I was driving home Sunday when I realized my battery light was on. A little later my car was a little difficult to start but started. Next morning, it was completely dead. I thought my battery had finally died because it was from 2006. So I put it on the trickle charger and drove it to work the next day. I meant to pick up a battery after work but got stuck in the office. When I got home I decided to check it out with my multimeter. It seems may battery is holding a charge fine, but with the motor running it is not charging. Ie. Charge at battery was 12.7 volts with the car on or off. I cleaned the terminals and looked at the fuses on top of the battery but didn't see anything suspicious. After searching it seems like it could be my alternator, my alternator pulley or my regulator. How can I track down what it is?
Battery / generator light on, alternator not charging, battery holds charge
- Thread starter Keven
- Start date
I just went through this with 2002 jetta with 100,xxx miles. It is almost certainly your alternator/generator. I recommend a complete rebuild of the alternator with bearings, pulley, regulator and brushes.
The only difference in symptoms is that my battery light did not come on.
The only difference in symptoms is that my battery light did not come on.
CopaMundial
Veteran Member
- Joined
- Jan 30, 2011
- Location
- Southeastern PA
- TDI
- 03 Jetta Wagon 5sp (New to me Oct 2014) 03 Jetta 5sp (RIP Aug 2014)
Good troubleshooting guide for the overall charging system here:
http://www.myturbodiesel.com/1000q/multi/alternator-troubleshooting-voltage-regulator-repair.htm
One key indication to focus on is when does the battery light come on?
It should come on when you're starting the car, and then go off shortly after the car is running. If it DOESN'T come on at that time... but it DOES come on while you're driving, then you should focus on the alternator exciter circuit section of the page linked above.
Contrary to popular belief, the alternator will not provide electricity just by spinning. It needs this initial exciter voltage. That is provided through the light on the dash, along a cable through firewall, under the battery tray, to the electrical connector on the alternator.
While the alternator itself or the pulley are obvious common failures, that wire for the exciter circuit will sometimes fray under the battery tray, so it's worth a look.
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It comes on when I first turn on the ignition and continues after the car has started. It never turns off. I've got the hood up and a replacement alternator on stand by right now. Trying to troubleshoot to see if it is in fact the alternator before I take it out. I have 12.5 volts coming out the back of the alternator with the car on.
vanbcguy
Veteran Member
It is almost definitely the alt if that light is on. The only other possibility is a short to ground on the blue D+ wire at the alternator.
That light can only come on (assuming no wiring faults on the blue wire) if it is grounding through the alternator. That should only happen when the alternator isn't producing any power, as the alt will feed power back up the wire causing the light to turn out if it is working properly.
The fat wire on the back of the alternator will always read battery voltage as it is basically connected straight to the battery.
That light can only come on (assuming no wiring faults on the blue wire) if it is grounding through the alternator. That should only happen when the alternator isn't producing any power, as the alt will feed power back up the wire causing the light to turn out if it is working properly.
The fat wire on the back of the alternator will always read battery voltage as it is basically connected straight to the battery.
I noticed that (doh!) after turning the car off. Still trying to troubleshoot if it's in fact the alternator and not something else.
Some food for thought. On Sunday of last week I was in a minor minor car accident. Monday I drove to the airport with no issues and left my car there for 5 days. On friday, the car took a couple more turns then normal to start but started. This is when I first got the battery light. Not even a scratch from the accident and we drove away thinking there was no damage. Well, there was enough force to make it difficult to open my hood now. At first it wouldn't open at all, but after some lubrication it will open but not as smoothly as before. I wonder if this is somehow related and I have a bad connection or something somewhere.
vanbcguy
Veteran Member
It definitely should not be grounded, if it is the alt won't work...
There are 3 terminals on your alternator. One in your case I think goes to the ECU, one goes to the light in the dash (or really comes from the light in the dash) and a big fat one that goes to the battery. The alternator is grounded through its mounts to the engine so that would be the "fourth" connection really. The alternator needs to see power on the D+ terminal (the wire in the harness is blue) in the 2 pin connection in order to "excite" itself. This is super easy to test - just turn the key to "on" and measure at that terminal with it unplugged from the alternator. You should see battery voltage there. Also the dash light should go out when you unplug that connector, if it doesn't then there is a short in the wire somewhere.
