There is a key on most charts like a map, some charts will show the location of the electrical controls, some charts will show how the controls are conected.
You just need to understand how these controls work then work on how there connected
Just remember, Source, Switch , Load, Home.
Every electrical circuit starts with a source.. Breaker, disconnect transformer etc. Goes through a switch (Or series of switches), Light switch , Thermostat, Low pressure switch, Contacter, Sometimes a low pressure switch and a high and a overload.. Then it goes to the load, Light bulb, Compressor, fan motor whatever.. Then it goes home, Common.. Disconnect, Breaker Transformer whatever.. If you always remember source switch load home. It helps alot to avoid being scatter brained.
It is easier to read schematics as well just focusing on one part of it, one circuit, And most times you need a sequence of operation to go along with the schematic. But with this method you can see where the power starts at its source and see where its waiting and for what switch to close. For example you would see constant power from the transformer (Source) waiting at the stat (Switch). When the stat calls it closes, Sends power to the Solenoid (Load) and goes back to common (Home)., Then the power waiting at the low pressure switch, The low pressure switch closes as the solenoid energizes, Goes and pulls in the contacter (Load). Goes to common, Then the power waiting at the contactor (Switch) goes to the compressor (Load) .. Goes home. (Disconnect).
In trouble shooting you will want to have a procedure that you follow each time. Try to start in the same place eace time then move to the next step. This way you will not have to cover the same ground twice.
There are established trouble shooting proceedure's know them.
Find work in the trade get to know safty issues
get a copy of a wiring diagram for a typical unit you will be working on --
it shows how the devices are wired.
grab 'highlighter' of several colors = 3+
or use marks= circle | dot | hash |square | dash
mark the 120v 'phase | hot'
mark the 24v lead coming from the transformer which feeds the t-stat
now draw a schematic= how the devices function, which affects the next.
when troubleshooting, start with a known voltage source --
measuring at the main elec panel is always good =
then one knows whether the voltage is 240/ 235/ 208 for phase to phase, and 115/ 120/ 127 volts phase to NEUTRAL.
know that unless you have several meters, the voltage may be slightly different from minute to minute, lots due to just what all appliances are running at any given moment
then get one in the shop & identify the elec devices.
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!