The following topics related to the VW fan are discussed in this treatise -
Paraphrased from John Muir* -
The Volkswagen engine is air-cooled, which means that the heat of combustion in the cylinders and heads is carried away by the flow of air. The cylinders and heads are cast with fins to aid in the removal of heat. A centrifugal blower (usually referred to as a fan) is provided to force air down through the spaces between the fins to carry the heat to the atmosphere. The entire engine is shrouded by sheet metal (usually called the engine tin) to hold the flowing air to the most efficient path. The fan is attached to the front (front) of the generator or alternator and is driven (along with the generator or alternator) by a belt from the crankshaft pulley. The fan is encased in a fan housing (often referred to as the fan shroud) which is attached to the engine tin to make an air-tight assembly.
The cooling air is drawn from the outside through slots in the car's body under the rear window (and, in later models, through slots in the engine lid) by the suction of the fan in the front (front) of the fan shroud. From there the air is blown primarily down through the cylinders but also (in cold weather) through nozzles on the sides of the fan shroud, through the heater boxes and to the front of the car to provide warmth in the winter.
Also in the fan housing, bolted to the top of the crankcase with appropriate seals, is an oil cooler which stands up in the stream of air like a radiator and cools the oil which is pumped through it. In later models the oil cooler is located just forward of the main body of the fan housing, with the housing tin molded around it. The fan housing for such models is referred to as a doghouse housing.
*How to Keep Your Volkswagen Alive --
A Manual of Step-by-Step Procedures
for the Compleat Idiot,
1976 Edition, page 14.
Note: Air flow for the non-doghouse 1300/1500/1600 cc engines is 20 cubic feet per second at 4000 rpm. The doghouse cooling fan is wider and blows around 25cf/sec @ 4000rpm, which is why cars with this cooling set-up NEED a slotted engine lid - the slots under the rear window are simply not large enough to provide the doghouse fan running with enough air at higher speeds.
Question -
Please just advise me on the best way to get the fan off (that is, loosen the big nut on the forward end of the alternator shaft). Some say that I need an impact wrench; others say you can put the pulley in a vise (padded with an old drive belt) and then remove the nut with a 1-13/16" socket.
John Connolly responded -
You need an impact driver. Those guys that use a vice are also the ones that have gone thru 5 fans in 3 years. If you don't have an impact driver, take a 6 pack to the local garage. It's a 3 minute job (don't forget 10mm socket and ratchet to get the backing plates off). Also, the vent MUST GO DOWN to cool the alternator.
Dave proposed the following method -
Take a big block of wood and drill a shallow hole in it just larger than the pulley nut, then placed the alternator/fan assembly down on the block, pulley end down, nut in the hole, so that it's resting on the pulley rather than the nut. Then, go after the fan nut on the other end with the impact driver.
Is this liable to damage the pulley?
Rob indicated that so long as the pulley is on tight and the shaft is upright so the pressure is even around the rim, I doubt you'd damage the pulley. And it would certainly make it easier to strike downwards rather than sideways, and at the same time prevent marking the end of the alternator shaft on a concrete floor.
Dave asked -
Now, a question about re-installation. When putting the fan nut back on it will be possible to hold the alternator shaft with a wrench on the pulley nut. The torque specification for the fan nut is 40 ft-lbs -- not excessively tight. I suppose you have to use the impact drive to remove the nut simply because it's difficult to keep the shaft from turning when trying to remove the nut (counterclockwise). And then too, the nut would tend to freeze over time anyway, so it may take a bigger whack to loosen than the bare 40lbs.
Anyway, holding the pulley nut with a 19-mm wrench while tightening the fan nut will torque both nuts to 40 ft-lb. Then, of course, once the whole assembly is reinstalled in the car, the pulley nut must be removed to get the fan belt on.
Rob responded -
I haven't needed to do this job, but I think "loading" the pulley end of the shaft by gripping it in a vice would not do it any good at all when the other end is torqued to 40lbs. I suppose that with an impact driver you'd only need to get someone strong to wrap a towel round the pulley and hold it, whilst the second person gives the impact driver a good whack or two. Inertia of the fan and alternator should make it easy enough to loosen.
Dave recorded the following experience -
I successfully took my fan/alternator assembly apart, thanks to my new handy-dandy impact driver. Piece of cake. Obviously never taken apart before -- it was FILTHY in there!
Next a thorough cleaning, then I'll install the fan and associated bits on the beautiful new alternator. Then its reassembly time, at which point I need to address the alternator/accelerator pump linkage interference in earnest.
