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How can I make sure I have a bad altenator?

my battery doesn’t charge on my eclipse 2002 RS I checked the ground connections with a multimeter and they are all good. when I check the voltage on the battery with car started it just stays the same and slowly goes down and when I turn on more electrical lights dim and turn off. could a wire be unplugged from the altenator or a bad fuse or something else? I want to make 100% sure before I just buy an new altenator and replace it myself. Also the battery is only 9 months old and is in great condition. (I’m only 17 so all help is greatly appreciated!)

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Call Auto Zone and ask them if you can bring your alternator and battery in for testing. That way you will have your answer.

While it sounds like the alternator may be bad, do as @VOLVO_V70 suggests.

But your title reminded me of an old joke - two hunters out in the fields, on accidentally shoots the other. Calls 911 “What should I do??? I think I killed him!!” 911 operator: “First, make sure he’s dead.” Hunter: “OK…BAM!!! Now what?”

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In the engine compartment fuse/relay box, check if fusible link #8, 30 amp is blown, or if multi-fuse #13, 10 amp in the junction box is blown.

If either of these are blown, the alternator can’t charge the battery.



When I called AAA to jump start a car in which I had left the lights on, the tech came and he used a diagnostic device to supposedly check the “charging system” and battery. Even printed me the readouts to take to my mechanic because I told him I would have it checked just to be sure it was the lights. That particular vehicle can be left with the lights “on” and will normally shut off when the key is removed. However, it can be tricked in a couple ways. I was impressed, but I cannot speak to the underlying technology.

Good advice above to just take your car to a place that will do a charging system and battery test. Many shops & even some auto parts stores have a special test system gadget to do that job in a jiffy.

When you say the alternator doesn’t charge the battery, on what basis do you say that? Is the battery going dead after driving a few miles, the car stops, and you have to use a battery charger to re-charge it to get the engine to run again? If the alternator completely stopped charging the battery, that would be the symptom. Do you have any warning lights on the dashboard saying there’s a battery or alternator problem?

Ill add if I thought my Corolla’s alternator wasn’t up to snuff, the first thing I’d do is measure the battery voltage at the battery posts (leaving everything connected) with the engine off. It should read between 12 and 13 volts, usually around 12.6 volts at room temperature when the engine hasn’t been run since the night prior. If that measured ok, I’d start the engine and do the same measurement. With the engine running it should now read in the 13.5 to 15 volt range. As the engine runs longer amount of time after first starting a cold engine, and the engine warms up, that voltage will decrease over time toward 13-13.5 volt range. Once the engine warmed up and the voltage was in the 13’s, I’d then turn on some electrical stuff like the headlights to bright and the rear window defroster. The voltage shouldn’t drop and may go up somewhat. If all that tested out ok, I’d say my alternator was working correctly.

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Sorry, but I disagree with Volvo and texases :frowning2:

If you want a proper diagnosis, you need to test the whole charging system, with everything installed

That way, you’re taking the belt, wiring, battery, fusible links, alternator, into consideration

If you bring the alternator and battery to a shop to be tested, you’re NOT getting the big picture

If autozone says the battery is okay, but the alternator is not, then hopefully that is your only problem. But I’ve seen enough vehicles where multiple problems exist.


I read it as bringing the car in, which is what I’d do. Looks like I misread Volvo’s comment.

It doesn’t charge the car at all I have to charge it with a charger. And no there is no warning lights on for the charging system.

Well, this shows you that the charging system has a problem. Follow the wire trail all the way from the alternator to the battery, and also the wire from the battery to ground, and make sure ithey’re all connected at both ends and not broken somewhere in the middle. If they’re in good shape, then that’s a pretty good indicator, assuming the fuses @Tester told you about are good, that the alternator is probably bad.

Get it tested at an auto parts shop, but you might want to get it tested at more than one. I had my alternator tested by a parts shop once, and they diagnosed it as bad. Thing kept going until I sold the car 5 years later.

