GMC Western States

 Tech Center Number 2 - June 1992

Electrical Basics

One of the basic maintenance functions is the ability to troubleshoot problems in order to properly repair them. One of the areas people tend to shy away from doing their own work is in the electrical department. This issue of Tech Center will cover some basic electrical and trouble shooting principles. We'll start with some definitions:
Alternating current flows alternately in opposite directions through a circuit, whereas, direct current flows through a circuit in only one direction at a constant voltage. Generally speaking, AC is used for houses and DC for automotive use. The Motorhome has two separate systems, one AC and one DC. They interact through the converter when the coach is plugged into an external power source. The converter is an AC powered device which supplies DC power to the 12 volt system in the coach. The alternator attached to the engine generates alternating current (it's more efficient to generate AC than DC from a belt driven device), which is converted by diodes (electrical traffic cops) inside the unit to DC before it goes on to the rest of the system.
Voltage is the electrical equivalent of pressure in a fluid system. It is the electromotive force that causes electrical current to flow in a conductor (wire). It is expressed in volts (V). The Motorhome uses a 12 volt-DC and a 115 volt-AC system.
Current is the measure of the quantity of electricity flowing in a conductor per unit-time (flow rate). The fluid system equivalent would be a flow rate such as gallons per minute. Current is expressed in amperes (A, or amps). Fuses are rated in amps and will melt and break the circuit when more than the preset amount of electricity flows through which is why they are used as circuit protection devices. Circuit breakers are nothing more than semiautomatic fuses.
Resistance is the opposition to flow of electricity in a conductor. It is comparable to restrictions, orifices, or irregularities that would slow or resist fluid flow. Resistance is expressed in Ohms 0. Dirty electrical connections increase resistance and cause the amount of voltage passing through the system to decrease.
Power is the rate which work is being done and is expressed as watts (W). Electricity is composed of two elements: Voltage and current. Power is commonly calculated (rather than measured) by the formula W=V times A, as volts and amps can be easily measured. In a 12 volt system, wattage requirements of light bulbs, fans, and other devices are usually expressed as the current (amps) at 12 volts. In a 115 volt AC system, as we've all seen at home, devices are rated in watts rather than amps (100 watt light bulb, 1200 watt hair dryer, etc.).
A Circuit is a closed conductive path through which electricity flows. It must contain a power source (+ or positive), wires to the loads), and return conductor (typically the chassis or metal body of the RV) back to the (- negative side) of the power source.
Series elements in a circuit are in a single conductive path through these elements. An example being 6 lead-acid battery cells each connected positive to negative which form a 12 volt battery (each being 2 volts). Another series application would be a switch in a wire leading to a device. The older style Christmas tree lights were wired in series, when one went bad, the circuit was broken and the entire string lost power.
Parallel elements in a circuit furnish alternate paths for the current to flow through branches of the circuit. The voltage across each load path will be the same, but the current will be determined by the resistance present in each parallel path (A=V/R). Modern Christmas tree lights are now wired in parallel. If one light bulb burns out, the rest still have power because the circuit is not broken. The Motorhome living area electrical system is made up of several individual circuits, each wired in parallel.
Voltage drop as used in RV electrical trouble shooting is simply the difference in voltage between two points on a circuit. Each element in a circuit has a normal voltage drop. Parallel circuit branches have a common voltage drop since they are connected into the circuit at common input and exit points. The sum of the voltage drops around a circuit must equal the terminal voltage of the source (battery, converter, or alternator). Dirty connections, inadequate wire, or corroded wires and connections can have a significant affect on voltage, sometimes as much as 3-4 volts. Enough to keep your furnace or refrigerator from functioning properly!
Electrical schematics or diagrams are drawings of circuits using symbols to represent circuit paths and elements. They usually contain all connections, components, and component values. Internal schematics of elements within the circuit such as alternators, starters, switches, relays, and motors are useful for troubleshooting and should be kept with the main system schematics. We are very fortunate that section 12 of the GMC maintenance manual, Chassis Electrical Schematics, also includes wire sizes and color coding. The majority of the RV manufacturers do not have schematics available, some even use one color wire for the entire coach!



