Air and refrigerant aren’t the only things that circulate through a truck’s air conditioning system. A third element also needs to flow smoothly and unimpeded — electricity.  The compressor’s clutch needs at least 11.5 volts to make the electromagnetic field that activates the clutch pulley, allowing refrigerant to flow. Without 11.5 volts, the clutch hub will slip against the face of the pulley, generating enough friction and heat to melt the clutch bearing seal. Then, lubricant leaks out, the clutch quickly fails and the entire assembly must be replaced. 

Unfortunately, the replacement will fail, too, unless you can find out what is causing the voltage drop.  Here are some tips to help troubleshoot AC electrical problems.

1.  Improper Ground Wire Gauge.
   Most technicians start their electrical diagnostics with a check of the ground path. A loose connection, corrosion or fleck of paint can create resistance, and they’re relatively easy to spot and fix. But sometimes the issue is the ground wire itself.
   The ground wire is part of the complete electrical circuit, just like the power line.  The longer the power and ground wires, the more electrical resistance it will have. If you’re still getting voltage drop after a thorough inspection, make sure the ground wire is the appropriate wire gauge size for the length.  Check out class8truckparts.com for a selection of truck electrical wiring.
2. Poor Connections Loose electrical connections can cause heat to build up between the conductors.  At best, the result is wasted energy. At worst, a bad connection can cause a fire.  Inspect all connections and replace any poor connections with new wire connectors, or run new wires being sure to use heat shrink.
3. Recreate the Demand Take a voltage reading with the engine and air conditioner running, as well as lights, wipers and radios — anything that draws from the truck’s electrical system. Ideally, you want 13.5 volts, but not less than 11.5 volts, at the clutch coil.   Don’t forget to check inside the cab or sleeper for an aftermarket power inverter with 110-volt AC outlets.  It’s evidence that the driver may be drawing power the truck’s electrical system is unable to handle.

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