Alternator, alternator pulley removal, and alternator troubleshooting for VW and Audi

This article shows how to remove the alternator, alternator pulley, voltage regulator, and troubleshoot the charging system

difficulty: 2/5


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Your car's electronics run off a 12 volt direct current system and is recharged by the alternator as the engine runs. The alternator converts alternating current made by the alternator to direct current used by the car. The alternator is driven by a belt coming off the crankshaft pulley (there's no such thing as free energy). If there's an alternator problem the battery will drain which could cause the car to not start or shut down. If you are experiencing a battery problem while driving, find a safe spot to pull over and call for assistance. Depending on how much battery charge is left and how poor the charging issue is, the car could continue to drive for a while further or shut off within minutes. Batteries and alternators are wear items and should be replaced as needed. Some tips here are specific to Volkswagen and Audi but the basic troubleshooting principles apply to most modern cars.

NEVER remove the battery cables and try to run the engine off the alternator without the battery. This will blow the alternator or your car's electronics by producing very high voltage.

SAFETY WARNING: remove any conductive rings, jewelry, or watches when working on the alternator, starter, or battery system! Make sure your wrench doesn't contact any live wires and ground or else it will act like a welder. The battery contains hazardous chemicals and can generate explosive hydrogen gas! Always wear eye protection when working on the car. Keep any from any running belts or fans or else you can lose a finger! Refer to your service manual for further safety notes and see the TOS Agreement for the full website legal disclaimer.

Bad alternator symptoms, checks, and testing

Use a voltmeter to test for at least 13.8 at the battery when the engine is running and 12.6V when the engine is off. If voltage is below about 14 with a running engine, the alternator isn't charging the battery. Leave the car for a few hours and check battery voltage. If it's dropped then the battery is bad or something is draining it. Batteries usually last 5 years depending on use and ambient conditions. In extreme desert heat they may only last 3 years and I would change any battery over 7 years old as preventative maintenance.

If the car can drive, many auto local parts stores will test your battery and alternator on the car for free. You can't test the amp output without a load tester. VW-Audi alternators are usually 90 or 120 amp, depending on the model. It should have a label on the back indicating the part number and amps.

If the car is idling and the battery warning light comes on, try revving the engine higher. If the warning light goes off during this test or while driving at higher engine rpm, this means something inside the alternator isn't charging the battery when engine rpm is low. You may also notice the headlights dimming.

Measure voltage drop between the alternator battery terminal and the battery positive post. Test ground between the alternator housing and the negative battery post. Both should be under .2 volts.

Check for good contact on the battery. Make sure the battery terminal clamps are tight, fully seated on the posts, and not cracked. Check the ground wire and the other end which connects to the chassis. If the battery posts are covered in green crusty corrosion, it isn't making good contact.

If you just replaced the alternator with a rebuilt unit from the local auto parts store, it's possible the replacement is faulty. I've had a number of cheap replacement alternators which were DOA or failed in a month. For this reason, I recommend OEM or equivalent parts (like a Bosch brand rebuilt alternator).

The serpentine (alternator) belt could be loose. This is unlikely but possible if the tensioner fails.

All mark4 generation and newer VW-Audi (around 1998 and newer) use a clutched alternator pulley. This reduces wear on the alternator but if it fails, the pulley will fail to turn the alternator shaft. To test the pulley, remove the serpentine belt. Use a wood or plastic pin (to avoid damaging the alternator) to hold the alternator fan inside the alternator. Spin the alternator pulley by hand - you should feel resistance in one direction and none in the other. If it rotates freely in both directions, the pulley is bad. Here are some videos explaining the function, check, and replacement of a one way pulley.

It could have a bad voltage regulator. The voltage regulator limits the maximum voltage coming out of the alternator. It also contains the carbon brushes which contact the rotating part of the alternator shaft (slip ring) to the non rotating part of the alternator. While the prices of new voltage regulators might be high compared to a cheap rebuilt alternator, the original alternator will be much higher quality than any local auto parts rebuild. Therefore, it's recommended to replace only the voltage regulator if that's all the alternator needs. It can also be replaced without removing the alternator from the car. The brushes should have at least 5mm movement (6mm is preferable). While the brushes can be replaced very cheaply, it doesn't rule out another problem with the voltage regulator. See below for details on voltage regulator repair.

