THE SHOTGUN METHOD
As stated previously, most technicians will adopt their own troubleshooting methods that are developed with time and experience. The shotgun method involves testing the circuit at various locations to determine trouble areas rather than following a step-by-step procedure as outlined in the hopscotch method. In this example, the circuit to be tested is a central air conditioner and electric heating system. Probably the best place to start is at the thermostat because it is readily accessible.
1. Check the power supply. To do this, set the fan switch to ON or MAN to see if the blower fan turns on. If it does, you have determined that the 24-volt supply is working. If the blower does not turn on, the problem could be the thermostat, fan relay, blower motor, run capacitor, 24-volt trans- former, or main power supply to the trans- former. At this point you have determined if the problem is with the inside unit or the outside unit.
2. Test the thermostat. Remove the thermostat from its base and check the wires connected to the thermostat base with a voltmeter to determine if 24 volts is available. If 24 volts is available, use a fused jumper to test the circuit components controlled by the thermo- stat. Connect one lead to the power terminal (R) and make connection to each of the other terminals to determine if there is a response. If there is a response to each of the terminals, the thermostat is defective.
3. If there was not a response to a particular circuit component, replace the thermostat on the base and check that component starting with the power supply. In this example, assume that the air conditioning unit did not respond.
4. Check the 240-volt power supply to the unit.
This can be checked at the breaker, disconnect switch, or main contactor depending on which is most accessible. In this example it will be assumed that power is present at the main contactor.
a. Check the output of the main contactor to determine if power is being supplied to the compressor. If not, check the 24-volt supply to the coil of the main contactor. If 24 volts is supplied to the coil, the contactor is defective.
5. If 24 volts is not present at the coil of the contactor, check the thermostat wires where they enter the outside unit. If power is not present, check the wiring between the thermostat and the outside unit.
6. If 24 volts is present at the unit, check any components between the 24-volt supply and the coil of the contactor. Components such as high-pressure switches, low-pressure switches, and so on are connected in series with the low-voltage circuit.
Now assume that instead of no response at the outside unit, the condenser fan started but the compressor did not.
7. If 240 volts is available at the output of the main contactor, check all components, such as run and start capacitors, between the contactor and the compressor.
8. If all components between the contactor and compressor are good, check the power sup- plied to the compressor terminals. If power is present at the compressor terminals, disconnect power to the outside unit by opening the disconnect switch or circuit breaker.
9. Disconnect the power terminals connected to the compressor. Use an ohmmeter and check between each terminal to determine if there is an open circuit. Also check between each terminal and the compressor case to determine if there is a grounded circuit. Note: It is possible for the motor windings to be shorted and not be open or grounded. Shorted windings will cause the motor to draw an excessive amount of current or may not permit the compressor to start when power is supplied. An ohmmeter generally will not reveal this condition.
10. If the ohmmeter indicates an open circuit in the compressor, note if the compressor is hot to the touch. If so, the internal overload may be open. It cannot be determined if the compressor winding is open or if the internal overload is open until the compressor cools. This overload cannot be bypassed. If the compressor is hot, it may take hours for the over- load to reset, depending on the temperature of the compressor, the ambient temperature, and whether the compressor is located in direct sunlight. The only way to know if the compressor is defective or if another problem caused the overload to open is to wait until the overload resets. It is recommended to leave the power disconnected to the outside unit until the compressor cools and allows the overload to reset. This will allow the technician to observe whether the compressor restarts or not.
11. Some of the circumstances that can cause the internal overload to open are:
• Defective windings in the compressor, causing it to draw excessive current.
• A stuck compressor.
• A brief power interruption, such as a loss of power or someone opening the thermo- stat contacts and reclosing them.
• Lack of air flow across the condenser and compressor. This can be caused by a dirty condenser or anything blocking air to the condenser. The condenser fan can also be defective and thus prevented from obtain- ing full speed.
• Low voltage supplying the compressor.
• Over charge of refrigerant causing high head pressure. This would cause the compressor to draw excessive current.
• Low charge of refrigerant. The compressor could overheat because it depends on cool vapor returning from the evaporator to help cool the motor.
• Very high ambient temperature and being exposed to direct sunlight.
12. If the compressor eventually restarts, check the current draw of the unit and compare this reading to the nameplate cur- rent rating. If the current draw is greater than the full-load-amp (FLA) draw listed on the nameplate, determine if the problem is a defective compressor or one of the other causes listed.