Fundamentals: It Was a Fine Yesterday
The apprentice: A second-year mechanic.
The technician: A 15-year veteran.
The diagnosis and repair sequence:
A homeowner advises them that the system was working fine yesterday.
“I change the filter every month,” he says. “I don’t know what could be wrong with the unit.”
The team goes to the unit and finds the following during their visual inspection:
Technician: “Do you think we should check the power supply?”
Apprentice: “No.”
Technician: “Good. But tell me, why not?”
Apprentice: “Well, the condenser fan motor is still operating. I think that means we still have power to the unit.”
Technician: “OK. Let’s shut down the unit so we can open it up. What should we check when we get the unit open?”
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a) The operating head pressure.
b) The operating suction pressure.
c) The operating oil pressure.
d) The system idle pressure.
ANSWER:
d) The system idle pressure.
b>Apprentice: “I can’t check the operating pressures because we will have the unit shut down. And I think that we should check the system idle pressure to make sure the system isn’t discharged.”
After properly connecting the service gauges, the team determines that the system has pressure. The veteran asks the apprentice to momentarily touch the compressor. The apprentice tells the veteran that the body seems to be at ambient temperature.
Technician: “What does that tell us about the compressor?”
a) It is normal for the compressor body to feel about the same as the ambient temp.
b) It is normal because of the liquid refrigerant that comes back to keep the motor cool.
c) It is not normal because the compressor should be warm due to operation.
d) It is not normal because superheated refrigerant should make it hot.
ANSWER:
c) It is not normal because the compressor should be warm due to operation.
b>Apprentice: “I know that we have voltage to the system because the condenser fan motor operates. And I know that the system is calling for cooling because on this type of equipment the condenser fan motor comes on at the same time as the compressor through the contactor.”
Technician: “Tell me, which one of the following is the most likely cause of what we are sensing?”
a) Nothing. Power is properly applied.
b) The compressor is broken up.
c) The compressor is off on internal overload.
d) The compressor capacitor is defective.
ANSWER:
c) The compressor is off on internal overload.
Technician: “How are you going to check the internal overload?”
a) Attach my voltmeter to the wires connected to the contactor and check the voltage.
b) Attach my amp meter to the wires connected to the contactor and check the amperage.
c) Attach my ohmmeter to the wires after disconnecting them from the contactor and check motor resistance.
d) Attach my wattmeter to the wires after disconnecting them from the contactor and check motor wattage.
ANSWER:
c) Attach my ohmeter to the wires after disconnecting them from the contactor and check motor resistance.
Technician: “Why are you going to disconnect the wires from the contactor?”
a) To make sure that only the compressor motor windings are checked.
b) To make sure that the fan motor amperage doesn’t affect the readings.
c) To make sure that the contactor coil voltage doesn’t seep into the lines.
d) To make sure that the system has a good ground.
ANSWER:
a) To make sure that only the compressor motor windings are checked.
Technician: “Now that the test is done, tell me what your findings were and what you think they indicate.”
Apprentice: “The compressor motor windings test out properly with the normal differences between the start and run windings. The internal overload is completing the circuit. I also checked for shorts to ground and didn’t find any. I’m lost.”
Technician: “We’re going to have to check the system with the voltage turned on. We have to be extremely careful, as the condenser fan will be running and there will be live voltage at the terminals we have to check.”
Technician: “OK, with the power back on and the system calling for cooling, we are going to check the voltage at the contactor terminals. What should the voltage be between Terminals T1 and T2?”
a) 480 volts
b) 240 volts
c) 120 volts
d) 24 volts
ANSWER:
b) 240 volts
Technician: “It seems that we have the proper voltage at the terminals of the contactor. What might we examine next?”
a) The voltage at the compressor terminals.
b) The amperage draw of the compressor.
c) The contacts of the contactor.
d) The holding coil inrush current.
ANSWER:
c) The contacts of the contactor.
Technician: “We could check the compressor terminals, but for safety’s sake, we will check the contactor’s contacts first. What do you think we’ll find?”
a) Perfectly flat contacts
b) Slightly convex contacts
c) Slightly concave contacts
d) Pitted contacts
ANSWER:
d) Pitted contacts.
Technician: “Smart call. We had voltage by the meter yet the compressor wouldn’t run.”
Technician: “Can you tell me why the compressor wouldn’t run?”
a) The pits on the contact surfaces, which don’t fit together, will let enough voltage pass for the meter but not for the compressor motor to operate.
b) The pits groove together perfectly, creating a reduced current flow.
c) The pitted scorching creates an insulating effect that increases amperage requirements.
d) The pitting increases the contactor surface and therefore more voltage is needed to run the compressor.
ANSWER:
a) The pits on the contact surfaces, which don't fit together, will let enough voltage pass for the meter but not for the compressor motor to operate.
Technician: “Well, it looks like we have determined the problem. Let’s tell the customer and get permission to change the contactor. Then we’ll do a complete operational check on the system to make sure there is nothing else that may have caused the contactor to become pitted.”
Publication date: 08/27/2001