Franks' thoughts on the bmw K series Fan

My ideas on the bmw K series Fan.
This fan is used on all the K series... well at least to about '95.

To get to a subject fast click on the subject.
What is it?   How does it fail?   Determining its condition.   Lubrication.  
What to do if it seizes?   Removal:   Fan:   Fan motor:   Bearings:  

What is it?

This is a multi bladed fan driven by a permanent magnet motor used to provide air flow over the engine radiator. The motor has 2 plain bearings that are self aligning and 2 brushes that contact the comutator. There is also a thrust bearing on the rear of the motor to take the thrust of the fan (more of a pull than a thrust but you get the idea).

How does it fail?

The bearings fail by increasing friction, causing the motor to stop, this motor then over heats, melting the brush holders. As the fan is used to cool the engine this too could over heat leading to an expensive repair!
You could extend the life of the bearings by lubrication them... but how?

Determining its condition.

You can manually spin the fan blades to see if they are 'free'. On nonflaring models look for the fan and radiator from the right side below the fuel tank around the air inlet pipe. Reach in through the right side below the fuel tank and you should be able to spin the fan.
You could measure the current of the fan motor - more current means increased friction. This quantifies (gives it a number) the friction but until 'we' have a good sample range then 'we' wont know what is good and bad.

Running Current Amps
K11 before lubrication	K11 after lubrication	K75
K11	K11al	4.4

This is what I did, I do not recommend anyone do this, if you feel that you have the skills to do this without risk then that is up to you.
I removed the temperature/fan circuit module and connected the battery positive to the violet/yellow wire on the connector through an ammeter. The fan operated, the current reduced as the fan speed rises to its normal speed, I recorded the operating (normal) speed current.
I measured the stalled current on one fan as 11 amps - it may be more than this as the ammeter I was using has a maximum of 10 amps, 11 is well over range. I did not repeat the measurement on the second fan as I only wanted an indication of the fans' maximum current. If you want to measure the stalled current, and I strongly recommend that you do not, then limit the time to say 30 seconds when measuring the stalled current. Longer than this and you risk damage due to excessive heat in the rotor and/of the brushes. Again I say - don't do it!
I'd take say 5 amps as a limit - higher than this you risk the motor stopping - lubricate at this level and check that the current drops to an acceptable level?
This may well be another thing for EFI computers to monitor and signal a fault if the fan is seized?  

Lubrication

The rear bearing and thrust bearing can be accessed by removing the plastic cover! Very easy. Remove the fuel tank, undo the fans electrical connector at the fan (the connector clips to the fan mounting), free the fans connector from the fan body and its cover. Press the 4 cover clips inwards (they are located top and bottom (vertically) and left and right (horizontally)) then pull backward. You will need to feed the electrical lead through the cover to give enough room. Apply say 2 drops of engine oil to the thrush bearing, the excess will go through to the plain bearing. Do not over lubricate as this will contaminate the brushes and comutator.
The front bearing is covered by the fan. You will need to put the oil drops onto a piece of wire (dunk the wire into the oil) and carefully place the wire and oil through the gap between the fan and motor then the bearing housing and motor body. You should be able to feel the wire touch the shaft, the oil will follow along the shaft to the bearing. You may like to feel around using the wire without the oil to find the shaft and bearing. There may be a felt washer on the shaft towards the rotor bindings.

What to do if it seizes?

If your really luck you may fine the fan motor stiff to turn or impossible to turn. Most people have the over temperature lamp turn on (or if your lucky enough to have a temperature gauge that it goes over its' normal upper limit). Pull over! You may have a coolant leak or some other cause for the engine to get that hot to start with. If you have water then I spray that on the front of the engine and finally on the radiator, but take a look first for coolant leaks.
If the coolant has leaked out then you need to replace it, about 4 litres max. Do you have that amount of water? If you do then I'd let the motor cool but turning the motor off unless you can cool the motor with water. Be careful removing the coolant cap - ware your gloves, jacket and helmet! Have the new water ready to go in. Restart the motor before you poor the water/coolant in, and do the poring slowly.
Once the engine is cooled check why it has got hot and then check the fan motor. If the fan motor has seized or become stiff to turn try some lubrication. Before you try this check the plastic that houses the brushes, if the brushes are aligned correctly to the commutator and are free to move under the spring tension then you have a chance.

Removal:

I found I had to remove the radiator - drain about half the coolant (about 2 litres) then remove the radiator hoses from the radiator (but not the engine). I left the filler on the radiator - just undo the 2 nuts that hold it to thee bike and it comes away with the radiator. Remove the mount screw from the radiator top and pivot it on the bottom mounts and remove - don't damage anything. The fan comes off the radiator by removing the 2 bottom screws, then the metal washer at the top so you can move the fans top mount over the rubber bit.

Fan:

The fan is a simple push fit onto the motor shaft, a few blows to the motor shaft while holding the fan will have it off. But before you do - mark the fan to shaft position so it can be realigned. The fan may have balance weights added so the relationship between it and the motor may be important.

Fan motor:

The fan motor can be seen by removing the cover at the rear - see the lubrication section above. The front self aligning plain bearing is hidden by the fan, it is the same as the rear bearing but has a felt washer that may have contained oil a long time ago.
The motor has permanent magnets as the stator so any metal filings will accumulate here.
It is held to the housing by 2 spring clips.

Bearings:

These self aligning bearings are simple plain bearings with the outside of the bearing shaped into a sphere. The shaft of the bearing can be angled away from the normal and the bearing will operate correctly.
Replacing these with sealed ball bearings may be a good idea - but the physical size is a problem, as is the alignment of the bearings. Physically the thickness is restricted at the rear. The thrust bearing at one end and the comutator at the other set the limits of the rear bearing thickness.
The other problem is mounting the new bearings, the present mounts are spherical for the self aligning bearings. They are crimped to the main body, they can be forced off by filing (watch the filings go to the magnets!) or using a cold chisel to reform the metal. The rear mount bracket has the brush mounts on it.
The shaft is 6 mm diameter, ball bearings to suit are available, cheap ones are 10 mm thick and 15 mm OD (about $aud6 each).

To get to a subject fast click on the subject.
What is it?   How does it fail?   Determining its condition.   Lubrication.  
What to do if it seizes?   Removal:   Fan:   Fan motor:   Bearings:  

Comments? all welcome just e-mail me at Yahoo.

If you quote me I would like an acknowledgment. Dated 28 June 2002.

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