My Mitsubishi montero does not start. The battery checked out good as well as starter and alternator. It did that once the otter day but then started up later.

So where all 3 Alternators returned & tested?

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

Check the starter and if it is functioning properly check your grounds and do a starter draw test

check for power at starter and if possible at batter terminal of alternator

the squeal when you start is the drive belt, it is slipping on the alternate you have to remember that the starter is the largest load on the battery so when the engine starts the alternator goes into a full output mode so there is a lot of load on the alternator belt right after start up also this probably why you are having a low charge at idle the belt is slipping enough to allow the alternator to slip at low ingoing speeds, take a good look at the belt tensionor and the drive belt best guess is that you will need both a new belt and a new tensionor.......HAVE FUN

Hit the starter with a hammer while some one is trying to start it and if it starts up after this the starter is bad most likely if you already checked the battery contacts in it get stuck if theres is power down to it ...starter bad....

I would start with checking the battery and starter connections. If they seem tight and clean I'd look at the starter itself as the possible culprit.

easy test on alt start car loosen pos cable.then take off cable from batterie if alt is good it wont cut out.if it does,more than likely your alt. if the belt was the problem it would be noticeable by squealing and audible slipping.and most voltage regulators are built into the alternaters these day.

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.

that indicates a dead battery. Your alternator may have failed. Check the battery reserve power, and if it is under 8VDC the car will not have enough power to crank the starter. A healthy battery will be around 10-12VDC when the car is off.... and 13-14VDC while running.

replace your battery