Low voltage situation

Hey guys, I'm hoping for some help with something, I've been reading a lot of old posts and there's a big difference in my situation that has no explanation as far as I can tell.

My battery has been dying, won't keep a charge. Had the system checked by AAA and it shows low voltage on the charging system: No Load-11.88V, Loaded-12.15V. With a multimeter and the car running voltage fluctuates fast between 12 and 15 V. Battery just showed it needed trickle charging, didn't fail.

Battery is 4 years old so I originally thought that might be the issue. Alternator is 1 year old, and was a new Bosch unit. A quick check of wiring and grounds didn't reveal anything weird but maybe I need to take a closer look.

Here is the bizarre thing: My charging indicator light has not turned on. I have verified that it illuminates when I turn the key in the ignition. A year ago when my alternator went bad the battery light alerted me to it. Anybody have ideas? Thanks.

First things first... What Yr, make, model of car do you have?
Your signature says you don't have any TDI's (yet)

The battery light is controlled by the alternator not by voltage. Recently i saw a explanation of which wire runs this but I forgot the post.


Where did this "new" bosch alternator come from? Also do you know if it had the proper clutched pulley on it?


From what ive seen, the bosch units are only really out there in a reman form now. Unless they are just so expensive nobody even offers them as new.

ffemtp said:

First things first... What Yr, make, model of car do you have?
Your signature says you don't have any TDI's (yet)

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Sorry, 2002 2.0L Jetta Wagon, gasser. I used to have a B4 Passat Tdi and found people on this forum pretty knowledgeable so I still come around.

AnotherPerson said:

The battery light is controlled by the alternator not by voltage. Recently i saw a explanation of which wire runs this but I forgot the post.


Where did this "new" bosch alternator come from? Also do you know if it had the proper clutched pulley on it?


From what ive seen, the bosch units are only really out there in a reman form now. Unless they are just so expensive nobody even offers them as new.

Click to expand...

What does that mean though? That if the battery indicator isn't lighting up and I have confirmed the bulb is good, it can't be the alternator?

The new Bosch alternator is from German Auto Parts, it does have the slip clutch pulley. I'll doublecheck but I'm pretty sure it was new as there was no core charge.

EDIT: Yes, it was new.

With a swing like that I'd check the ground points. There are a lot of them on that car! If it is set up at all like the TDI there is one under the battery tray that can be susceptible to debris, and corrosion a bit more than the rest. I'd check as many ground points as you can find and clean and retighten them. This would be a cheap first step before diving into the alternator / voltage regulator. Most of the big name auto parts stores will do a starting / charging test for you for free. That may also give you some guidance, but I'd check the ground points first, personally.

ffemtp said:

With a swing like that I'd check the ground points. There are a lot of them on that car! If it is set up at all like the TDI there is one under the battery tray that can be susceptible to debris, and corrosion a bit more than the rest. I'd check as many ground points as you can find and clean and retighten them. This would be a cheap first step before diving into the alternator / voltage regulator. Most of the big name auto parts stores will do a starting / charging test for you for free. That may also give you some guidance, but I'd check the ground points first, personally.

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Planned on it, will do. Thanks.

intro said:

What does that mean though? That if the battery indicator isn't lighting up and I have confirmed the bulb is good, it can't be the alternator?

The new Bosch alternator is from German Auto Parts, it does have the slip clutch pulley. I'll doublecheck but I'm pretty sure it was new as there was no core charge.

EDIT: Yes, it was new.

Click to expand...

Bosch doesn't take back cores even on the reman units. Confirmed thru idparts on there page it says the same thing.


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Hope this stuff helps you out. Be sure to keep us updated....

For what it's worth the alternator can easily be bad and not turn the dash light on. Some failures will trigger the light, some will not.

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There are a few things you can do to determine where your problem lies. The easiest to do is a voltage drop test from the alternator output stud to the battery positive post. Then from the alternator case to the battery negative post. Both tests are with the engine running, meter on DC volts. Any reading above .5v is excessive and indicates a problem between where the leads are placed. A negative reading means the leads are swapped.
Charging voltage normally runs from 13.5 — 14.5 volts. Your initial reading indicates the battery isn't being charged.
Also check all connections to the alternator - non should be lose.

ffemtp said:

With a swing like that I'd check the ground points. There are a lot of them on that car! If it is set up at all like the TDI there is one under the battery tray that can be susceptible to debris, and corrosion a bit more than the rest. I'd check as many ground points as you can find and clean and retighten them. This would be a cheap first step before diving into the alternator / voltage regulator. Most of the big name auto parts stores will do a starting / charging test for you for free. That may also give you some guidance, but I'd check the ground points first, personally.

Click to expand...

stamp11127 said:

There are a few things you can do to determine where your problem lies. The easiest to do is a voltage drop test from the alternator output stud to the battery positive post. Then from the alternator case to the battery negative post. Both tests are with the engine running, meter on DC volts. Any reading above .5v is excessive and indicates a problem between where the leads are placed. A negative reading means the leads are swapped.
Charging voltage normally runs from 13.5 — 14.5 volts. Your initial reading indicates the battery isn't being charged.
Also check all connections to the alternator - non should be lose.

Click to expand...

Looks like you guys were right. I cleaned the grounds under the battery tray, though they looked ok. When I went to remove the + strap from the alternator it felt loose a bit, removed it and it looked like it needed a little sanding. Retightened and everything is well again.

One thing I was wondering about, and I feel like I made this mistake at some point years ago doing an alternator on another car, as I tightened I heard the black plastic casing of the alternator begin to give/crack, so I backed off a bit. If memory serves when I cracked the piece years ago it didn't matter much as far as the function of the alternator, but it's possible I was extra careful installing this one. It wouldn't loosen if it was torqued to spec I'm guessing. Thanks everyone! Relieved the solution was simple.

Glad it worked out!

Just for future reference... the battery idiot light is called an idiot light for a reason... it often lies about charging system issues, particularly if the issue is subtle.

A voltmeter is your best diagnostic, as you found out. 12.5V or better, engine off, 13.5V or better, engine running. Else something's going on, regardless of what the idiot light says.