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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!
Safety
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.
Troubleshooting
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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