1999 Mitsubishi Montero XLS Sport has an electrical problem. I've replaced the alternator 5 times in the last 24 months, and a new battery was installed 17 months ago. A local Mitsubishi dealership has checked it and they disconnected and bypassed the anti-theft device and a local auto-electrical shop could find nothing wrong, either. Alternators were purchased at Advanced Auto (3) and NAPA (2). Any suggestions???

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Not easy to get to or change according to this.

https://www.justanswer.com/mitsubishi/5693h-crankshaft-position-sensor-located-2003-montero-is-diagram.html

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Montero Sport Alternator Replace 1 of 4 - YouTube

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www.youtube.com/watch?v=Dwp14p6vBRU
Jun 22, 2011 - Uploaded by JesseHafen 2003 Mitsubishi Montero Sport alternator belts pulley auto repair steps. ... My 1999 Montero Sport Limited has 245000 miles and now is giving ...

Answered

So where all 3 Alternators returned & tested?

Do you have wiring diagrams out ready to go
& a volt meter to start your testing?

check fuse box under hood for alternator fuse and relay /connections and cables

Verify your connections at the battery, not just for being tight but for corrosion as well, on the terminal and the cable.

Also check for oil on the bottom of the alternator. The right hand cam seal is notorious for leaking directly onto the alternator, killing them quickly.

That all being said, I haven't seen an aftermarket alternator last on a Montero for more than a month or two; so if your new ones are not OEM, you might consider getting a genuine one if your corrosion and oil leak checks are OK.

How new is the battery? Is it a maintance Free? It sounds like it needs to be replaced. Take your vehicle into your local Autozone and they will test your battery and charging system for free.

I have had the exact problem. I've changed altenator 3 times also and it has the same leak, but I've changed altenator an batterys over 2 year span. It just happend to me today after not driving it for 5 months. It's been in storage while I was away. Drive it fine for 2 days but today the radio kept shutting off and finally the car died right in the middle of the road. Bought a new battery because I had a warranty on last battery, but bought it just to get it home and in the driveway. Did you ever find out what was wrong with it?

Replace the key alarm battery.