Viscous fan coupling

Changing a viscous fan coupling takes less than half an hour. Alisdair Cusick shows us how to tell if yours has failed, and how to replace it

How to replace it in 30 minutes

THE two main forms of cooling on an engine are simply air forced through the radiator by vehicle movement, and the viscous fan assembly. The viscous fan assembly comprises a set of fan blades mounted on a hub which is, in turn, fixed (in the case of this Range Rover Classic V8) to the coolant pump. The coolant pump is driven by the ancillary drive belt from the crankshaft, and the hub of the viscous fan assembly turns with it. When the engine is sufficiently cool and no fan cooling is needed, the fan (via its viscous coupling) is uncoupled from the hub and is not driven by it. As the engine coolant temperature increases, the viscous coupling reacts to the heat and engages drive to the fan to cool the engine. This occurs, for instance, in standing or slowmoving traffic when there is no air flow through the radiator – the fan pulls the air through to cool the coolant as it flows through the radiator. The fan will engage

whenever the prescribed upper temperature is reached.

A viscous fan works mechanically, basically using two components. The inner part is mounted to the water pump (or idler pulley on other engines), rotated via the fan belt as the engine rotates. The outer part is attached to the fan blade assembly, and works using viscous fluid. The fluid can go in two areas, according to the opening and closing of a valve controlled by a bi-metal thermostat attached to the inner part where the fluid usually sits. As the engine temperature rises, the thermostat opens the valve, allowing silicone fluid to flow into the outer part, causing both sections to lock. Thus, the fan is now driven and extra cooling effect is provided. It is a relatively simple fail-safe to prevent engine temperatures getting too high.

So how do you know if the viscous coupling has failed? Well, from cold the fluid will drain down, and levels out

between both inner and outer reservoirs. Starting from cold, because there is fluid in both areas, the fan is coupled until the engine centrifuges the fluid into the inner chamber, whereby the fan de-couples and is no longer driven. You’ll know this entire process is happening, as the fan roars characteristically like a jet engine on start up, until the fluid redistributes and the fan stops, along with the roaring noise. That is the first sign all is well with your viscous fan. As soon as you notice it not kicking in briefly from cold start, or that it continues roaring, that suggests it needs checking out.

With the car switched off, and with a cold engine, there are a few checks you can do on the viscous coupling. Firstly, try moving the fan back and forth, along the axis it rotates. You should feel good resistance, as the fan should be coupled with the engine having stood a while. If it spins very freely, then suspect an issue. Secondly, try wiggling the fan gently forwards and backwards, from radiator to engine bay. Any play at all in this plane suggest the viscous unit has developed play between the inner and outer parts, and should be replaced.

Replacing the unit is one of the simplest jobs on the vehicle. You will need a special viscous fan spanner with offset jaw: 36 mm AF on my Range Rover Classic, but check your vehicle’s spec (32 mm on a 200Tdi, for example). The most difficult thing will be unbolting the viscous fan assembly from the water pump, but I have a cunning tip to help. I’m showing you a 3.9-litre V8 with serpentine belts but, broadly speaking, all Defenders, Discovery 1 and 2s, Range Rover Classics and P38AS are almost identical. The difference will mainly be the left or right hand thread on the fan nut. Get the thread wrong, and you’ll definitely struggle!

Get to the job

1 Improving access to the fan is simple, just pop two clips on the shroud and off it comes. You can now check the fan or remove it easily.

Cold check

2 First check is the rotational resistance. Holding a fan blade, rock it back and forth. You should feel good, firm resistance. Free spinning is bad.

Any free play?

Next, try rocking the fan from 3 radiator to engine block. There should be no play at all. Any loose rocking suggests the viscous is past its best.

Start-up test

4 Start the car. The fan should roar, sounding like a jet engine. Thirty seconds to one min later, it should stop as the fluid centrifuges out. Keep fingers clear.

Crucial tool

5 This is a typical viscous fan spanner, with 32 and 36 mm jaws, but note the cranked head. You could modify an old spanner if you wished.

Know your car

6 Be careful with the thread direction. Mine is actually right-hand to undo, but there are right and left-hand depending on model and engine, so check the manual.

Release the nut

7 Spanner securely located on the viscous nut, try and undo it. It may come off, but most likely you’ll eventually rotate the water pump. Worth a try anyway.

Shock and awe

8 Try a little encouragement. Check the spanner is secure, then with a hammer give a sharp tap. The shock can sometimes undo it, but if it doesn’t, don’t worry.

Tools work for you

9 Slide a long screwdriver to work against the water pump pulley and its mounting bolts. You can now work the spanner whilst locking the water pump.

Remove the fan

10 Spin the fan around to thread the viscous coupling off the water pump pulley, then remove the fan assembly upwards. Note the fan blade direction.

Split the unit

11 Using a hex bit socket, the four mounting bolts securing the fan to the viscous coupling are undone. They’ll be tight, so take care not to crack a fan blade.

Check all is well

12 Old unit out, new unit ready to go in. Take time to check the fan over for any cracks, mud or damage. A blade coming off can destroy a radiator.

Fan to the front

13 Mount the new viscous unit to the fan the correct way round. One face is usually marked as FRONT, if you do forget (rear side shown here).

Equal tightening

14 To ensure it goes on square, just like fitting a road wheel, fit one bolt, then fit and nip up the opposite one. Finally, check all four are tight.

Refitting

15 Slide the reassembled unit back into the engine bay and start the viscous unit nut onto the water pump thread by hand. Don’t cross the thread.

Swap tools

16 To tighten, you’ll definitely need to be able to secure the water pump pulley. Reverse the screwdriver and spanner as you used them before. Refit the shroud.

Check it works

17 Start the car. The fan should spin, characteristically roaring for a minute, then stop (keep hands clear). Mine now did that, confirming my original unit had failed.