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Resolved 2G Alternator works but it won't charge the battery..

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randizzle420

10+ Year Contributor
224
19
Sep 20, 2009
El Centro, California
I just bought a new battery, and it kept going dead when I was driving it. I was pretty positive it was the alternator because I tested the battery when the car was on, and it was only 12 volts. I took the alternator off, took it to pep boys, they tested it, and the alternator is fine, was putting out 14. something volts. Any ideas what the problem might be?
 
i was having a similar problem, except i have a new alternator AND battery. i never attempted to fix the problem, but my guess is probably wiring somewhere.

my car would have trouble turning over or whatever else in between, then i hooked up an air fuel controller and have it monitor my volts.

when you run electronics in the car (cigarette lighter, radio, turn signal, headlights, etc.) obviously it uses the battery power...but overdrawing power things (an amp, 55w silverstar headlights, holding on the brake at a long light, etc) drains power faster, dims light, and hurts performance

my advice to u if you are concerned is buy a cheap voltmeter, and monitor it yourself. wanna spend a bit more? get a voltage gauge or an air fuel controller.

*edit* also, try just driving it around constantly for about a half hour at high speeds (get on the highway and then turn around) to see if the alternator charges the battery. at off, the battery should be between 12.8-14.8. if it doesn't then you may need a new alternator.
 
The battery is new and the alternator was just tested at pep boys and the alternator works. I did drive it on the highway and the battery died on me and I had to get my car towed.
 
something in your car is drawing alot of power to cause it to die. now if its dying while it's running but stopped...thats one thing. if it's dying while its moving, that's another. you probably have a short somewhere. check all alternator wiring, make sure the battery terminals are clean and that the negative is properly grounded.
 
well I put a new ground in for the battery, cleaned the posts and everything. I'm still having the same problem with the battery not being charged by the alternator. The battery light just stays on in the car. Also, if I turn my headlights on then turn them to the off position the headlights stay on. Only way to get the headlights to turn off is to unplug the battery.
 
then you have some type of crazy wiring problem. i'd get it checked out
 
to me it sounds like you have a problem with your voltage regulator within the alternator.
- the issue with your headlights could be when your alternator is putting out around 16v-18v the relays in your car like your headlight relays act very funny and do crazy things.

- i got a "rebuilt" alternator from a parts store and the voltage regulator was only charging 12.3v at idle but above that at around 2k and up it would act normal aka why pepboys thinks its good. but if you let your car idle for a few seconds it will drain your battery. so it will not start back up once you shut it off but if you hold the rpm at 2k for a few mins and shut it off at 2k then go to restart it, it will start up.

- like stated above your car shutting off WHILE driving sounds like a short or a poor ground that can be more than just at the battery. when it shuts off do you lose all power to accessories also?
 
Aftermartet alts are known to have voltage regulators that don't work right with our cars or somethinhg to that effect. I put 3 pep-boys alts in my car in 2 mth period and have read many times about other dsm'ers doing the same thing with atouzone alts, advance, etc. Best bet is to get an oem alt and pm me if you want a good used oem mitsu alt cheap because I'm parting out one of my dsm's.
 
I found the culprit! It was a bad alternator relay. I replaced it and now everything runs good.
 
Solution
I know you took your alternator off and to pep boys to test it, but you should test it with it still in the car to figure out what your problem is. Use your multimeter (if you don't have one get one). Turn it to dc volts put your leads on your battery (with the car running), if your alternator is charging it should read from 13.5V-14.5V(thats a good charge). If your not getting a reading from there you can check your output from your alternator. Make sure you ground your multimeter, and put the power side to the stud that is coming out from the alternator(should be nut holding your wires that go to your battery). If your getting 13.5-14.5V there then the connection between the alternator, and battery is bad. Hope that helps! :thumb:
 
I found the culprit! It was a bad alternator relay. I replaced it and now everything runs good.

I was just reading through your post and thinking, sounds just like a blown ALT fuse. Glad you figured it out. Wish I'd seen your post earlier, I would have pointed you in the right direction. Good luck!

Also, don't forget to mark this thread as solved/resolved.
 
So my alt spikes up to 17v to 18v for a few min at a time then it will run the normal 14.v. Would this be the alt relay too. It is a new alternator. Or would it be the voltage regulater is bad in my new alternater.
 
That's a bad regulator hands down. Sounds like its not regulating anything anymore and its sending full field voltage to it causing it to max out on the charge. Anything over 14.8 is abnormal usually, anything over 16 can start to eff up electronics, you'll be lucky if your ECU/TCU survive too many of those spikes
 
Just for future reference, a quick easy way to check if an alternator's dioeds are bad is to get a Multimeter, put it on AC VOLTAGE. put a lead on each terminal while the car is running. If you get anything more than .5volts one or more of your diodes are going bad.

Diodes convert AC voltage to DC voltage which can be stored in your battery.
 
I have the same exact problem on my 1995 Tsi but I can't find where the alternator relay is on my car, can you please point me in the right direction. My car has been down for about 4 months now with this problem and I've replaced the alternator, battery and ecu. I would appreciate any help. Thanks in advance.
 
I'm having kind of a similar problem. But my accessories and lights don't work. The car runs and starts up fine but the battery will die after running it for a while. I can't even roll up my windows. What do u think it is
 
I was having a similar issue, turns out that the bolt on the alternator rattled off and was causing it to not start after i turned it off. in which i had to jump it every time i started it. hope this helps someone
 
Yea I have the same problem too, it hella annoying. Having the the Batt and alt. Monitored while driving its fine but at idle with headlights radio radar ect ext. it bites.....even with a new OEM 70 AMP ALT. what bites even more still acts the same way with the 90 amp alt.
 
Any info on the location of the relay would be helpful to me as well. I have the same issue. New battery and alternator tested fine and it still doesn't charge
 
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