Regarding recommendations that Dave replace the fan, Rob wrote -
I'm a little puzzled about the need to replace the fan. It spins to a max of about 8500rpm (4800 engine rpm), and I guess it could be damaged on a stock engine by overrevving the engine, but unless it's causing vibration, I would think it's still OK. Any reason given for the suggested replacement of it? Mine's done 248,000 miles without any problems at all.
At Rob's suggestion Dave did not replace the fan, and has had no troubles (after initial problems).
There is a vent on the alternator backing plate that attaches to the fan housing. This vent is used to cool the alternator/generator -- it sucks air through the alternator into the fan. So the main cooling fan does the alternator cooling job without the need for those little fans found on the outside of most alternators. On your alternator you'll see some slots just behind the pulley -- this is where the air enters.
High pressure air enters that gap and runs through the alternator or generator to cool it. The vent, which is on the edge of the backing plate, MUST GO DOWN to cool the alternator. You can point this feature out to anyone that has their unit mounted 90 degrees off (there are a lot of them).
Rob wrote, regarding the gap between the rim of the fan and the shroud -
Put your hand into the fan opening in the front of the fan shroud (engine OFF please!), and see if there is a gap between the rim of the fan, and the shroud. It should be no problem feeling over the top of the shroud above the alternator and feeling the opening lip of the shroud and the fan rim just inside it. There should be only a tiny gap between the two if the fan and alternator attachment plate are correct. If the fan is the older type there would be a bigger gap as it is 5mm narrower than the later type. I doubt this is the problem with the alternator clearance, but since it's a 1 minute job to check. If you have the wrong fan you would be loosing cooling air. Very unlikely, but might be possible, and MIGHT be the cause of the interference.
With the engine out, looking at the fan will show instantly if it's the right one and correctly mounted. It should completely fill the fan shroud opening, with almost no gap all the way around between fan rim and shroud. A gap of 5mm or more and I'd suspect that the fan was the wrong one. (Unlikely though -- as I've said before).
Dave reported -
I checked the clearance between the fan and the housing -- it is indeed five mm. Obviously the PO installed the fan designed for use with the alternator in the '71 housing.
Rob responded -
If the gap is that big my fear is that you have the thinner non-doghouse fan inside the doghouse shroud. It's obviously working, as you've taken the car on a couple of long trips, but if I am correct, then it may run a little short on cooling in hot weather when under load. The later wider fan pumps out roughly 10% extra air to allow for dumping the oil cooler air overboard, whilst still maintaining the same or better airflow and pressure to the heads/cylinders. Since the oil cooler air channels in the shroud are at the front of the shroud (closest to the main air inlet), the oil cooler may be getting a little less air than it would like.
It may not be likely to cause any real problems though (the doghouse fan is enough to cool 2 liter type 4 engines installed in Beetles, so the smaller fan must have a slight over capacity for it's usual engines too), but it's worth checking out.
Dave wrote -
The engine tin sticking up around the base of the fan housing -- not remembering whether the housing goes inside or outside of this vertical tin, I did a little research in the Haynes Manual and found a picture that clearly shows the vertical engine tin on the outside of the fan housing.
While putting on the fan belt I discovered that the fan is rubbing slightly on the inside of the fan housing. It definitely was not rubbing while the assembly was on the workbench -- I don't know why its decided to do so now.
Rob responded -
Rubbing on the front or rear of the fan? Are there supposed to be any spacers between fan and alt backing plate or something? Tension on the fan belt pulling the alternator shaft slightly rearwards perhaps? Slightly possible that you put the belt on too tightly. Remotely possible that you didn't center the outer fan cover exactly right, though there isn't much play there -- those holes act as positioning holes for the fan/alternator. One possibility -- you might be able to lift or push the shroud down just a tiny amount on the two outer fixing screws, leaving the fan backing plate firmly bolted. The shroud might be "bending" fractionally, enough to rub.
Dave reported -
I took off the piece-of-junk alternator strap I got from XYZ (broken already) and just lifted up a bit on the alternator -- end of the rubbing fan. When held down tightly to the stand, however, it does rub. Strange. I wonder if there is something under the fan housing that is keeping it from seating all the way down, because the alternator won't fit snugly down on the stand -- a "high shroud" problem. And that's exactly what it turned out to be. I just loosened the two side bolts on the shroud and pushed it down firmly, then strapped the alternator in place, tightened up the bolts, and it spins freely!
