There's been a lot of speculation over these couplings but i think we have an answer. If you've already jammed it up read on, you can fix it as good as new for just a few pounds. Better than reducing your MPG with a noisy fixed fan or giving Daihatsu 200 quid for a new one.

If you just want the gist -

The problem is most likely that the pin at the middle of the bimetallic coil has seized where it passes through the body of the unit. This needs to turn to open a gate inside that allows the viscous fluid to move around. If the pin doesn't turn when you heat the coil open the unit and tap the pin back and forth to free it. Close it up and test it. Job done! It really is that simple

Or the long winded explanation -

The cooling system on my Fourtrak seems easily up to the job but crawling up a welsh mountain in 4WD low range i boiled it up. The dash gauge isn't very good as it was boiling up way before it was in the red. If your needle ever moves above it's usual position watch out!

The radiator fan would turn freely even when the car had overheated so i knew the coupling was at least part of the problem. As it only overheats with extreme load i'm pretty sure the rest of the system is ok, no need to re-core the rad etc. If yours overheats with normal driving your problem is going to be more than the viscous coupling, though it could still be part of the problem. Take out the thermostat and check it opens fully and at a low enough temperature. If that's ok, try flushing the radiator with the bottom hose off and then fix the coupling. IMHO DO NOT just put a new rad cap on before you've found the problem, that rusty old spring might be saving your engine by releasing pressure easily. It won't be the original cause of overheating as you'll only lose large amounts of water through it if you're overheating in the first place. If you put a stronger spring on there the pressure will try to find a why out somewhere else, an old hose bursting won't make your day any brighter...

By the way you don't need to stick a broom handle in the moving fan to see if the coupling's engaged just try and turn it with your hand when you've just turned off a hot engine, you're not looking for fine degrees of stiffness, just pretty stiff or very loose. Or watch it stop when you turn it off, if it stops almost straight away it's locked, if it spins on slowly to a gradual stop it's not engaged. The difference is very obvious. It's only designed to come on when the engine is a fair bit above normal operating temperature though so you'll have to be sure the engine is properly hot, not just after a normal drive, or you'll not be any wiser. Best to use an infrared thermometer or the like and rev it up until the top of the thermostat housing is +90 degrees C. If it takes an age, and it probably will, you can be pretty sure the rest of the system is ok.

If you can't get the coupling to lock up you can fix this supposedly sealed, throw away, 200 pound unit for nothing in under an hour.

The coupling is a very simple design and unless it's been damaged there are only 2 possible things that can go wrong, either the viscous fluid has escaped which if the coupling hasn't come apart i think would be very unlikely. (There isn't much fluid visible normally so don't worry if you can't see much when you open it and the O'ring seemed too big a diameter but went squarely back in with a bit of fiddling.) Or the other thing is the bimetallic coil is not functioning properly, this is what happened with mine and is very likely the problem with yours.

I don't think the fluid gets old and stops working, i.e. the fluid is not some magic stuff that gets more viscous with heat. It is silicone fluid and silicone is known to resist heat very well, i think the secret is that it STAYS VISCOUS even when hot where other fluids would quickly become thin. (With a hot air gun on it the fluid still moves around at about the same speed as when it's cold.) It is the amount of fluid at the outer edge of the inside of the coupling that increases and causes the 2 parts to lock, it is not the increase of the fluid's temperature.

The amount of fluid is controlled by a thin plate (the blue steel bit in this one) inside the unit that rotates to open 2 small holes which allow fluid to flow outward and lock the 2 sides of the coupling. The plate should rotate when the bimetallic strip on the front of the case heats up, by hot air from the radiator, and turns a pin at it's center that passes through the casing. If this pin is seized, the plate will not turn, the fluid will not flow and the coupling will not lock.

If you take the coupling off the engine try heating the coil on the front of it with a hot air gun (only takes a few seconds) or a hair dryer might do it if you hold it close for a while. Watch the pin at the center of the coil, it should gradually turn by about 30 degrees and turn back when it cools down. If it doesn't move you've found the problem.

Before i started I got hold of some silicone oil from a model/hobby shop, used for RC car diffs, but i might not have needed it. This stuff will do it - http://www.rccarsandtrucks.co.uk/610_1_1055189.html but you have a choice. Toyota, who don't pretend their couplings should be disposable, quote using 3,000 cps for cold countries and 10,000 for Australia etc. I went for 7,000 to account for global warming and ended up using less than half a bottle for what i spilled before i'd worked out what to do. If you've drained it and bunged it up with rubber you would probably need no more than a bottle but thats a guesstimate, i couldn't find a figure for it. If you add too much you'll find the fan is always on so you could drain some out and try again. (Actually that's probably a kinder fix than rubber if you want an always on option or you can't free the pin)

If you have the time you might be able to save all the fluid by keeping the closed unit coil side up overnight (it moves slowly this stuff) to drain the fluid into the other side which doesn't need to be worked on. I thought i'd be able catch it when it came out in case i needed it but it's harder than it sounds, like treacle and tricky to handle, i was glad i had some spare. Might be best to crack the screws loose before leaving it to stand. You need a number 3 crosshead/philips screwdriver for the 4 screws which will probably be plenty tight.

Once you got the halves apart place the top half coil side up, spray some WD40 under the coil and tap the middle of the coil with a hammer, it should move in a mm or 2. Turn it over and hit the other end of the pin using a punch or head of a nail etc so it moves back through, turn it over and repeat until you can push the coil side back in with your thumb, until it's loose. Try heating the coil, if it now rotates add oil if you lost some, screw the 2 halves back together and get it back on the motor and try it out. When the engine gets up to temperature, took ages at fast idle, it should suddenly come on with a roar. A very satisfying fix

I think the problem is that this coil so rarely needs to function, only with towing or heavy off-roading, it has a lot of opportunity between times to seize up. Might be worth occasionally getting some heat on it or letting the engine get some heat up.

Great car these Fourtraks, well worth keeping them out of the scrappy for as long as possible so i hope this fix helps someone out!

All the best
Abdullah