On any circuit ther is a beginning and an end.
The car battery is typically the beginning and the end. ( Yes I know if the car is running the alternator is the source, but the car is not running right now)
What is in between is the load.
Electrons flow from the battery, most get used in a load and the remainder go back to the battery (just a simplified version of what happens)
To trace out a cir you first have to know where the path goes from the battery plus terminal to the load then back.
The back is the ground which in our (and most older cars) is the car body.
Take the head lights.
The path (simplified) is:
- Battery plus terminal
- Bulkhead connector
- Light switch
- Down to the highbeam /lowbeam switch
- Back to another terminal on the bulkhead connector.
- To the head light connector
- Into the headlight filament
- Out of the headlight on another terminal
- To a ground source (typically the sheet metal of the core support)
- Through the metal of the car body
- To the ground strap from firewall to the engine block
- From the engine block to the heavy cable on the front of the engine
- To the negative battery post.
It's just a loop.
If the loop is broken anywhere along the route the lights do not come on.
Like when the headlight switch is in the off position the loop is broken and the lights are off.
As for testing with a multimeter.
You can test to see if you have 12 (nominal) volts and every junction along the way
One end of the tester goes to a suitable ground. The battery negative post is the best but your leads might not be long enough, so a bare metal part of the car body will work (assuming the ground straps are all intact)
Then check for voltage going into the headlight switch.
If you have voltage there move on to the next point in the path.
Some times it is easiest to work back from the load. Till you find power
Example you have a burnt out headlamp,
You could trace the voltage all the way from the battery plus post to the bulb only to find that there is power at the lamp socket, had you checked there first and noted voltage, changing the bulb might have been your first thing to try.
A word on multimeters.
A 12v battery and a wire that is just barely making contact can show 12v on a multimeter but the connection is so bad that if you hook up a load it will drop to 0 or near 0, making the load not work.
A test lamp can find that type of failure easier. If you get a bright light on one end of a path segment and a dim one on the next segment then there is most likely a bad connection in-between.
There is A LOT more to learn but that is the BASIC version of things. Reviewing and understanding the path of each circuit is the first thing to do.
Keep asking questions, post photos, one man's bonnet is another's hood. Less confusion with photos