When reinstalling the fan shroud Dave had trouble with the cooling vane connecting bar -- it wouldn't clear the doghouse oil cooler. Rob wrote that it was designed that way. The connecting bar has appropriate jigs and jogs in it to get around the oil cooler and things. It all has to go together in the proper sequence, but it all fits together very nicely.
When Dave's son bought his '73 SB, there was a circumferential crack all the way around the alternator pedestal, through which oil would pour whenever oil was added to the crankcase. Dave went through two "pot-metal" pedestals before resolving this problem. Dave's mechanic said the he had "never heard of a failure like this...never." Rob's mechanic told him that the cheap ones don't last very long. Dave was delighted to find an original VW alternator stand at Russ' Recycling in Duarte, California, and he has had no trouble since.
For further discussion, see our article devoted entirely to the Alternator Stand.
Dave encountered a very strange problem with the forward half of the alternator pulley rubbing against the alternator. He found that a spacer of some sort is needed between the front pulley half and the alternator, otherwise the pulley will rub against the alternator as it rotates. Rob was skeptical and said that such a spacer really shouldn't be needed. If it IS needed, the question is why? Incorrect pulley wheel (using a 6-volt one perhaps, or is the pulley distorted? It should be close to, but not rubbing, on the alternator. The various VW catalogues, which have quite clear pictures, do not show any washer between alternator/generator and front pulley half. The pulleys aren't expensive, and perhaps a new one will fix the problem in a flash.
And indeed it did. Dave discovered that not all alternator pulleys are created equal!
Dave wrote -
Dave had a little trouble with the pulley nut. He tried putting the extra shims under the nut, but then he couldn't get the nut on. Rob reported that he has usually found it a bit difficult to start the nut as the slack belt tends to fall into the centre of the pulley halves, but once the nut is started and tightened a few turns, Rob turns the pulley by using a screw driver in the slots in the front half, and the belt climbs out of the valley to it's proper place so the nut can be tightened. An alternate method would be to just keep turning the nut so it tightens a bit, then pulls the engine around to the next compression (has to be in neutral of course), and the belt climbs higher at the same time, then the nut tightens some more, and so forth. Once the nut is snug, use the screw driver in the slot to hold the pulley whilst the nut is fully tightened. It's a bit of a fiddle, but not really difficult.
Make sure that the fat pressure washer is immediately under the nut. The genuine article is about 1/8" thick "plain" washer with a slight dish in the middle and flat rim, so the nut presses on the raised dish and applies pressure to the rim of the shim stack. I think it also has a "flat" in the hole so it won't rotate.
Dave had a split washer was had a very difficult time getting the nut started with the washer on. He could hardly get it started with the washer OFF! Apparently a PO had put in the extra split washer. Such a washer is definitely not needed -- the design of the thrust washer prevents the nut from undoing, and the direction of spin tends to do it up anyway.
Someone asked -
>Is there such a thing as an over tightened fan belt? Mine deflects less
>than half an inch. all the shims are in the inside. I cant seem to find a belt
>that fits just right. Either too loose or too tight.
Rob responded -
Yes, you can have a belt too tight, and this puts extra strain on the generator bearing nearest the fan belt. You can use either 900mm or 905mm belts, but they must be the correct narrow type of belt one that is too fat will not ride in the pulleys correctly.
Someone wrote that he doesn't ANY shims between the two pulley halves, as the belt is just right without them. Rob said that seems a little short. It should have at least 3-4 shims for a 905mm belt, and maybe 5-6 for a 900mm belt. In either case you then still have room to add shims to tighten it as it wears. The Beetle normally comes with about 10 shims altogether, and of course ALL shims should be on the generator axle, either between the pulley halves or under the nut, so the stack remains the same depth and the lock washer and nut fit correctly.
Dave wrote -
The replacement belt I have (and most likely the one that's on the car as well) is 920mm in length.
Rob responded -
That explains it. With the longer belt you have to make it climb right up to the top of the pulley to get it tight. Much better if you can find a 905mm belt.
"Speedy" Jim wrote to ask -
Are you also aware that the alternator belt is different from the generator belt? The alternator fan belt is supposed to be 912mm, versus 905mm for the generator. This is because the 912mm alternator belt is 11.3mm wide whereas the 905 generator belt is 10mm wide.
Rob elaborated -
That means the longer wider belt will ride in the same place in the alternator pulley (with an extra shim or two) as the shorter thinner belt, but it will be stronger for the bigger doghouse fan and the higher output alternator.
