Cherokee Pilots' Association
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Cherokee Pilots' Association |
The Cherokee alternator system is
extremely simple. There are just two main components - the alternator and
regulator. Yes, there are some other items - wiring, switches and an ammeter.
But basically you are stuck with only two major components.
So, with such a simple system,
troubleshooting should be a no-brainer, right? Well, not exactly. In fact,
charging system problems constitute one of the major headaches in Cherokee
maintenance. Sometimes the problem is that the technician does not really
understand how the system works. Other times, the troubleshooting process is
complicated by some baffling symptoms which, on first glance, just don't seem to
fit.
Nonetheless, here is an overview of
the alternator system which should provide guidance in most normal situations.
With one caveat, of course - the cases which are extremely frustrating are far
from normal situations.
The System
As we said, the alternator system
contains just two main components - the alternator and the regulator. In most
cases, one or both of these components will be defective. Of course, there are
some other components to consider - lots of wiring, switches, relays and circuit
breakers which all connect to the battery bus. The alternator is a modified
version of the automotive unit, which most people are somewhat familiar with.
The regulator may be either the older mechanical type or the more modern
transistorized version. The system is far from exotic.
Although the transistorized
regulators were a marked improvement over the mechanical type, many of the
latter continue in service today.
The purpose of the alternator is to keep the battery charged. How this is
accomplished requires a little elementary physics to understand.
Electricity is generated when a
rotating coil of wire is passed through a magnetic field. That field could be
from a permanent magnet, but in an alternator it is produced by another coil of
wire called the field coil.
In operation, current is put to the
alternator field coil via the F terminal on the alternator. The field coil is
then rotated (via the alternator belt) and a current is induced into the three
stator coils which are bolted into a fixed position in the alternator housing.
The amount of current generated in
the stator windings is dependant on two things: the speed with which the
alternator is turned and the amount of current passed through the F winding. The
speed, of course is not fixed. The alternator is connected via an alternator
belt in such a manner as to run faster than the engine RPM. Normally, an
alternator spins from 2,100 RPM at idle speed to about 9,500 RPM during
take-off. It is the regulator's job to control the current in the field winding
to compensate for the various speeds the alternator is driven. Engine speeds up
- the regulator must lower the field current. Engine slows down, the field
current must be increased.
The regulator measures the output of
the alternator and adjusts the field current to stabilize the alternator output.
Some regulators send regulated power to the alternator field, while others
simply adjust the frequency at which the field circuit is closed. Shorter
duration periods at which current flows has the same effect as lowering a
constantly-connected current through the field.
The regulator adjusts the field
current to maintain proper charging voltage - normally 14.2 volts. The voltage
must be higher than the battery voltage because batteries are charged by running
electricity through them - it requires higher voltage than battery voltage to do
this. Field current, however, must also pass through a switch (the second half
of the master switch) and/or a circuit breaker. Turning off the switch or
pulling the breaker cuts off all field voltage disabling the entire system (no
matter what the regulator is doing at the time).
Some Cherokees are also equipped with
an overvoltage relay (most models since 1965, 140's since 1969). This relay
automatically opens if charging voltage exceeds a specified number shutting down
the charging system. To reset this relay (and turn the charging system back on),
generally you must shut down the alternator and master switches, wait a few
seconds and then turn them back on. Cherokees generally do not have an
overvoltage warning light built into the system to indicate that the relay has
tripped.
System Failure
As we said, generally you are looking
at a defective alternator, regulator or both. And it is not uncommon for both
units to be bad, especially where transistorized regulators are used.
Transistors are highly prone to failure due to voltage spikes - when the
alternator goes, it often takes the regulator with it.
Failure in the alternator system may
occur in two different ways. First, there may be no power generated at all. This
is the easiest problem to troubleshoot. Or, the system may charge, but at a
reduced rate, resulting in sub-par performance and premature battery death.
Let's first look at troubleshooting where there is no alternator output.
Before doing anything embarrassing, make sure the simple things are checked
first. Check the alternator belt for security and also check for popped circuit
breakers or blown fuses. If this is the problem, you will be really chagrined
when you have paid for a new alternator or regulator unnecessarily. With these
preliminaries out of the way it is time for troubleshooting in earnest.
Troubleshooting should not involve
simply mindlessly replacing components without making further checks. If you do
this, it becomes inordinately expensive. Often you simply end up trashing the
substitute component without determining what the problem is. The proper method
requires the use of a volt-ohmmeter. These devices must be used with care, but
they are available inexpensively from a wide variety of sources, including
aircraft supply houses and even your local Radio Shack store.
If you suspect total system failure,
measure the bus voltage while the engine is running (be careful the plane is
properly tied down and care is made to avoid propeller injuries). If the meter
reads battery voltage (approximately 12 volts) the charging system is not
working. Check for loose terminals or broken wires. Then, you need to check the
field for continuity. Use the ohm meter's low scale. Disconnect the field wire
and check the resistance from that terminal to ground. There should be some
resistance (Normal field resistance is 10 to 18 ohms). If you have 0 resistance
(a dead short) the alternator is bad (and almost certainly, so is your
regulator.) If you have infinite resistance, you have an open field and the
alternator is bad.
