GenVibe - Community for Pontiac Vibe Enthusiasts

Vibe wouldn't start this morning and had to get a AAA jump. Started right up with boost.

Tech ran their test, which said the alternator was fine, but the battery had to be charged and tested again, so I drove it around for an hour, then went to O'Reilly's.

With the car on, O'Reilly tested the battery which came back as good but the voltage was dropping as he tested it. It started at 13.5 V but after 5 min. had dropped to 12.4 V.

After sitting for 4 hours in the cold, the battery voltage is sitting at 12.6 V. After starting the engine with all the electrical draws off, voltage reads at 14.2. Turning on the radio, A/C, headlights, fog lights, and all the wipers, voltage drops to 12.6. After 5 min. of monitoring it, voltage is now at 11.9.

Does that sound like an alternator that is on the verge of failing completely?

Sounds like a bad alternator to me if:

1)The battery is relatively new and the connections are tight
2)It's the original alternator (what is your model year?)

Battery is 8 months old and I just cleaned all the terminals before checking with a voltmeter.

As far as I know, its probably the original alternator.

The alternator test while installed in the car is iffy at best. Battery test is ok but not perfect.

Salsa Guy wrote:The alternator test while installed in the car is iffy at best. Battery test is ok but not perfect.

Would you suspect that the voltmeter wasn't lying though?

wintersilke wrote:Vibe wouldn't start this morning and had to get a AAA jump. Started right up with boost.

Tech ran their test, which said the alternator was fine, but the battery had to be charged and tested again, so I drove it around for an hour, then went to O'Reilly's.

With the car on, O'Reilly tested the battery which came back as good but the voltage was dropping as he tested it. It started at 13.5 V but after 5 min. had dropped to 12.4 V.

After sitting for 4 hours in the cold, the battery voltage is sitting at 12.6 V. After starting the engine with all the electrical draws off, voltage reads at 14.2. Turning on the radio, A/C, headlights, fog lights, and all the wipers, voltage drops to 12.6. After 5 min. of monitoring it, voltage is now at 11.9.

Does that sound like an alternator that is on the verge of failing completely?

The voltage drop from 13.5v to 12.4 that you witnessed is normal. As far as the alternator load test you performed, when you turn on accessories and check voltage you always want to have the engine running at 2000 revolutions per minute. It is not uncommon to have low voltage at idle will all of the accessories on. Try the load test again with the engine at the proper RPM.

It sounds to me like the alternator and battery are fine. If I had to guess I would say you have something draining the battery. I would see what kind of amperage is being drawn with the car just sitting there.

But shouldn't the alternator mostly keep up with the load on it?

I reran the test later and after 20 min. the voltage had dropped to 11.1 V with everything on and the engine was sputtering.

wintersilke wrote:But shouldn't the alternator mostly keep up with the load on it?

I reran the test later and after 20 min. the voltage had dropped to 11.1 V with everything on and the engine was sputtering.

That still doesn't seem out of the ordinary. It wasn't designed to hold a full load at idle. You really just need to perform a proper alternator load test if you're worried about it.

Okay...so now I've got to ask. What is considered a proper alternator load test? Is it something I can do with a simple voltmeter or do I need something more complicated?

wintersilke wrote:Okay...so now I've got to ask. What is considered a proper alternator load test? Is it something I can do with a simple voltmeter or do I need something more complicated?

Perform the whole test with the engine at approximately 2000rpm. First turn all the accessories off and check battery voltage. It should be around 14v. Now turn all the accessories on and check voltage again. It should be no less than 12v.

My more knowledgeable neighbor from across the street came over to help.

We did the 2000 rpm test. With everything off, voltage was 14.20-14.24.

With everything on, voltage was 12.1 but began dropping. After about 4-5 min. it was below 11.9 and still dropping.

It's hard to tell without being there. At this point you need to use an amp clamp on the alternator cable to see what kind of amperage it's putting out. Autozone will perform this test for free if you don't have one.

I would also check for a draw. Make sure all the dome lights are off and they key is out of the ignition. Disconnect your negative battery terminal and set your DMM to mA. Connect one lead to the disconnected negative battery terminal and one to the negative post on the battery. Draw should be less than 20mA (0.02a).

the key word i keep paying attention to is the voltage dropping below 12v while running.

i would suspect the alternator itself. the internal voltage regulator might be failing after running a while, hence the voltage dropping to less than 12v.

for proper electrical operation the alternator MUST generate a minimum of 12 or more volts while running/driving. any time the alternator is generating less than 12v while running/driving then the remainder of the required voltage would be pulled from the battery. continued operation of the vehicle under this condition would lead to a discharged battery.

i am suspecting the alternator is failing.

We replaced the alternator and with a full load with everything on in, the terminals show 14.1 V so I call that a success.

wintersilke wrote:We replaced the alternator and with a full load with everything on in, the terminals show 14.1 V so I call that a success.

Glad you got it fixed. For future reference, a fully charged battery is 12.6v or more. Any time the charging system is outputting less than 12.6v the battery is being discharged and that is NOT normal.

beemerphile1 wrote:

wintersilke wrote:We replaced the alternator and with a full load with everything on in, the terminals show 14.1 V so I call that a success.

Glad you got it fixed. For future reference, a fully charged battery is 12.6v or more. Any time the charging system is outputting less than 12.6v the battery is being discharged and that is NOT normal.

As an engineer, I knew that last part.

Thanks everyone.