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.

1) Alternator.

• 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 checked.
• 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 connector. 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 alternator.
• Broken stator coil. The output of the alternator will drop, probably not to zero. Indicator light will probably glow.

• 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.

• 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 contacts.

    The 3AW is very delicate, dropping it can cause the insulators to break and the wiring to short. Use the following test procedure:

• Remove the 3AW from the car.
• 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 used.
• 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.

Copyright © 2003 CoolCat Express Corp. All Rights Reserved.