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alternator removal and testing: mk3 Passat and Jetta TDI
Alternator removal for VW Jetta and VW Passat mk3 TDI
This DIY shows how to test and replace the alternator on VW Passat TDI or VW Jetta TDI, 1996, 1997, 1998, and 1999.
The alternator charges the battery which provides the electrical needs of your car. The alternator is considered a wear item and should be replaced on an as needed basis only. Symptoms of a bad alternator would be the battery warning light coming on, less than 14V at the battery when the engine is running, or dimming headlights when the electrical load is higher than normal (and you know the battery is good). Luckily, the TDI has relatively low electrical load since there are no sparkplugs.
If the battery is weak, engine rpm when starting can be too slow. The engine computer sees this and does not inject fuel.
The parts that see wear are the brushes and copper contacts. These are the parts that connect the rotating part of the alternator's electricity to the non rotating part of the alternator. In these 120amp alts, the brushes are held by the voltage regulator. Since the voltage regulator and brush costs about $95 and a rebuilt alternator costs $110, getting a rebuilt alternator may be faster. Some auto parts rebuilt alternators tend to be low quality so if the rest of the alternator is in good shape, a new voltage regulator and brush should extend the life of the original alternator. The spec on the voltage regulator states that, at minimum wear, it should have at least 5mm movement. If the diodes start to go bad that convert the AC to DC you can also lose amperage output and still be at 14V. The bearings and pulleys also see wear but they should have very long lives if properly tensioned.
If you are having a charging problem, it's either the alternator or instrument panel light, battery, or grounds. The battery voltage when the car is off should be over 12V. The battery voltage when the engine is running should be at least 14V even at idle. If it's the alternator, the problem could also be in the instrument panel. There is a low voltage warning light in the instrument panel of the mk3 cars. If it burns out, the alternator will not charge the battery properly. This is not likely since it's an LED so the connector or instrument panel can be suspect. It could also be a loose battery terminal or ground that prevents proper charging of the battery. If you are not sure, take the alternator to an auto parts store because many places will check it for free. Many places will check it on the car.
Jetta also have a one way clutch on the alternator pulley. If it fails, the pulley will spin but the alternator internals will not. To test it, remove the serpentine belt as described below. Spin the pulley by hand - you should feel resistance in one direction and none in the other. Use a wood or plastic pin (to avoid damaging the alternator) to hold the alternator fan inside the alternator and try to spin the pulley by hand - it should move in one direction and not in the other. If it fails these tests, the alternator pulley is bad.
Related links: 1000q: Passat/Jetta cold chirp fix - if you have a chirping belt slipping noise when cold, it could be the crankshaft pulley
6mm allen wrench
DIY alternator removal procedure
Remove the negative (-) battery terminal cable. Set it securely aside.
Release the tension on the alternator by using a wrench to move the tensioner arm lever. The service manual says that you can use the pulley bolt to move the lever arm but I found this to be stressful on the bolt and may tighten it. Remove the alternator belt.
Remove the 2x 6mm allen bolts (circled in red below) that hold the tensioner spring so that you can get access to the lower alternator bolt. Removing the belt first so that the spring is not under tension will make removing those bolts easier.
Remove 2 bolts that hold the alternator in place, marked with red circles below on the left side. You can use a socket that is larger than the metal bushing pressed into the alternator, to tap the alternator away from the bracket, marked by the red arrow. This will loosen the tension on the bushings to loosen the alternator. You can try to pry it out without releasing this tension but it'll be harder.
Remove 2 more bolts that hold the alternator cable in place. The alternator should now be easily removed. Clean all grounds as necessary.