According to the acknowledged VW electrical guru "Speedy Jim", re-building alternators is quite a chore, and it may be difficult getting the proper bearings. Jim says he would opt for a rebuilt unit if it's bad.
There are two bearings, one at each end, and two brushes which ride on "slip rings" - on the spinning part. There should be a brush-block on the side or top which may be removable so you could see the state of the brushes (the old style generator brushes are replacable in the car, but I haven't had a REAL close look at the alternator). Alternator brushes wear much slower that generator brushes, so it may just have a slow deterioration of these. If they are worn, also check that the slip rings aren't too worn down into "valleys" where the brushes ride. If this happens the alternator would definitely need to go to an overhaul place to have the rings machined or replaced so new brushes will ride properly.
Alternator Brush Refurbishment
First an interesting saga, then Dave's fix procedure -
Dave had a worrisome prolem with his "Alt" light. He wrote to Rob - The light was on all the way home from work (~20 miles) tonight, with the headlights on. I checked the fan belt a couple of days ago - seems to be all right.
Rob responded - If the fan belt had come loose enough to cause the "Alt" light problem, then the engine would be overheating, as the fan runs off the belt too. Check the battery voltage with the engine off - it should be 12.6V or therabouts, and with the engine running 14.2-14.5V. If the voltage doesn't change and the fan belt is good, then it might be the rectifiers inside the alternator or something like that. If so it would be an odd one since alternators are very reliable (as an example, my old truck has 900,000km and the alternator has never been touched).
Dave wrote - The fan belt is not loose, cracked or frayed. Voltage across the battery terminals (engine off) is about 12.5V. With the engine running (charging voltage) it's about 14.5V. The alternator was replaced on 4/29/08 when I had the engine rebuilt.
This morning when I started the car (it starts instantly) the "Alt" light came on and immediately went off, just like it
Rob responded - If the voltage is still at 14.5 with the engine on, then the alternator is charging the battery OK. But maybe it's an intermittent problem, like a dirty brush (yes - there are two small brushes inside all alternators - they normally wear very slowly as they run on smooth slip rings at low amps, not like the generator which has large brushes carrying the full amp load on segmented commutators which spark/wear a lot). Just keep an eye on it and if the Alt light starts coming on more, then you might need to do some more checks. It might also be an intermittent fault with one or more of the four diodes they have inside (which change the alternating current into direct current) - maybe getting hot and going open circuit. I think they are changeable if needed (I have never needed to find out).
Dave wrote to Speedy Jim - I´m having a strange situation with the "Alt" light in my instrument cluster. Sometimes (but not always) it will stay on, but will become dimmer (but not go completely off) as the engine speed increases. Other times it will come on when the engine is started, then go off normally. I have checked the fan belt - it is the proper size (912mm x 11.3mm wide), in good condition, 1/2-inch deflection. The alternator was replaced when I had the engine overhauled in April 2008. With the engine off, the battery reads 12.5 volts across the terminals; with the engine running, it reads 14.4 volts, which tells me it's charging.
Jim responded - It could be a flaky regulator. I would connect the multimeter to the battery semi-permanently,
with the meter in the cab where you can see it. See if the voltage is 14.4V even when the light appears to be
acting up. If the voltage *is* correct when acting up, then the regulator is OK and there is some flaw elsewhere.
Dave lengthened the leads on his VOM and set the meter on the passenger seat so he could read the voltage across the battery terminals with the engine running. He found that when the "Alt" light came on the voltage across the battery terminals dropped to about 12.5V, indicating that the battery was not being charged.
The problem really had Dave baffled. He wrote to Speedy Jim - I just went to see my doctor in Pasco, a distance of about 20 miles, highway driving. The "Alt" light didn't come on at all, either going or returning. It did flicker briefly in town... Then the other morning on the way to work the "Alt" light was on and the voltage remained at 12.5V. Half way to work (about 10 miles) the "Alt" light went off and the voltage jumped to 14.5V, indicating that the battery was being charged. Same situation on the way home, only this time the "Alt" light was off and the voltage read 14.5V on the first leg of the trip. Then I went into a store; when I came out and started home, the "Alt" light was on and the voltage read 12.5V for the duration of the trip.
It seems (at least to me) that there is something wrong with the alternator which causes it to charge only intermittently.
Jim responded - It may be that the brushes are making poor contact, intermittently. You can pull the brush holder out (along with the regulator, if internal) and check them. You can also polish the brass slip rings by spinning the shaft and holding a pencil eraser against the slip rings. If that's no help, replace the regulator. I doubt that there is a serious problem with the alternator innards.
Dave approached the job with fear and trepidation, as always with a procedure he hasn't done before. But this one turned out to be a piece of cake. Here's Dave's procedure for refurbishing the alternator brushes -
- Take a look at Dave's alternator -
Dave's Motorola Alternator
- Remove the ground strap from the battery.
- Remove the air cleaner to provide access.
- Note the white ceramic box on top of the alternator (voltage regulator?). There is a Phillips screw on either side of this box. Remove these screws.
- Pull the box straight up (there's a wire that will prevent you from moving it up too far). Note the two brushes that extend down into the alternator and rub on the slip rings. (See the picture below.)
- Look down into the alternator and note the two brass slip rings.
- Remove any deposits from the ends of the brushes with the eraser on the end of a pencil.
- Polish the slip rings by rubbing a pencil eraser on the rings as you turn the alternator over (19mm box-end wrench (ring spanner) on the alternator pulley nut).
Note: I used 200-grit sandpaper on the slip rings as well as on the brushes. Both Rob and Speedy Jim recommend using the eraser - I hope I didn't do my slip rings any harm!
- Slip the two ears on either side of the ceramic box down into the slots in the alternator body.
- Replace the two Phillips screws on either side of the ceramic box; tighten them securely.
- Replace the air cleaner.
- Replace the grounding strap on the battery.
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