TM 9-2320-280-20-1
2-15. GLOSSARY OF ABBREVIATIONS AND COMMONLY USED TERMS (CONT’D)
Functional flow schematic - a diagram which is much like a normal electrical circuit diagram, except
that its purpose is to show the flow of information through the system (or the flow of a signal or the
flow of some material such as the fuel). This kind of diagram shows how each component or device
depends on the others. It is called functional flow because it shows the function (purpose of each
component) and how the output of one component “flows" into the next. For troubleshooting, the
functional flow schematic is better than the usual circuit diagram because it allows you to quickly see
how the system works and what to expect when you make measurements on a system that has
faults. You can’t expect the output of a device to be good when it has a bad input.
2-16. ELECTRIC CIRCUIT DESCRIPTION
An electrical circuit is a collection of electrical devices which are connected in a loop from a positive
voltage source (the battery positive) to a negative source ( the battery negative). It must be
continuous, with no breaks (no opening in the loop) so that electrical current can flow from the positive
to the negative. You can think of it like the plumbing in your house. There must be a source of water
under pressure or nothing will flow through the pipes. Water pressure is like the positive voltage of
the battery. There may be branches (tees) in the pipes going to several different places, but if you
don’t connect the pipes, you don’t get water. The same thing is true with the electrical circuit. If the
wires aren’t connected, no electricity will flow through them.
In the plumbing of a house, all of the water must go to the drain (you won’t permit it to be spilled on
the floor). With the vehicle electrical circuit, the drain is the negative terminal of the battery. With the
water pipes, the water always flows from high pressure to low pressure (another way of saying that
water always flows downhill). The electrical current is the same as the water flow - it always goes
from positive to negative voltage. Voltage is to electricity what pressure is to water. Just like the
pressure in the water pipe, the greater the voltage, the more electricity will flow through the wires.
Unlike the water pipes that will spill the water if they break, you can’t "spill” the electricity. The closest
thing to this in an electrical circuit is when two wires touch that aren’t supposed to and the current
flows to some place that it shouldn’t (this is called a “short circuit” or a “short"). Shorts often happen
where the wire touches the vehicle body (the body is connected to the negative terminal of the
battery). Since the current always flows through the easiest path to negative, it will bypass the rest of
the circuit where it was supposed to go, and go through the short directly to the battery. Because this
new path to the battery negative is shorter the malfunction is called a “short circuit” or a “short”.
If you put a valve in a water pipe, you can control how much water flows by closing the valve. What
you are doing is pinching off the pipe with the valve which restricts the flow. If you shut it off
completely, you can stop all water from flowing. In the electrical circuit, a resistor acts like a valve. If
you make the resistor extremely large, you can stop the current from flowing. The resistance is
measured in “ohms”.
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