My battery light came on...

[runningallday]

mk4 jetta TDI, 1999.5 manual, 155k miles

My battery light came on the other day, it was ridiculously hot and humid and I was stuck in stop-and-go traffic, so hot that the radiator fan turned on.

So at first, I just figured that the battery was just under stress or something? Since then, I've learned that the "battery" light is actually more an "alternator" light.

The battery soon died and would no longer start when I parked at home.

I took the battery off and charged it up - a day and a half later, I put it back in the car, expecting to be able to drive around until the battery died. But something strange happened.

The battery light stayed on - and then, once I revved the engine above 1500 rpm, all the gauges turned off and went to zero. I googled this, and they suggested the voltage given off by the alternator would be too high.

And so I checked this with my multimeter - and upon revving, the voltage across the battery went up to 17 volts really quickly. I haven't even turned the car on again since then.

I read online that "the voltage regulator can fail open or closed" - so it seems like at first, it failed closed, and now, for some reason, it failed open? I tried to read more about the brushes on the voltage regulator and the mechanism behind how they could fail - but couldn't find an explanation for this.

I checked the connections/wires/fuses on top of the battery - all very good. I checked the resistance of the wires between the alternator and the battery - both pretty low?

Unfortunately my car is not in a strategic place to have the alternator taken off (in my apartment parking lot), so I hope I can solve this problem without lifting up the car. So I decided that I'm going to try to replace the voltage regulator on the alternator, this looks doable (only 5 bolts from an awkward angle?). A new voltage regulator is coming in the mail later this week and I'll tear apart the car then.

Is "failure of the voltage regulator" a reasonable explanation for these symptoms?

Is there anything I missed, could it be anything else like some bad wiring?

Could anyone speculate on whether I'll end up needing to replace the entire alternator?

My first post - this community has helped me out so many times but previously I haven't posted anything - thanks everyone!

 

[Mavrick]

If connections are good, voltage regulator should fix it. Alternator replacement is necessary if the bearings or the rotor are damaged/worn out.

 

[PDiesoiler]

A word of caution!!!

An overly charging battery can boil the electrolyte into hydrogen gas. One spark and BOOM!

Seriously...I had this very thing happen once and when I fired off the ignition it blew the whole top of the battery off. All the neighbors came out to see what the explosion was. Check the electrolyte and Be safe with your face.

 

[oilhammer]

I would replace the alternator with a Bosch reman unit. It will come with a new Bosch regulator and a new INA clutch pulley, along with of course everything else all fixed up perfect. 120A if a manual car, 90A if automatic.

Also be sure the battery is the proper 80Ah size, and that the connection in the fuse link box is good.

 

[runningallday]

If connections are good, voltage regulator should fix it. Alternator replacement is necessary if the bearings or the rotor are damaged/worn out.

So far I haven't heard any problems, the belt seems fine... (crossing my fingers for the regulator to take care of everything)

A word of caution!!!
An overly charging battery can boil the electrolyte into hydrogen gas. One spark and BOOM!
Seriously...I had this very thing happen once and when I fired off the ignition it blew the whole top of the battery off. All the neighbors came out to see what the explosion was. Check the electrolyte and Be safe with your face.

How long did you charge it before it exploded? I've only left it running for < 1 min for diagnostic reasons, definitely not turning it back on now haha!

I would replace the alternator with a Bosch reman unit. It will come with a new Bosch regulator and a new INA clutch pulley, along with of course everything else all fixed up perfect. 120A if a manual car, 90A if automatic.
Also be sure the battery is the proper 80Ah size, and that the connection in the fuse link box is good.

I upgraded the battery many years ago, to help it start on the coldest days in the winter (although after I replaced the starter last year, it's been starting a lot better year-round)... You're saying my battery might have a compatibility problem?
Just checked the prices, the cheap alternators are $180, the bosch remans are $237, yeah I'll definitely get the bosch for that price difference. The labor is what will kill me if I have to end up replacing the whole alternator.

 

[PDiesoiler]

"How long did you charge it before it exploded? I've only left it running for < 1 min for diagnostic reasons, definitely not turning it back on now haha!"

In my case the alt was charging at 24V for an undetermined amount of time. There was no over charge system warning light.

You should be ok if it hasn't been over charging for long. You can always top off each cell with distilled water. Most sealed batts have screw caps under the label on top. should be 6 of them. This too will let any hydrogen gas that has accumulated out.

I've learned to always protect my eyes around batteries since then.

 

[ffemtp]

Just as a side note, if you have an AGM battery (like an Optima, Odyssey, or the like) do not attempt to open the battery. They are 100% sealed. This might seem obvious, but thought it was worth at least mentioning it.

 

[runningallday]

The regulator came today in the mail. I need to find a few sockets and a small screwdriver, and then I'm going to try to install it tomorrow morning.

I went down to look at my car, and noticed that the battery was dead. I measured the voltage... it was 1.5 V.

So it took a few days - but the battery went from 12.5 volts (after being recharged in my apartment) to 1.5 volts.

