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David, VW Master Technician
Category: VW
Satisfied Customers: 2366
Experience:  VW Master Technician Certified for 9 years, ASE Master Technician Certified
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Volkswagen Beet;e: I have a 1999 VW Beetle that battery light

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I have a 1999 VW Beetle that battery light just came on in. I tested the output voltage of the alternator and it's only 11.59 VDC. The battery has 11.62 VDC. I currently have the battery on a charger. I took the alternator off and took it to a local autoparts store to see if it's still good or if I need to replace it. They said it was still good. What do I need to look for on the vehicle that would cause what seems like a short or grounding issue?
Did you test the voltage on the out put back side of the alternator or at the battery?
Customer: replied 3 years ago.

on the output side of the alternator, the post of the alternator that has the wire the runs from it to the fuse box above the battery. Goes to fuse 177 if I am not mistaken.

Not sure how Auto parts tested this, but if you are getting no voltage out of the back, then it is not charging. The alternator has a small wire that comes out of the side and it is the cluster signal, the other wire, large black goes to the battery fuse box. If you have no charge voltage, Over 12.6 on the back of the alternator then the alternator is bad. If you have 14 or so volts on the back but not at the battery, I would say the charge lead is bad, seen a bunch of those, but you would see burning on the fuse box wire side. I say replace your alternator,.
Customer: replied 3 years ago.

I have 0.1 ohms of resistance from the wire/lead that connects to the back of the alternator to the fuse box. I have 0.1 ohms of resistance on the fuse itself and the wire that connects from the fuse to the positive battery post terminal. I even went as far as testing resistance from the positive batter post terminal to the end of the lead that connects to the alternator and have 0.1 ohms resistance in that also. There are two wires that plug into the alternator in a standard clip connector. I take it that one has power at all times and that there is some type of relay or resistor that lets voltage go through to illuminate the cluster signal when the alternator is not performing as it should. Is that correct?

Ohms tests do not tell you anything. You can have one strand of wire and have good connection, but under load it will have a very high voltage drop. To properly test circuits, they are done under load and a voltage drop test is performed. Simple way to do this is start the car, with the running and the DVOM, put on volts DC. Place the positive lead to the back of the alternator, then place the negative lead on the Positive battery cable. Anything more than 0.10 volts you have a problem.

FYI, ohms are used to test sensors, and items like that, volt drop tests are done on large circuits

Customer: replied 3 years ago.

Makes sense, I remember that from my automotive degree plan I am currently taking. Brain fart on my end. However in a slim defense, I didn't have the alternator in the vehicle to perform the procedure you mentioned, so I utilized the ohms test. I work on diesel equipment in the military and I am used to there being a activation wire for a voltage regulator. I am a novice to say the least on the civilian side so not seeing a activation wire threw me off, because I was wondering what activates the alternator regulator and controls the voltage that the alternator produces. I undertand that this isn't automotive electrical 101, lol, but if you wouldn't mind I would like to know.

Everything is self contained in the alternator. Very similar to big rig alternators. Once it starts to generate an A/C pattern the voltage rectifier converts it into DC and the regulator controls output. Does not need an activation wire, as it is all self contained. The wire coming out is the signal to the cluster when something is wrong. You should be able to place the alternator on a test bench and spin it up, and get 14 volts out of it. But it really needs to be tested under load, to make sure it is working correctly. Again you can have 14 volts coming out, but it can crash under load, which would indicate a bad alternator.
Customer: replied 3 years ago.

Thanks for your patience and help!! Have a good evening!!

You are very welcome, and please let me know what you find. Have a good night.
Customer: replied 3 years ago.

Just got done putting everything back together and the alternator is putting out 14.24 vdc. Did the load test as your described earlier and got .004 as a readout. Don't know what the issue was, but it is fixed for right now. Once again, thanks for you help!!

could have been something not making a good contact on the charge lead. Keep an eye on it and if it stops charging again, then do the voltage drop test between the back of the alternator and the battery. Might be a bad charge lead cable
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