Bryan walks through a schematic of a Carrier straight-cool air conditioning system. He shows exactly how the real system lines up with the connection diagram and the ladder schematic.

Before getting started, Bryan pulls the disconnect and ensures that there is no power to the unit during the demonstration.

The connection diagram and schematic diagram show the same components, but they illustrate the components differently. Schematics show the relationship between all components and the two legs of power and ground, and connection diagrams show the connections in their real-life orientation.

We start by looking at the field connections of high-voltage power (L1 and L2) into the contactor. There is also a metal bar across the contactor (point 23), which exists on both diagrams but in vastly different places. Points 11 and 21 are normally open contacts that break L1, and they are also labeled on the contactor.

The crankcase heater and crankcase heater switch have stars by them, which indicates that they may be factory OR field-installed. In this case, we don't have either component. The capacitor wiring is also color-coded based on the compressor terminals. (Herm on the run capacitor has a blue wire leading to the compressor start winding; the start winding is active all the time.) Two black wires connect to the compressor common and the condensing fan common. There are also two yellow wires; one goes to the run winding from the capacitor, and the other one goes to C on the capacitor.

The connection diagram also shows the start gear; each component also has a star on the diagram, indicating that it may be factory or field-installed. Again, the unit does not have any additional hard start gear.

The schematic diagrams also show the low-voltage wiring. Many of the components in those circuits have stars, meaning that they may or may not already be on the unit. The wiring goes straight to the contactor, so none of the optional accessories are in place.

Inside the unit, the black, brown, and yellow wires each go to start, run, or common on the condensing fan motor. At the compressor terminals, the blue wire goes to the start winding, the black wire goes to the common terminal, and the yellow wire goes to the run winding. The schematic shows how each wire is routed (albeit the wires aren't colored).

Overall, you can get a solid start by paying special attention to the legend and notes as you begin reading schematics.

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