The fan belt works quite hard for such a thin one and it's been the same from '49 to '72. The 36hp engine had 24 blades on the fan and 160watt generator. The 40hp and 1300/1500/1600sp had 28 blades and during that time the generator output rose from 160 watts to 360 watts. The difference between DIN and SAE hp ratings indicates roughly 4hp on the fan belt on early engines to about 5hp on the 1500/1600sp engines. The doghouse fan is about 10% bigger (it's 5mm wider) so will absorb maybe 5.5hp ('71 and '72). Then add 1/3 hp for the extra alternator output (220 watts extra) ('73 up) and you have close to 6hp driven by the belt.
VW they found that the thinner belt was breaking more often or wearing too fast on the doghouse/generator models ('71, '72) and so when they introduced the alternator they thought they'd better have a bigger belt. The 912mm belt is about 10% bigger than the old one, and the longer fatter belt would also result in a greater side area to the belt - it's the sides which grab the pulleys. So the bottom line -- the shorter, thinner belt on an alternator will work OK, but it probably won't last as long as otherwise.
Regarding belt tension adjustment -- more shims inside for a shorter (newer) belt, and less shims for a longer (worn) belt. Dave was surprised to find six shims between the pulley halves, which he thought perhaps could be making the gap between the pulley halves too wide and the belt too loose and perhaps slipping. Certainly a loose belt will slip a little - it runs about 5hp through it at high rpm to drive the fan, and that's a lot of load on a small belt, so it needs to be just tight enough, but not so tight it stresses the alternator/generator bearing.
Rob reported that the 905mm belt can't be had for love nor money in Australia - Bosch and the others only supply a 900 x 9.5mm belt, which Rob has found buries itself so far into the pulleys it almost touches the shims in the generator pulley. The 900mm belt works OK - it just doesn't look quite right. I'm sure I used to be able to buy the 905x10mm belts which would sit a little higher in the grooves, but none of the belt distributors or VW shops know of them.
Rob asked John Connolly (Aircooled.Net) -
I have a generator beetle which uses the 900 or 905mm belt. In Australia, only the 900x9.5mm belt is available, but this belt sits VERY low inside the pulleys, almost touching the adjusting shims in my 71dp engine - it just doesn't LOOK right (though it seems to work OK). I note that you have both 900 and 905s, and if I can't find the 905s here in Aus, I think I'll get some from you.
Question - can you confirm that the 905 x 10 is likely to ride a little higher in the pulley grooves than the 900 x 9.5? And a further question - will the 912x11.3 alternator belt fit on the generator models too, or is the alternator engine pulley wider to take the wider belt? I've heard both sides - yes it fits; and no it needs the alternator engine pulley. My thought was that if the alternator belt fits the generator models, I'd use the wider stronger belt on my two generator VWs.
John Connolly responded -
Yes, run the 11.3 X 912 instead, it's just the ticket for you.
Rob responded -
So there is no difference in the engine pulleys (generator, alternator models), then. I'm pleased to hear it -- I always thought that the thin/short belt was working very hard. It's a wonder it lasts as long as it does.
What John is essentially saying, if we're understanding correctly, is that the 11.3 x 912mm belt is applicable to virtually all Bugs. But one VW place in Australia said the alternator bug engine pulley had a wider groove. Yet 912 x 11.3 belts are unobtainable in Australia, so the presumption is that they are using the 900x9.5 beltson all bugs - which would mean a shorter life on a alternator bug with it's bigger fan and alternantor.
Dave installed a new 912x11.3mm fan belt on his '73 Bug. With five shims between the pulley halves the belt was just a little tight. Rob recommended adding another. You can go up to 8 -- that's usually how many are in the stack.
The only absolutely essential spare you need to carry in any VW (besides the spare tyre and the means to change it) is a spare fan belt. Rob replace the belt as soon as he sees it start to crack -- usually about 50,000 miles -- and then keeps that one as the spare, (throwing out the previous used spare which has then been sitting in the front of the car for 3-4 years). He's only needed to use a spare once, but even that once is enough to make keeping it worthwhile. Even a cracked spare is better than no belt!
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
We hope you find this information useful, but we don't take any responsibility for anything which happens to you, other people, your VW or any other property or goods resulting from your use of this material.
Feel free to print off any of this information for your own use. If you intend to link this material to another site, reprint it, or in any other way redistribute it, please leave the information complete, including this disclaimer section, and provide a link to this Web site.
Last revised 4 May 2004.
