The MGA With An Attitude
CHARGING SYSTEM Test and Repair Notes - ET-120

At 06:58 PM 4/4/2009 -0600, Matt Szechenyi wrote:
"The ignition warning lamp came on while on a road trip and remains lit while the engine is running, no fluctuating intensity when engine starts".

If you're not sure about fiddling with the generator and regulator, then read this first:
What Generators Do and Regulators Ought To.

Any time it's not charging, always test the generator first. See Workshop Manual Section N.2 page N.3. If you don't have a shop manual, it starts on page N.3 here: wsm_n_electrical.pdf

If the generator is bad you have to fix that first. Very often the only problem is worn or sticking carbon brushes, and that is very cheap and easy to fix. If brushes are okay, then refer to the test procedure to see if it might be a bad armature or field winding. You might be able to re-connect a broken wire on a field coil. You most likely would not be able to repair a bad armature. A replacement armature used to be almost as expensive as a rebuilt generator, but the armature got cheaper while a generator got more expensive, so now you can afford to buy an armature.

If the generator tests okay, then be careful not to fry the thing while fiddling with the control box. Refer to Workshop Manual Section N.9 for testing and adjusting the control box. That starts on page N.8 of the same book section (link above).

When adjusting the regulator relay to get the right output voltage, a very small turn of the adjustment screw makes a relatively large change of output voltage. You might only turn the screw 1/8 of a turn to change output by a couple of volts. It is also sensitive to touch, so make a small adjustment and get your hands and tools away from it while checking output voltage.

See electrical diagram here: ET-101 circuit c
Power for the ignition lamp comes from the ignition switch and sinks to ground through the control box. When the generator is not charging, the most common cause is bad carbon brushes (worn out or sticking) in the generator. Generator brushes is a $3 part set. Replacement runs 20 to 60 minutes (depending on how handy you are).

Disconnect two wires, remove three bolts, and take the generator to the work bench. Remove two long screws and pull off the rear plate (which carries the carbon brushes). R&R one screw to replace each of the 2 brushes. For reassembly use a flat screwdriver to nudge the brushes back into the holders while you reinstall the rear plate. Tighten two long screws securely so it will not vibrate apart in use. Reinstall on the car, tension the drive belt, and reconnect the two wires.

A word of caution here. Any time you do a generator or control box repair it is important to check output function of the generator.

In recent years replacement control boxes have been notorious for poor or marginal quality. They may drift out of adjustment over time toward lower output, which can cause the battery to run down while driving with lights on. The control box might also fail to operate without notice, which can cause overcharging, which in turn can burn out the generator (internal melt down). In short, failure of a $40 control box can take out a $100 generator at the same time (and they only get more expensive with time). If you replace the generator without checking function of the control box, it might subsequently burn out the new generator as well.

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