
amp draw while the compressor is trying to start.