Next, check for power to the
alternator. With the master and alternator switches on, check for voltage at the
switch and at the regulator. If you have power at the switch but not the
regulator, check to make sure the overvoltage relay has not been tripped. If it
does not reset normally, it needs replacement.
If you get voltage at the regulator,
but not the F terminal of the alternator, suspect a bad regulator.
Unfortunately, the only real test of a regulator is substitution of a known good
unit. But, if there is a problem somewhere else in the system which "trashed"
the regulator, you will probably soon have two bad units rather than just one.
Finally, if you get voltage to the F
terminal of the alternator but still get no output, the alternator is bad.
Either an internal break or, perhaps, all three diodes or windings in the stator
have burned out.
Low Charge Rate
What occurs when the system charges,
but not enough to keep the battery properly charged?
If, during operation, the alternator
output is greater than battery voltage (12 volts), but less than system
operating voltage (14.2 volts), the alternator has a low output. The alternator
belt may be slipping, there may be higher than normal resistance in the brushes,
the voltage regulator may be misadjusted, one of the diodes in the alternator
may have burned out or one of the three stator windings may have opened up.
Obviously, check the alternator belt.
The service manual specifies the method. For example, on the Warrior II, a
torque wrench is applied to the nut on the alternator which attaches the pulley
and the torque is read at the time the pulley first slips. A new belt should
slip at 11 to 13 foot pounds of torque, while a used belt should slip at 7 to 9
foot pounds of torque. (The higher tension of a new belt is to compensate for
the stretching which occurs when a belt is first used.)
To check the alternator, first remove
the wire from the F terminal and attach an Ohm meter across the field (red to F
terminal, black or green to ground.) The reading should be between 12 and 18
ohms. More than 20 ohms means trouble.
The alternator should be checked for
signs of overheating. If either a stator winding or a diode has shorted out, you
should see signs of heat damage. A shorted stator winding will slightly lower
the output voltage, but will result in a major decrease in output current.
Other problems sometimes plague
alternators. Brushes riding on the slip ring can wear out and power to the field
is interrupted. Poorly seated brushes, brushes that bounce, and cracked plastic
brush holders can all lead to failure.
In addition, sometimes drive bearing
failure permits the rotor to move about within the alternator housing resulting
in mechanical binding and eventual destruction.
Probably one of the most likely modes
of failure involves the burning out of one or more diodes. When this occurs
charging will still occur, but at a greatly reduced rate. The purpose of each
diode is to convert alternating current from its winding to direct current. When
one diode fails opened, the current from that winding is interrupted and only
the remaining windings are working.
When diodes burn out, they may be
replaced individually in lieu of replacing the entire alternator. Diodes are
readily available, but a special pressing tool, basically a modified C clamp, is
recommended for pressing them into and out of the case without causing damage.
One overlooked item of
troubleshooting is system grounds. The alternator, regulator and other system
components must be properly grounded. Even if the "hot" contacts are properly
connected and their circuits are good, bad grounds will cause the regulator to
increase system voltage, eventually resulting in the overvoltage relay kicking
out or in damage to system components.
If all else is ok, you might try
adjusting the system voltage regulator. Adjustment of the older mechanical
regulators is covered in the aircraft service manual. The later model regulators
are, according to Piper, not field serviceable. However, CPA members have
discovered that the part which generally fails is an internal transistor,
TIP-2955, and some electronically inclined members have replaced that transistor
at a tremendous cost saving. And for those not electronically inclined, the
manufacturer can repair the unit by doing the same thing (Hazotronics, 1622 E.
Whatley St., Longview, TX 75601 (903) 758-6661.)
As to adjusting voltage regulators,
remember to make sure that the system is under load - you want a 30 percent load
(11 amps on a 37 amp alternator) which has stabilized, and you need to make sure
the regulator has warmed up to permit a stable ammeter reading.
That Oscillating Meter
One more failure mode often afflicts
Cherokees - oscillating ammeter, also known as ammeter dance. Although some
fluctuation is normal with the Cherokee "loadmeter" system, especially with high
charge rates and low loads, when the oscillations continue, they can be quite
annoying.
Some oscillation occurs from
regulator "hunt" as the regulator cycles in an attempt to control alternator
current with field circuit voltage. However, there is another source of ammeter
dance which has an unusual source - the master switch.
As a master switch gets old, its
contacts develop resistance. As the switch passes current, the current, passing
through this resistance results in heat. As heat builds up, the contacts act as
a bi-metallic switch, eventually opening up the circuit momentarily. As the
circuit opens, the heat source disappears and the switch rapidly closes again,
beginning the cycle anew. Voila! The Christmas tree lighting effect. The cure,
obviously, is replacement of the master switch with a new one.
So, to sum it up, the alternator is a simple one - deceptively simple. Any
troubleshooting should begin with a plan and with the use of some simple tools
and the application of some common sense. Although the system is simple,
troubleshooting can be frustrating and carelessness can result in damaging other
system parts while the troubleshooting process progresses.
And although this article gives a
good overview of the system and some basic troubleshooting concepts, remember
that the section in your aircraft service manual has a lot more good information
and is a good place to start when the troubleshooting begins.