Basic Electricity applies to any type of machinery running on a 12 Volt Direct Current system. Once you see how it works it's much easier to understand, instead of making it complicated I will explain the basics in layman's terms. This is the chasis electrical panel on a 1995 Thomas Saf-T-Liner[pusher]. note: Click on the Photos to Enlarge
It looks a lot more complicated than it really is. Any 12 Volt electrical system has a source of power from the batteries, in this case we run on 3 top post 900 CCA [cold cranking amp rated] batteries.
That's a lot of juice available to supply voltage to the body and chasis circuits, but there are environmental and mechanical factors that will cause a severe voltage drop if not detected.
Voltage is like a river, the heavier the flow the stronger the current. If there's a dam down river this will cause resistance and restrict the flow. If the source [the river] is cut off so is the flow.
Any resistance between the battery and the load will cause a voltage drop, this can be caused by a short circuit, open circuit or just plain resistance from corrosion. A good rule of thumb when troubleshooting a 12 volt circuit is to:
Step 1...Check the battery -Sufficient Voltage?
Step 2...Check the load i.e. A Light or Relay, is it working?
Step 3...Check the connections
Step 4...Check the wiring
Very basic steps but most electrical problems are found using these steps, check the simple things first.
Say for instance you have 13.5 volts at the battery and 0.0 volts at the load. This means you have an obvious Open Circuit problem [a rubbed through wire possibly]
Using a digital ohmmeter you can ground one end of the circuit and using the two ohmmeter leads at the other end connect one lead to the faulty wire and the other end to ground. What you're doing here is checking for Continuity [the ohmmeter has it's own battery voltage which runs through the faulty wire].
By the way, the multi-meter you use for newer equipment must be Digital to safeguard the electronic systems that are much more common in new trucks and buses. If the circuit breaker is popping on and off then there is obviously a Short Circuit which can be hunted down using a Short Finder [a very slick tool to have which saves a lot of time and grief]
You hook up the auto-reset breaker then slide the meter along the wiring until it finds the exact spot where the short is [the indicator needle stops moving] along the frame or body.
Clean the connections and check the load i.e. marker light or tail light for poor grounds & connections.
That's a Quick Overview on Troubleshooting a Basic 12 Volt Electrical Circuit.