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Among other things, the heater fan is the Spider's Achilles' heel. As most of us know. And as the rest of us will one day find out... :) Some BBers have already posted helpful hints on the subject: see Eric's article about S3 repair here.
However, I found nothing about repairing or even opening the fan motor, so here are a few hints for those of you who won't search for a spare part if (or should I say when) the original fails. This is about the S1 and S2 Junior (with Duetto-style dashboard) fans, which are different to S3 and later. I can't say if this is would be any help when dealing with a later motor.
The following will deal with mechanical problems only. No help to offer on electric mishaps. To tell the two apart, try manually rotating the fan: if it turns over easily, but won't respond to fresh 12V fed directly on the wires (to check the operation of the single-speed electric motor, polarity is unimportant) you have an electrical problem. Obviously, you have to check the wire brushes. You'll find instructions on how to get to them, but I can't help you with blown rotating parts and stuff like that.
However, if you try to rotate the axle and the motor offers commendable resistance, the malfunction is mechanical. That will be our hunting ground today.
On pic 1 you see a rough drawing of the motor as it sits in the heater box. The fan is on the upper end, therefore I’ll refer to the side with the fan as the upper side and the opposite is the lower end. Let's begin!

1. Open up the motor from the lower end. To do this, just remove the two 7mm nuts. Gently pry the two halves apart. If you’re lucky, the lower part will come off and the wire brush assembly (plastic round part) will remain in place (see pic 2). If not, at least you can see if your wire brushes are worn. (Note: the wire brush assembly isn't very hard to install on the axle, but it gets annoying after a time, so I suggest to keep the assembly on the axle whenever possible. This requires some attention, but it's still better than reinstalling the stupid thing over and over. But if you follow the instructions, maybe you won't have to do it as often as I had to.) Anyway, try to rotate the fan again manually, with the lower part hanging on the wire, out of the way. If it turns over easily, go to point 2. If there's no change, go to point 3. If it's easier but still-not-quite-as-you-had-planned, go to point 2 and then 3.

2. Now, the important thing to look for are the bronze bushings on both ends of the axle. They can go wrong in two different ways. The more obvious kind is rust due to non-usage and water entering the system. Then the axle will cement in place. The lower end of the axle, located at the bottommost part of the heater where water is almost certain to accumulate (don't forget to drill a hole at the bottom of the heater box!), is more prone to this. You've already removed this end, so clean up the axle with fine sandpaper or with wire brush; gently but firmly. As for the bushing, try rolling the fine sandpaper into a cylinder and insert it into the bushing. The simplest way to check operation is to insert the end of the axle from the outside of the engine cover and turn it manually. No resistance is allowed. When you're satisfied, go to point 5.

3. If the axle turns with difficoulty within the upper part, you need to separate them. Remove the fan, the securing ring, the washer and the two 7mm nuts if you like. The nuts are optional; if you do not remove them, separate the wire brush assembly from the threaded bolts and remove it. If you do remove the nuts, remove the assembly and the threaded bolts together. Theoretically it's possible that the assembly is blocking the axle, but that should be fairly easy to detect... Not likely, though.
Pull out the axle. The magnets will pull it back; don't let them win. Clean the bronze bushing according to point 2. If the axle turns easily now (when correctly installed in the upper part), go to point 5. If not, go to point 4.

4.a Your motor certainly is a tough customer. The axle turns easily when installed into the upper part from the outside, but has difficoulty when installed in its correct place? You might have a problem with the magnets, but it's highly unlikely. Not impossible, though; one of the magnets in my motor have simply fallen off. You are unlikely to encounter such a problem, but if you do, then clean both surfaces (that of the magnet and that of the upper part) very thoroughly and simply glue the magnet back. (This requires some faith in the glue. Use high quality.) It had been glued there in the first place, therefore if your cleaning is sloppy, the remains of the previous glue will keep the magnet from fitting neatly against the upper part of the motor. Then the rotating part of the axle won't have enough space to turn around and it'll seize.
After the glue has set, reinstall the axle and try to turn it manually. If it makes all kind of noise or won't turn easily, check the outside diameter of the rotating part against the inside diameter between the magnets. If the space is inadequate, you (or the PO) messed it up with the magnets. Gently remove the suspect magnet, clean, reinstall... . (Note: I even managed to break off about 1/5 part of the magnet. I glued back the remaining part and it still works fine.)
4.b But if the space looks and measures OK, you might have encountered the second way in which the bronze bushings can fail. The bushings are in fact ball joints and therefore meant to compensate if the axle holes in the bottom and in the upper half of the motor don't line up perfectly. They can and will turn to make the holes line up. However, once they had been turned in the right direction, they won't turn again for decedes (why should they?), meaning they are likely to be stuck in that position. Once you take the motor apart for repairs or cleaning, not to mention messing with the magnets, odds are, you won't put the two halves back together again just exactly the same way, therefore the bushings will no longer line up perfectly and now they are likely too stuck to turn on their own accord, therefore the axle will seize and the motor won't turn over easily.
So get a screwdriver or something, stick it in the upper bushing and rotate it to one side as much as you can (pic 3). Clean exposed surface with fine sandpaper. Repeat to clean as much surface as possible; after that, grease it as well as you can. Now try to line it up so the rotating part of the axle will sit nice and comfy within the magnets. Don't be afraid of some trial and error. Eventually your motor will run out of excuses; if the magnets are installed properly and the ball joint bushing is lined up OK, the axle will turn easily. Go to point 5.

5. You need the axle in the upper half of the motor (secure it with nuts) and the wire brush assembly installed. See pic 2. At this point you must be able to turn it over manually quite easily. If not, start over from point 1. Give it some amps! It's supposed to run at high speed. (If your motor was in a pretty bad shape, this is one of those rare moments of absolute triumph over the Forces of Evil that come once in a blue moon.) If it doesn't do anything, contact your local Professor of Electric Sciences, as it’s mechanically OK. Re-install the lower part (don’t forget to neatly arrange the wires) and check operation again. If performance drops noticeably, check the lower bronze bushing by the method described in points 2 and 4.b. If the motor runs fine, congratulations! Re-fasten the lower side with the two 7mm hex nuts and washers. Install the fan onto the other end, check it again with voltage, and it's good to go!


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