I suspect that's the voltage regulator and that diode's probably a Xener Diode although schematically it's not the correct way of drawing it.

Your voltage drop explanation makes perfect sense fasteddy!

I will also check the battery to make sure it allows full charge, but it was replaced after the first alternator failed so I'm reluctant to blame it.

Here's the official mitsu diagram for you purists! But it is from a 96...closest i can find online.

Both volumes of the 98fsm is downloadable from mitsubishi links dot com. <img src="/forums/images/graemlins/kewl.gif" alt="" />

I took the battery to Autozone for a charge and test, and to my surprise it was bad! Got a free replacement. (it was holding a charge, but, just barely 12V and not a completely FULL 12.6+ charge, thanks for suggestion sasnydley)

I then figured out why my dash battery light wasn't coming on. That light gets its ground THROUGH the internal wiring of the alternator. My alt was so fried that it was not grounding the circuit when the key was switched on - so no light!

Installed new alt and battery. Started up fine, dash light works. While running, with lights and some accessories turned on, I did a voltage drop test from + battery terminal to back of the alternator terminal. 0.2V, within spec.

Then did the same test from Neg battery terminal to the alternator case. Under heavy load with lights, seat heaters, etc., getting a 0.45V drop on that...kinda high, right?.

Then as a quick test I ran a new 8gauge grounding wire from one of the alt mounting bolts directly to the neg battery terminal. This time the engine seemed to start with a little more gusto, dash lights seemed a little brighter. The voltage drop with the same load just barely hit 0.3 V. So a good improvement.

Conclusion...weak grounds creating slightly high resistance, slowly frying my alternator and battery over time, right?

Problem is I really can't see how to reach a few of the grounds to clean them up or replace the wire. Neg cable to frame and neg cable to motor mount seem particularly inaccessible.

I think for now I'll just permanently install my extra ground wire from alt bolt to battery neg...no harm in doing that is there?

I forgot to write the moral of this story. If you're like me, you're always overcomplicating things. When you have an electrical problem like this you're hoping someone comes back and says "I had that exact problem in MY 1998 beige Montero and it was the secondary resistor plate in the dopwaggle that went bad. Happens all the time. Only costs $1 - replace that and you'll be all set!"

But the advice you really get is pretty boring. "Check grounds." "Check battery." Yawwwwn! You then go out, open the hood, glance at the battery cables, check for 12ish volts, jiggle em around a bit, and then proudly report back that "Battery and cables and grounds are good...what else could it be!?!? What about the dopwaggle and the microplate transistor?!?".

So novices like me - there's a reason the advice you get is often very basic. Sometimes you have to do some extra research to figure out what short statements like 'check grounds' truly involve. (But if this alternator dies in short order I hope you guys will finally admit to the known manufacturing defect in the dopwaggle.) Thanks all. <img src="/forums/images/graemlins/cheers.gif" alt="" />

Glad I could help. Also the Alternators tend to come with Chinese made diodes which don't seem to hold up well. I had to get Alternator rebuilt like 3 times in 2 yrs (Warrantied at shop) This last time has been running for at least 2 yrs. But since my daughter has been driving it the Montero doesn't see much water <img src="/forums/images/graemlins/scared.gif" alt="" /> <img src="/forums/images/graemlins/lol.gif" alt="" />

Whenever I get stumped, I usually find that it's something I already "checked", or something I assumed was fine. I have to remind myself that I still miss stuff and screw stuff up (rarely, my ego interjects). A wise old mechanic who taught me a whole lot once said, "90 percent of all auto problems are electrical - the other 10 percent are mechanic errors..." He was a drunk, but a wise drunk...

Success was short-lived. Ran beautifully with rock-steady 14+ volts during a full weekend of camping, about 100 miles of driving. But as soon as I got close to home the gauge started slowly dropping again. Put a meter on it and sure enough, already down to 13.6V at idle.
<img src="/forums/images/graemlins/frown.gif" alt="" />

More research makes it sound like my problem is related to frying diodes in the alternator. I'm now going back to my original thought, that there's a problem with in the circuit involving the dashboard battery light.

As I said, with key ON, the dash light illuminates, but it is much dimmer than the other dash lights. It was dim even when I first installed the latest alternator.

We established that the 1 volt drop I was seeing at the white wire of alternator connector could be due to the power going through the 'generator relay'. That made sense, however, going the through that relay is kind of a backup/secondary path.

Wiring diagram shows that the power SHOULD also be able take the primary path, which is basically a direct shot from key to dash light to alternator connector. If that was working properly, seems to me I should see full battery voltage on that white wire, not 1 volt less.

When alternator is running properly, it applies voltage to the white wire. This is supposed to 'cancel out' the voltage coming in from the key/dash light. So +12V fighting with +12V on the same wire will result in net 0V at the dash bulb. Bulb has 'no voltage', so it then goes off to indicate the system is working properly.

But if there is a short somewhere in that white wire, alternator would essentially be sending at least some power straight to ground. My theory now is that this situation could be short-circuiting or overloading the diodes and causing them to gradually burn out one by one until all are dead.

Charging voltage should drop with increased temperature to avoid overcharging the battery.

Whats wrong with 13.6V at idle? Sounds about right to me, depending on the idle RPM and state of your battery charge.

check for continuity with a multimeter between the "white wire" and the chassis if you believe there's a short in that circuit, but I doubt it.