TM-9-2320-280-20 HMMWV Technical Manual Unit Maintenance Volume 1 Download
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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”. 2-39