HoW Do I diagnose alternator problems in 1990 mercury sable

Hi,
Diagnosing alternators are about the same on all vehicles. You will need a digital multimeter to do the tests. ( WalMart has cheap ones for around $9)
First, you need to make sure it isn't really a battery problem.
One at a time, take off each battery cable at the battery and thoroughly clean the terminals and cable ends and retighten securly. NOTE: take off negative (black) first, leave it off, then do the positive (red) cable. Re-attach positive and then negative last. This way you will avoid any contact spark on the positive side which could damage your computer or wiring.
Next, check the voltage on your battery. With multimeter on Volts DC 20 setting, you should get around 12.65 Volts DC.
NOTE: Always use red lead on meter to positive (red) cable / battery post and black lead to negative (black) cable.
If your battery was discharged some due to your alternator suspition, you can go to the next step and start / jump start the engine.
With engine running test voltage at the battery again. You should get 13.4 to 14.2 volts.
Wiggle your test leads, scratching at the metal battery / cable terminals to ensure a good metal contact.
If you get more than 14.2 volts DC, your voltage regulator in the alternator is bad and the alternator must be replaced.
If you get less than 13 volts DC, move your black test lead to an engine ground (metal bolt which holds the alternator on, or clean metal surface / bolt on engine itself) and see if your readings are the same. If you get a higher reading of 13.4 - 14.2 volts your negative connection (cable or attatch point on engine) is faulty.
If still the same (less than 13 Volts) Move your red test lead to the connection on the back of the alternator itself where the red wire connects and black lead to ground. If your voltage is higher in the 13.2-14 volt range, your positive (red) cable or connection in that circut is faulty.
If none of these test produces at least 13.2 volts DC, replace the alternator.
If you get good alternator output voltages, but your battery won't hold a charge, have your battery load tested (free at auto stores or WalMart) as it is probably bad.
Hope that helps!
Mike

One is by disconnecting negative cable from battery for an instant with car running, if car stops running, your alternator is bad, other way is to read voltage at the battery with car running, what your reading is alternator voltage, should be between 12v and 14.9v any reading off means your alt. could be bad, but if your not shure, if you can, take the alternator out and have an autoparts tested for you they will tell you if is good or not. thanks