Why should a component be disconnected from the circuit before its resistance is measured?
- Parallel paths through other parts can give a false reading
- The ohmmeter's battery can damage the control board
- Resistance cannot be measured on a cold component
- The meter needs line voltage present to read ohms
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A. Parallel paths through other parts can give a false reading
An ohmmeter reads whatever paths exist between its leads. If the component is still wired in, current can also flow through other devices in parallel, so a good reading may hide an open part. Power must also be off, since an ohmmeter must never be used on a live circuit.
Reference: Electrical troubleshooting; resistance measurement
A compressor contactor has 24 V across its coil terminals but the contacts do not pull in. With power off and the wires removed, the coil measures infinite resistance. What is the fault?
- The contactor coil is shorted
- The thermostat is not calling
- The transformer is undersized
- The contactor coil is open
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D. The contactor coil is open
A coil with voltage applied that does nothing is either open or mechanically stuck, and an OL reading confirms an open winding. The thermostat and transformer are proven good by the 24 V present at the coil.
Reference: Contactor diagnosis
The low-voltage fuse blows each time cooling is called. The contactor coil measures 0.8 ohms; a new contactor of the same type measures 12 ohms. What is the fault?
- The contactor coil is shorted
- The contactor coil is open
- The contacts are welded shut
- The transformer is undersized
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A. The contactor coil is shorted
By Ohm's law a 0.8 ohm coil on 24 V would try to draw about 30 A, far more than a 3 A or 5 A fuse allows. Much lower resistance than a known-good coil means shorted turns. An open coil would read OL and draw no current at all.
Reference: Contactor diagnosis; shorted coil