Alternator Wont charge if plugged in, but does when unplugged

[Jarnutt]

Car died the other day from a dead battery while on the freeway, So i assumed bad alternator, HOWEVER, when I unplugged the 4 pin harness on the side, while the car was idling, I noticed on my SAFC that the voltage went from 11.8 to 12.1. So i bumped up the throttle to about 1700 rpm and the voltage holds at 14v, so the reguator is working, and flashing the feild off of residual voltage it appears, alternator is possibly good still. Ive checked all my connections, and tried some of the different things suggested when I searched on alternator problems, but What Im stuck with is this (all testing done with radio, A/C, and headlights on);

-Its an Autozone Alternator, (came on the car when I bought it, not by choice)
-Battery light on the dash works initial key on, bulb is good, but never turns on, even with the alt unplugged
-Has always had a tendency to charge more when revving the engine up during acceleration, for instance, pulling up my driveway I have always noticed my headlights get brighter when I gas it to pull into the garage.
-if it holds at 14vdc no matter how High my RPMs are, and settles down to 12.2ish when the car is idling, Id think the regulator works, it just doesnt have anything energizing its field at idle speed, like the 4 wire plug.

I had my girlfriend in the car holding the RPMS at about 1700 and watching my SAFC. As soon as I plug the harness to the alternator in, voltage goes DOWN to 11.7 or 11.8ish, when I UNPLUG it, RPMs go down (charging) and it shoots up to 14.1. Im lost, could it be the charge "relay"? or maybe that printed circuit board behind the gauge cluster effing up? I didnt just rely on my Safc for readings, I verified with my Fluke AT the battery as well.

 

[DSM1G90]

Forget about the SAFC reading nonsense.

And you don't ever have the alternator unplugged with the motor running, for plugging and unplugging it raises havoc with the regulator (in which, I do believe that it's gone now...) since the battery is supplying it constant voltage to operate.

Did you buy a new battery? I bet the old battery took your alternator out in a quick.

In idle or at any speed, you should be seeing 13.8VDC - give or take a volt or two. Is this what you got out of your FLUKE meter when you put the corresponding leads across the battery with the motor running and see what the output voltage is coming out of the alternator?

It shouldn't fluctuate like what the readings your getting. Take it out and have a good shop check it out - don't take it to Autocrapzone for testing.

Good luck - DSM

 

[90moneypit]

^ +1. It sounds like a bad regulator. Oh, and if your car dies on the freeway, it's not from a dead battery, it's from a crap alternator not providing the energy to run your engine. It a working electrical system, the battery is only used for starting the car, once it's running everything should be provided via the alternator.

When you replace the alternator, make sure you have a full charge in your battery or you'll just end up killing the regulator again. Look up 'alternator death' on google. Hell just look up google and play pac-man for the weekend for that matter!

 

[Jarnutt]

I guess I should have been a little more clear, I DID use my Fluke DMM to check the battery voltage, it matches up with what the SAFC was telling me the voltage was at the ECU.

As for the car dying, I wasnt implying that a bad battery is why it died, I was just saying that the battery obviously had enough to start the car and drive for a few miles with no charge going to it, but if the regulator is bad, then why would it hit a 14V peak? I dont know about DSM alternators, but usually with most Ive dealt with in the past a bad regulator means it will either charge past 16v or not charge at all because there is no regulation of the charge Field. All that the regulator does is apply more or less 12v B+ to the feild to either create more or less magnetism which in turn creates more or less charge amps. Im getting no charge at idle, but a perfect charge when the engine speed is over 1200 RPM, thats with the alternator unplugged, so the regulator is working off residual voltage in the armature, its possible its the regulator, I was just kind of hoping there was a more simple explanation.

 

[90moneypit]

It's a little more complicated than that but essentially yes, the regulator tries to allow no more than ~14v to pass through but it has other functions as well like acting like a one way gate to prevent drain. You can say a CPU only turns switches off and on but we can all agree (hopefully) that it's a little more complex than that.

The problem lies in your alternator not charging your battery while you drive and DSM's don't have external regulators, that leaves replacement of the entire unit as the only option. Now the question is whether to stay with a Mitsubishi alternator or look at the option of installing one out of a Saturn. There are plenty of threads about running the Saturn alt so I won't go into it here.

 

[Jarnutt]

Ill look into the saturn alternator, I saw a lot about it when searching around already, but lucky for me, steve left the lifetime warranty receipt in the glove box for my alternator, and even luckier for me, the new guy at autozone had a boner for Turbo Buicks and after about an hour of him telling me how awesome it is tghatg I own a DSM AND a grand national, I've pretty much got a "no questions asked" swap and return guy on the inside HAHAHA. If it turns out to be this alternator screwing with me, ill swap it at least this one time before looking into spending money on a different one. Guess ill find out this weekend.

 

[90moneypit]

I'm just afraid that you'll end up being best friends with the counter guy at Autobone. I went through... more than a few lifetime alternators before I got fed up with them and bought one from Advanced auto. That was back before I switched to Napa though. I can't say I ever got a bad electrical part from Advanced though, just some questionable t-stats and cheap tools.

 

[Jarnutt]

I'm just afraid that you'll end up being best friends with the counter guy at Autobone. I went through... more than a few lifetime alternators before I got fed up with them and bought one from Advanced auto. That was back before I switched to Napa though. I can't say I ever got a bad electrical part from Advanced though, just some questionable t-stats and cheap tools.

Im not too concerned about being best friends with him, Its just nice to have that guy on your side that doesnt make you wait an hour for something to test on the bench, just to tell you that their machine says its good when you know damn well its not. This applies to my buick that eats LS1 starters, Im good for one of those every year or so.

Im not a huge fan of Remans, but like I said, I have a receipt for this one in the glove box, so Ill give it a try, and if Im back here after I swap it this weekend, bitching that it took a crap, again, Ill try something else, fool me once or twice, but not thrice. Besides, I have an account with napa and a couple other places here in town that rebuild stuff for our diesel engines at work, Genstar and Turbo & Electric (which makes a kick ass T28 out of a stock T25 core). Ill figure this bi*** out this weekend hopefully and then resolve it.........I hope.

One last thing though is the charge light, does it not work when the alt is bad? I swapped bulbs around and checked that printed circuit board on the back, everything is good but the battery light just does not want to work.

 

[DSM1G90]

One last thing though is the charge light, does it not work when the alt is bad? I swapped bulbs around and checked that printed circuit board on the back, everything is good but the battery light just does not want to work

... it should - usually it pops on when the alt is not charging at all - (like bad brushes, or the like..)

Only thing that bugs me about the DSM and afterwards: no voltmeter in the gauge set. Chryslers had them in some of their gauge sets in their domestic lines, but the DSM missed the clue somehow.

 

[Jarnutt]

Only thing that bugs me about the DSM and afterwards: no voltmeter in the gauge set. Chryslers had them in some of their gauge sets in their domestic lines, but the DSM missed the clue somehow.

Thats why I just use my SAFC to check voltage, it matches up to my Fluke perfectly.

New alternator (reman from autozone thanks to warranty) is in and working, it was the problem I guess, all dash lights work again. Next step is, Im making a heat shield for the downpipe, or Im just wrapping it with some exhaust wrap, which would probably help with the under hood temps anyways.

Im marking this one resolved, stupid ass DSM alternators.