Always remember that those free tests are there for one reason: To get you in the door and sell you parts. Any time a tester has a financial motivation to find bad parts, you are at risk that they will “find” bad parts that are, in fact, perfectly fine.

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Just checked the fuses their all good so I’ll check the wiring. I did see that this wire isn’t plugged into anything and it comes from the same harness as the other wires going into the alternator but I didn’t see anywhere to plug it in last time I was under the car so I’m amusing its not needed.

I assume that red plastic thing under the hose under your hand is not that plug’s mate.

I don’t believe it does it came from the middle of the harness while the others are at the end. Also that rubber red piece under the hose is to protect the wires that are plugged into the alternator its a rubber neck.

I think that red thing is the rubber boot covering the hot positive lead connected to the alternator. That plug sure looks like it needs to be plugged into something though so I’d have a good look with a strong light and mirror. Any previous work done? Doesn’t look like it would have become unconnected on its own. Got a wiring diagram to see?

Just my little story to always check your work. It was about 20 below out and I had replaced or overhauled the alternator in the car (in the garage). A couple days later the wife was stalled at the grocery store with a dead battery. So bought a new one and put it in in the freezing cold in the parking lot. Only to discover I had failed to connect that positive lead to the alternator. Probably would have needed a battery sooner or later anyway.

How can I make sure I have a bad altenator?

Well, you could pound on it with a big hammer until it breaks.


If the alternator is working but not connected properly to the battery this symptom could result. The way to tell that is to measure the voltage between the big output terminal on the alternator and the battery positive post. When the engine is running, idling in the driveway. It shouldn’t measure more than 0.25 volts, usually less. When you do that you can also measure the voltage output of the alternator by probing between the big output terminal on the alternator and the alternator case.

My Corolla’s alternator wiring harness has a similar unused connector just left dangling. That might be the case for yours too. I’ve always presumed it’s configured that way for some option my Corolla doesn’t have, like AC or automatic transmission, cruise control, charging meters, etc.

I took a look at the wiring for your chargings system, looks to be

  • between the alternator output terminal and the fusible link to the battery there are two wires run in parallel, both are w/r (meaning white with a red stripe I think), but one is 8 awg and the other is 5 awg, so one is thicker than the other.
  • from that same fusible link there’s two white wires in parallel, 8 and 5 awg again, that run to a 7.5A dedicated fuse, from that fuse there’s a r/w wire that goes to the alternators voltage regulator (pin 2),
  • there’s two wires from the alternator voltage regulator (pin 1 and 4) to the engine computer , one is y/b, the other is b/r
  • finally, there’s a w/g wire from the alternator voltage regulator (pin 3), that goes to a charging meter then to the ignition switch . since you say your car doesn’t have that meter that may be the unused wire you are seeing.

I know Advance Auto offers a free battery and alternator test as well.

Most alternator circuits have a battery warning light in the dash to light up when there is a charging problem. The lamp light is also used to pass current to the alternator field windings. If the light doesn’t work for whatever reason the charging system won’t work either. I assume your vehicle is designed like this and if so make sure the battery warning light turns on when you turn the ignition switch to ON. If the lamp turns on then that part of the circuit should be okay. You then need to check the voltage between the main output lead of the alternator and the positive battery post while the engine is running and the lights are on. There should be less than .3 volts across that lead if the connection is good. That wire should be hot to the battery at all times so don’t let anything come in contact with it except the test leads of the voltmeter.

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I agree with GeorgeSanJose I test mine with a multimeter some are free at Harbor freight stores. anyway overnight and not started in the morning the volts should read about 13.5 -14.7 now start the engine the volts should read 14.9 - 15.1 if you do this AND GET THESE READINGS YOUR ALTERNATOR IS GOOD…

I think maybe that free voltmeter is a little out of calibration. 13.5 volts on a battery at rest is a little high, and 15 volts charging is also a little high and putting a big strain on the alternator. In my view anyway. More like 12+ and 13+.