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This is how a relay works. Ignition switch power causes an electromagnet inside the relay to close an internal switch, allowing high-amperage current to flow to the load.

When checking an air conditioning circuit, an ammeter should show three to four volts in the wire leading to the clutch coil. If the reading is lower, check along the wiring route for voltage drops.


Another common electrical accessory is the AC (air conditioning) system. When diagnosing a cooling problem, you need to determine whether it is electrical or if the system is low on its charge of refrigerant. The electrical part of an AC system has many switches hooked in series, so there are lots of places for high resistance, which can cause trouble.

The first step is to use an ohmmeter and measure the resistance of the clutch coil. Most AC clutch coils should have three to four ohms of resistance. Measure the battery voltage and then, using the Ohm’s law formula (current = volts/resistance), calculate how much current should be flowing to the clutch coil. If the battery is 12 volts, you should have about three to four amps of current flow.

Next, take a digital volt meter and connect the amp part of the meter in line with the clutch coil. It should show three to four amps of current flowing through the meter. If not, then you have a chance to try your voltage drop measurement skills and find out which of the switches has the high resistance (see a diagram below for detailed instructions on how to measure voltage drops).

Remember if an AC system is low on refrigerant, the low pressure switch will open and you won’t get current to your clutch coil. By the way, saying a system is low on “Freon” is like saying you are Ski-dooing on a Polaris. Freon is the refrigerant trade name used by DuPont.


Anytime you have an electrical accessory problem, remember there are only four types of electrical faults: open circuit, high resistance, grounded circuit or short circuit. If the lights do not turn on at all you have an open circuit. If the lights turn on but they are dim you have high resistance. If the lights turn on but another circuit turns on as well you have a short circuit. If the fuse blows you have a grounded circuit.

The following diagrams show how to use a meter to find common electrical problems.

The Shop Class series was created with technical assistance from the staff at Assiniboine Community College’s School of Trades and Technology at Brandon, Man. If you’re interested in more information about ACC’s training programs, check out



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