Either the battery is somehow broken and loses charge on its own, the alternator is sucking out power, or there is a power leak somewhere in my car?

I was really hoping I wouldn't have to replace the entire alternator but at this point, that looks likely.

Also looks like I'm going to have to buy a new battery.

 

[williambill]

Just bought a Bosch reman alt. for my 2002. Rock Auto has it for $192. If you have an Advance Auto Parts, they will price match Rock Auto, but be prepared to show them their own policy on price matching on your phone or have it printed out before hand. This is what I did so I could core out my old unit locally. Hope this helps.

 

[runningallday]

Ok update -

I just finished swapping out the voltage regulator (while the alternator is in the car). it took 1.5 hours... absolutely brutal!!!

In all, I believe there were 6 bolts? so you'd think it'd take about 5 minutes right? I had so much trouble getting my screwdriver bit at the right angle. I ended up using a bunch of different bits, screwdrivers, extensions, ratchets, and a vice grip to get it all done.

So much stuff in the way! (Also, I have an aftermarket oil sensor that was a huge obstacle.)

For the first half of this process, I felt certain that the whole alternator was bad, and that I was just wasting my time swapping out this voltage regulator. But over time, I slowly made progress, and had to keep reminding myself that I only had to swap out this stupid regulator. If it doesn't work, then I'll just have to figure out how to replace the whole alternator... But if it does work, then... fantastic!

Anyhow: the old regulator had brushes that were about half the length of the new one. I took a look at the poles that the brushes rub against, they looked like they might have shallow grooves in them? But at this point I ignored this and put everything back together. Next time, I will replace the whole unit.

I checked the resistance between the "power out" from the alternator and the case. I read that this is a good way to test to see if the diodes are fried? It passed! (in one direction it was "1", and switching the probes the other way, it was some random number)

So after putting it all back together, I threw my battery (fully charged again) back in it. btw, is it supposed to have a small spark when you reconnect the battery?

I tightened down the connections, then went to start the car.... and my alarm went off.

Ok I shut off the alarm, then started the car... and IT WORKED!!! no more battery light!!!

So now - tomorrow I will go check on the battery, see if it's drained down at all by morning (I noticed it was draining before, I don't know if that was because of the bad voltage regulator, or if I have some power leak somewhere).

if the battery's ok - then I will take it for a loop around the block and see if things are stable. i would do it tonight but it's 2 am.

I probably spent ~5-10 hours reading about alternators and how to diagnose the electrical system in mk4's... and i spent $40 on a new voltage regulator, $10 on a in-dash voltmeter (might be useful next time I have alternator problems!), and spent 1.5 hours leaning over my car, sweating like crazy.

so, all in all, not a bad deal! i feel pretty lucky right now. it seems like the battery might not even be ruined!

 

[runningallday]

quick update -

last night the voltage of the battery was 13.0 (charged to 100% by a charger). this morning, it was 12.8. so either i have a power leak (really hope it's not the alternator), or the battery has as bad cell.

after turning it on, though, it was charging at 14.5 volts, so looks like the voltage regulator is working better now!

 

[BobnOH]

Could be a power leak, not a bad cell. Could be nothing, maybe check after it sits longer

 

[Genesis]

12.8v is normal for a fully-charged lead-acid battery that has been allowed to rest after charging. If you let it sit overnight and have 12.8v it's still full. (A serviceable but somewhat-older battery can read as low as 12.6v and still be "full" after it is allowed to rest for several hours post-charge.)

You need at least 13.8 (and no more than 14.8) volts to fully charge a flooded lead-acid battery in order to clear normal sulfate deposits that form on the plates when it is discharged.

From the evidence at this point it appears your regulator was shorted.

 

[runningallday]

From the evidence at this point it appears your regulator was shorted.

Yep I wonder how many people replace the entire alternator, when they could just switch out the $40 voltage regulator without even removing the alternator from the car...

 

[Genesis]

All of those who take their car to a repair shop.

Oh, and all who have a bad pulley too.... and by the way, the Stealer wants close to a grand in parts and labor to do that job.

 

[roadhard1960]

Yes, you changed the regulator. I did that too. You forgot to change the alternator bearings. If it had enough miles to wear out the brushes, it has enough miles that the bearings are not long for this world. Yes, my bearings started making noises a year after changing the regulator. I could not figure out where the noise was coming from. I figured it out when smoke came out from the hood. The alternator started on fire. A splash of water put out that fire, I called the towing service and a few days later I was back on the road with a brand new Bosch alternator. I do need to change the bearings out on the truck's alternator as part of PM.

 

[runningallday]

Yes, you changed the regulator. I did that too. You forgot to change the alternator bearings. If it had enough miles to wear out the brushes, it has enough miles that the bearings are not long for this world. Yes, my bearings started making noises a year after changing the regulator. I could not figure out where the noise was coming from. I figured it out when smoke came out from the hood. The alternator started on fire. A splash of water put out that fire, I called the towing service and a few days later I was back on the road with a brand new Bosch alternator. I do need to change the bearings out on the truck's alternator as part of PM.

3 years later, alternator still working! (I don't put a lot of miles on it though)