E-Type Alternator System Explained (page 3)
The process of troubleshooting a Lucas alternator is really quite straight
forward. We'll break this up into components, and talk about possible problems
with each one.
2) 4TR Voltage Regulator
Slipping belt. This is
a common problem, yet can do a lot of damage. check the belt every couple
of months to make sure it's tight and in good shape. If the belt slips,
the output of the alternator will drop. The voltage regulator tries to
compensate by increasing field current, and eventually the field coil can
fail. If your fan belt ever fails, disconnect the connector on the voltage
regulator immediately. This will allow you to run for a while on the battery,
and will prevent further damage to your alternator.
Bad bearings. The bearing
will complain loudly before they actually fail. Disconnect the fan belt
and the voltage regulator to prevent damage from getting out of control.
Bad diode. A diode can
either short or go open. In either case, there will be a reduction in output.
The VR will compensate, and that can make the problem even worse, overheating
the rotor and burning out the 3AW relay. The dashboard light may or may
not light. This condition can be readily diagnosed if you have access to
an oscilloscope. The output of the alternator should appear as a rippling
line: a bad diode will interrupt the ripple with momentary sags or surges.
It's possible to detect this problem with a voltmeter set to the AC range:
if you can measure more than a couple of volts of AC, then there is a problem.
Doesn't work with all voltmeters, though. A quick, simple check is to disconnect
all the wiring attached to the alternator. Measure the resistance
between the B+ post and ground, then reverse your test leads and measure
again. The reading should be about 4 megohms in one direction, and infinite
when you reverse the leads. If it reads otherwise, investigate further.
Alternatively, the alternator can be disassembled and each diode individually
Brush problems. Any problem
with the brushes or slip rings would be seen as chronic low voltage. The
brushes should be at least 4mm long. If they are less, they won't work
correctly. Brushes should be available from the usual suspects. The slip
rings should occasionally be cleaned of accumulated carbon. This can easily
be done with the alternator in place. Disconnect the battery cables at
the battery. Remove the brush pack from the rear of the alternator. You
will see the slip rings inside the case. Insert the eraser end of a pencil,
and hold it firmly against the ring while slowly turning the alternator
by hand. Repeat for the other ring.
Broken rotor coil. The
light on the dash will come on, output will drop to zero. To test, remove
the connector from the voltage regulator. Check to be sure there is power
on the + lead. If there isn't, then check the 6RA relay. If there is power,
fabricate a short length of wire, and jumper the "F" and "-" leads on the
Start the car. If the alternator now charges, immediately turn the car
off and replace the voltage regulator. If it fails to charge, replace the
Broken stator coil. The
output of the alternator will drop, probably not to zero. Indicator light
will probably glow.
3) 6RA relay
The voltage regulator
is a sealed solid state device. The usual failure mode would be a
failure to charge. To test, remove the connector from the voltage regulator.
Check to be sure there is power on the + lead. If there isn't, then check
the 6RA relay. If there is power, fabricate a short length of wire, and
jumper the "F" and "-" leads on the connector. Start the car. If
it charges, immediately turn the car off and replace the voltage regulator.
If it fails to charge, replace the alternator.
4) 3AW relay.
Burned or dirty contacts.
This can cause low alternator output or inconsistent operation. To test,
turn the ignition key to "on", but don't start the motor. disconnect the
F+ lead at the alternator, and measure the voltage between this lead and
ground. Should measure very close to 12 volts (assuming the battery is
ok). If it measures less than 12volts, clean or replace the relay.
Bad coil. Alternator will
produce no output. The dash light will glow. Test is the same as for burned
The 3AW is very delicate, dropping it can cause the insulators to break
and the wiring to short. Use the following test procedure:
read about how to completely modernize the Lucas 11AC alternator and control
system, click here.
Remove the 3AW from the
Using an ohmmeter, test
the resistance between the AL lead and the E lead. This should be 16-20
ohms. Anything outside of that range indicates a bad relay. If there's
no conductivity between AL and E, the relay is defective and MUST NOT be
Measure the resistance
between WL and E. This should be close to zero. If it's more than a couple
of ohms, replace the relay.
Reinstall the relay and
turn the ignition key on. The indicator light should light. If it does
not, check the bulb.
Start the motor. The indicator
light should turn off. If it does not, the problem is elsewhere.
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