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Alternator replacement,3 times!

I have replaced the alternator in my '95 Mitsubishi Montero 3 times now,at $200 each!It lasts a year or less then fails .Belt is good,not too tight .Battery is good,no oil on it.when I first replace it works well.Any thing that could make it fail?Why?

I would suggest having the battery and charging system checked at one of he auto part stores that will do it for free.

How do you know each time that the alternator is failing or failed? Have both ends of each battery cable been checked to assure the contact areas are clean and tight? Where are the replacement alternators coming from (rebuilt?) Has the same shop replaced it each time?

I might add that Belts need to be properly tight and the alternator needs to be properly aligned with the belt.

Exactly what is the mode of failure of the alternators?

The mode of failure is that the volt meter will just start to register below 12,before that it reads 13,if I keep the car running eventualy the battery and car will die.No apparant reason.Cables and contacts are good,belt tightand aligned

Are the replacements new or “rebuilt”?? Many, many of today’s “rebuilt” parts are of dubious quality…

The fact the voltmeter drops to 12 volts does not mean the alternator is bad.
A poor connection in the alternator peripheral wiring can be a hit and miss affair in which an alternator can work fine one minute and quit the next.

There are 3 fusible links between the alt. and battery. A poor connection there could do it.
There is an alternator relay in the circuit that could be acting up.
A faulty connection in the dashboard warning lamp circuit or burned out bulb can cause the alternator to quit charging.

If that little red light does not come on when the key is turned the alt. will never charge.

Just to rule out any belt problems I suggest putting a socket on the alternator pulley and then attempt to rotate the pulley. You should not be able to turn it.
I ran into this a few months back with my car. A new alternator, new belt, new belt tensioner and it would not charge a lick. Bad pulley on the alternator was the cause. (and visually, there was nothing wrong with the pulley)

Thanks for your reply,it gives me the best options for pursuit,any other ideas are welcome

But replacing the alternator solved the problem for a year in three cases. So unless the connections at the alternator itself degraded each time, the alternator(s) would seem to be the culprits.

/Mr Lynn

Putting too much load (current output), for too long (duty cycle), can shorten the life of the alternator.
Larger loads can come from shorts, which have some resistance, which are too low in current draw to blow a fuse; but, still draw too much current. A battery can have internal shorts, as can other components.
Some components, from their design, draw a lot of amperage: electric motors, high power amps, large bass speakers.
Put an ammeter (multimeter) on the alternator with the engine running. With everything turned off, measure the amperage. Turn things on, one at a time and note, and record, amperage draw for each one. Are some components drawing high current? If so, that may be the cause of the alternator’s short life; or, it may be ALL of the currents, combined, which are excessive.