When you turn the car on, you should see the low voltage light turn on and then off within a second of engine start. If it never comes on, this means the exciter circuit is disabled. On VW-Audi, the instrument cluster sends electricity from the low voltage light to the alternator to get it started. The alternator can't start making electricity by itself. Once the alternator is outputting sufficient voltage that is equal on both sides of the the low voltage light, it turns off. If the wiring from the light to the alternator is damaged or if the light never comes on, check this wire. Since the low voltage light bulb is an LED, it's very unlikely for this to fail.

Below is the battery cable and the plug for the instrument cluster battery warning light. The alternator grounds to the engine block through its mount.

The alternator bearings are pretty reliable and shouldn't cause any charging problem. You should also be able to hear bad bearings.

If the battery is fully charged and cranks fine but isn't charging, there's no problem in the cable going from the alternator to the starter. That cable then goes to the positive terminal on the battery.

If the battery is weak, engine rpm when starting can be too slow.  The engine computer sees this and does not inject fuel which can cause a no start condition. Most of the time it won't crank the engine at all and you hear the starter solenoid clicking with no cranking.

If the diodes start to go bad that convert the AC to DC you can also lose amperage output and still be at 14V. If the windings are bad, the alternator won't charge. In these cases, I would replace the entire alternator.

It's very rare but I've heard of the starter getting stuck out and engaging the flywheel as the engine is running. In this case the starter will act like a generator and it can cause the headlights to get bright from too much voltage. You should be able to hear loud whining like a leafblower. Obviously this will not make low voltage.

Alternator removal

For mk4 VW, refer to 1000q: mk4 Jetta-Golf-Beetle alternator removal

For B5 Passat or Audi, refer to 1000q: Passat alternator removal

Voltage regulator replacement and repair

If your alternator just needs a new voltage regulator, remove the 2 nuts (1x 13mm, 1x 15mm) and 1 phillips screw (may vary depending if you have a Valeo or Bosch) holding the rear plastic cover.

Remove the 3x phillips screws holding the voltage regulator. (Bosch shown)

Remove the voltage regulator.  The slip rings (copper contacts) also wear but replacement requires alternator removal and at that point you might as well rebuild it yourself or replace with a quality rebuilt unit. Resistance between the slip rings should be around 3-5 ohm.

Measure the length of the carbon brushes (the part that contacts the shaft).  The wear limit is 5mm.  New brushes are 12mm. If you determine that the voltage regulator is bad but the rest of the alternator is good, replace only the regulator. If the brushes are below 6mm, I would suggest replacement. The Bosch voltage regulator is shown below and is not interchangeable with the Valeo. The rest of the regulator is a transistor and is not serviceable.

The Valeo voltage regulator is often more expensive but if you only need the regulator, I don't think the additional labor of removing the alternator is worth saving the price difference of switching to a Bosch alternator. Below is the Valeo with one of the brushes removed. The brushes maintain contact by the spring inside pushing it against the slip ring.

Alternator brush replacement

If the voltage regulator only needs brushes and you don't want to wait for parts, just replace the brushes. Brushes are very cheap (around $5) and compatible parts can be found in any local electric motor shop.

Remove any coverings and desolder the end of the brush wire. The brush wire length controls how far it can go out while out of the alternator (normally it pushes against the slip rings). The springs push the brushes against the slip rings.

Clean the brush guides and insert the new wire through the wire hole. You'll have to widen the hole to get the wire through as shown below. Solder and then cut the wire sticking out the back to length. Use silicone gasket maker to cover the solder points you just made.

Below is a worn brush compared to a new brush.

Alternator pulley removal only (B5 Passat -Audi A4 and Jetta shown, other models similar).

Remove the dust cap over the pulley if present.

Insert metalnerd tool MN3400 or VW tool# 3310 socket into the splined hole to counterhold the pulley.   Then stick the M10 triple square 12 point bit or T50 6 lobe bit through the tool to loosen it.  NOTE: The thread is reversed so turn MN3400 or VW tool# 3310 clockwise to loosen it!

To install, tighten the pulley to 48 ft-lb. Below left is a video showing how to do this on a B5 Passat or Audi A4. Below right is a video showing how to do this on a mk4 VW Jetta TDI.

How did this DIY job go for you?  Help improve this article by commenting in the VW and Audi TDI forum.