1998 Mercury Mystique Repair Question
1998 Mercury Mystique Charging System light is on
Do not remove the battery cables with the engine running. This is not the way to test an alternator. There are special instruments for load testing an alternator. We suspect that you have a burned out diode in the alternator, which would require replacing the alternator. A Mitchell, Chilton, Haynes, dealer auto repair manual or a specialty repair book for your make of car will have diagrams and or specifications showing what you are looking for. The manuals and books give step-by-step instructions.
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DO NOT PULL THE CABLES OFF when the engine is running! The battery is the main part of regulating system voltage. Without it, it is possible for voltage to go over 30 volts and destroy every computer on the car. You are lucky the generator is not working. That was a quick trick done many years ago on cars built with common sense and not all of these unnecessary, complicated, expensive computers, by mechanics who didn't understand how these systems work.
The proper test is to use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is higher than that, check for battery voltage on the yellow wire. That's where system voltage is sensed. If that voltage is present, the voltage regulator is defective and can be replaced separately. If that voltage is missing, there is a blown 15 amp fuse. That will result in a no-charge condition.
If you have battery voltage on the yellow wire, there is a "ground here to test" terminal on the back, as part of the voltage regulator. Watch the voltmeter or head light brightness when you ground that test point with the engine running. If it starts to charge, the voltage regulator is defective. If it still doesn't charge and there is voltage on the yellow wire, the generator is defective. Typically there will be an open brush. They are replaced as part of a new voltage regulator. To test for that, use the meter on the lowest ohms setting and measure for continuity between that test terminal and the other one across from it. That second terminal might have a gray plastic cap that must be pried off.
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