One interesting anomoly of Mitsubishi alternators is their voltage/temperature relationship.
Many vehicles have temperature compensation built into their systems, but for some reason, the 4g63t charging system carries it to an extreme. It's almost like they didn't spec a different alternator/voltage regulator/temperature compensation for the turbo cars, and just used a generic application without taking into account the insane heat levels in a turbo vehicles engine compartment.
I found this chart in the factory service/technical data sheets.
14.2~15.4v at -4 degrees (f)
13.9~14.9v at 68 degrees (f)
13.4~14.6v at 140 degrees (f)
13.1~14.5v at 176 degrees (f)
These are the voltages measured at the back of the alternator, on the stud where the wire attaches that runs to the fuse box.
What it means is that temperature increases, voltage decreases .
In the real world, it makes checking the factory charging system a little more complicated, as the technician must take temperature into account.
The chain store mullets have a little toaster that does a decent job testing voltage/current on the vehicle. Problem is, most of them have no idea how to use it, and routinely condemn systems that are within the factory tolerances... /ubbthreads/images//graemlins/bawling.gif. They will then sell you a total p.o.s. that doesn't have as radical a temp. comp. curve and send you on your way. (And you get to practice swapping alternators in "scenic" parking lots until you get a good one... /ubbthreads/images//graemlins/banghead.gif)
Checking the system can also be done with a decent voltmeter, and some forethought.
You want to check the system for voltage and current. Start with all the accesories off.
Stone cold start up should be mid to high 14v.
Warming up anywhere around 14.5v ish is good.
Hot, at a fast idle (~1500 +), it should be (hopefully) low 14v, high 13v...
Now, boost it around a bit and let it drop to the slow, hot idle. You'll see the voltage sag to ~mid 13v, to low 13v. (On a really hot day, at a real slow idle, it's not uncommon to see high 12v's with any kind of load on the system.)
(one interesting thing about the vr4 specific e-prom is that the *slightly* higher idle rpm speed programed in vs. the T/E/L/ helps hot idle voltage sag a ton!)
So, anyway, that chart is about the temp at the alternator case, as that's where the regulator is. If you're blasting it with some serious heat, (no heatshields/exhaust leaks, etc) I could see the voltage falling off the bottom of the chart, with a perfectly healthy alternator .
And, it's also about the voltage at the back of the alternator.
The battery voltage may be totally different than the voltage at the back of the alternator due to voltage drop.
With the engine off, the alternator and battery will read exactly the same
With no load, just sitting there idleing, the "hotel load" will cause the beginning of a loaded voltage drop. That load (fuel pump, ignition, etc) is ~around~ 10 or 15 amps (ish). The battery and alternator voltages will be still be ~close~, but the battery may be down a couple of tenths of a volt, compared to the alternator
Now, turn on the headlights, fog lights, a/c, with fan on high, rear window defogger, and you have a ~50~75~ amp load. This fully loaded voltage, measured at the alternator and again at the battery may vary up to .5v, or more.
That number is your charging system voltage drop.
If it's just a couple of tenths, you're good to go.
Around .5v and up, you can start to see some benefits from a rewire.
Above a volt, and you will definately be pleased with the result! /ubbthreads/images//graemlins/worthy.gif
With everything on, some cars with trunk mounted batteries will sag almost to resting battery voltage (~12.8v) at a HOT, slow idle.
If I had a dollar for every mysterious, unsolvable miss/fuel cut/high rpm stutter that was due to voltage sag, I'd be rolling a lot nicer bucket, that's for sure! (when the voltage sags, fuel pumps slow down, ignition gets weaker, etc...)