To be honest I'd bet it is either the regulator or the brushes in the alternator. The brushes are attached to the regulator so really you will end up replacing both at once if you choose to go that route. The brush / regulator pack is available at most parts stores.
Now, if the brushes are worn out, in particular if they are worn unevenly then there is a good chance the bearings in your alternator are on their way out. If the bearings are going you will end up ruining the new regulator's brushes fairly quickly.
There are 3 terminals on your alternator. One in your case I think goes to the ECU, one goes to the light in the dash (or really comes from the light in the dash) and a big fat one that goes to the battery. The alternator is grounded through its mounts to the engine so that would be the "fourth" connection really. The alternator needs to see power on the D+ terminal (the wire in the harness is blue) in the 2 pin connection in order to "excite" itself. This is super easy to test - just turn the key to "on" and measure at that terminal with it unplugged from the alternator. You should see battery voltage there. Also the dash light should go out when you unplug that connector, if it doesn't then there is a short in the wire somewhere.
To be honest I'd bet it is either the regulator or the brushes in the alternator. The brushes are attached to the regulator so really you will end up replacing both at once if you choose to go that route. The brush / regulator pack is available at most parts stores.
Now, if the brushes are worn out, in particular if they are worn unevenly then there is a good chance the bearings in your alternator are on their way out. If the bearings are going you will end up ruining the new regulator's brushes fairly quickly.
I just did the blue wire test. With the alternator plug off the light goes out. When testing for voltage I get 11.9 at the plug with the ignition on (12.65 at the battery). So there is a slight drop but seems normal. I'm going to replace the alternator.
For the record, the blue wire is the one on the rounded side of the plug.
For the record, the blue wire is the one on the rounded side of the plug.
vanbcguy
Veteran Member
Probably the best course of action. It is probably rebuildable if you have a rebuilder in town.
Update:
I went ahead and swapped the alternator with a cheap remanufactured alternator from Advance Auto. This one specifically:
ToughOne Alternator - Remanufactured - 120 Amps
Part No. 12048
Normally $175 but there was a $50 off coupon floating around.
The process took a while because I had originally thought that I most likely had a bad pulley. When that passed the spin test I was worried about a misdiagnosis so I did a lot of searching on tdiclub to see what else it could be. Took about 4 hours for the entire process but only 2 hours for the actual replacement of the alternator. If I were to do it again I bet I could get it done in just over an hour. Anyway, I'm back in action and will report back if this thing fails.
I went ahead and swapped the alternator with a cheap remanufactured alternator from Advance Auto. This one specifically:
ToughOne Alternator - Remanufactured - 120 Amps
Part No. 12048
Normally $175 but there was a $50 off coupon floating around.
The process took a while because I had originally thought that I most likely had a bad pulley. When that passed the spin test I was worried about a misdiagnosis so I did a lot of searching on tdiclub to see what else it could be. Took about 4 hours for the entire process but only 2 hours for the actual replacement of the alternator. If I were to do it again I bet I could get it done in just over an hour. Anyway, I'm back in action and will report back if this thing fails.
Thats good, but did you do measure the voltage across the battery to confirm that the VR in the new alternator is regulating properly? It only takes a minute.
Check it at idle, then at 2500 RPM and then at 2500 RPM with some loads on (rear window defroster, high beams, seat heaters, etc). You should be seeing at least 14V.
CopaMundial
Veteran Member
- Joined
- Jan 30, 2011
- Location
- Southeastern PA
- TDI
- 03 Jetta Wagon 5sp (New to me Oct 2014) 03 Jetta 5sp (RIP Aug 2014)
Also if the alternator you took out is OEM (Bosch or other) then you may want to repair it anyway to have on hand for when the cheap one fails.
^ Wagner Alternators and Supplies, Inc. is where you can find all the parts you need for a DIYer rebuild.