Electrical shock is not a primary consideration in 12 volt systems because it takes about 40 volts to penetrate dry skin. The exceptions are the ignition coil, primarily when the engine is running. Peak voltages of up to 300 volts and spark plugs which carry 30,000 to 50,000 volts. Chances are that you will let go of these things in a hurry if you grab them when they are "live". The AC side of the Motorhome electrical system merits all the respect that you give your home electrical system. 115 volts is definitely enough to give you a good "jolt".
Short circuits across 12 volt systems can melt wires or burn holes in tools. Either can cause serious burns or even fire. A dead short across a normal 12 volt battery or wires can draw hundreds of amps producing thousands of watts in a small area. It's good practice to disconnect the battery, and/or unplug the coach, when you will be working on the either the AC or DC system.
Battery fumes are explosive. Battery charging breaks down water into hydrogen and oxygen gasses in the same proportions used to propel the space shuttle into orbit. Introducing a spark in the vicinity of a charging battery, or inside a poorly ventilated battery compartment may provide you the same propelling experience without the benefit of a space suit or reentry vehicle!
dumper cables should always be used in the following manner: First, check both batteries for proper electrolyte (water} level, add water to bring the level up if necessary. Second, attach one positive (red or +) clamp of each end of the cable to either battery. Third, attach the negative (black or -} cable to the DEAD battery. Fourth, attach the other negative clamp to a metal source AWAY from the battery in the "good" vehicle, an alternator or radiator support bracket is a good solid ground source. The clamp will spark when you attach this last cable, but the spark will be away from either battery.



First, gather data, either you or someone who you hire will need it. Document the facts, write down the symptom that tells you that you have a problem. Document completely any noises you hear, how often, when did it start, what was happening before it started. Did someone work on the coach recently, if so, what did they do? They key is to start simple and work out from there. Too many people have paid good money to have an 'expert' repair and electrical problem, only to find out a wire had come loose or a fuse had blown. The proper sequence to follow when trying to find the cause of a power loss is to check the power source. Are the batteries charged? Is the coach plugged into a good source? If you have a power source, check the fuses or circuit breakers, a blown fuse or circuit breaker indicates either an overloaded or shorted circuit. Try replacing the fuse or resetting the breaker. If it trips again, then you probably have a short.
Decide what area the problem is coming from, the lighting circuit, the furnace, refrigerator, etc. If the problem is a short which caused a blown fuse or circuit breaker, then look over the area for loose wires, bare wires, broken wires, burned wires, or wires that might get damaged in the future. Look for wires touching metal (ground}, use a flashlight, or some engine cleaner if you are trying to see what's going on inside the engine compartment.
Take out your maintenance manual and follow the wiring schematics. Try to follow the circuit through the coach so you know where to look. Some circuits start on one side and than cross over the top and end up on the other (for example, the refrigerator and the hall light above the bathroom/closet on 26-foot side bath models are on the same circuit}.
One of the more common areas where electrical problems creep up is the living area fuse block, the connections get dirt built up on them and after some time the resistance builds to the point where it stops letting current pass. A fix for this is to disconnect the battery, and pull each fuse out, one at a time. Use some fine sandpaper or steel wool to clean the fuse ends and their respective sockets. Also disconnect each lead coming and going to the fuse box and clean the wire ends the same way.
Clean your vehicle and living area battery cables and posts. These batteries are the power source for the entire coach and their connections should be as clean as possible. If there is acid build-up on the terminals, use water with baking soda mixed in to neutralize the acid. Use a wire brush to clean the cable ends and posts. Re-connect the battery using some kind of corrosion inhibitor, auto parts stores carry a spray or some treated felt washers you can put on your battery posts.


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