The PAS pump on my Discovery failed fairly spectacularly last week, shedding it's pulley at the side of the road. In order to get at it to replace it, I need to get the fan off, which requires undoing a left-hand-thread nut in the viscous coupling.

The problem, however, is that I can't stop the pulley rotating in order to undo it (it's on it's own pulley rather than being on the end of the crankshaft). Normally this would be held in place by the serpentine belt, but without the PAS pulley I can't refit the belt...

Any suggestions? I can't get anything in there to wedge the pulley...

Edit: It's the big bolt in the middle of this part (thanks Paulster2 for the link): Viscous coupling

And it is located here on the car: enter image description here

The big pulley spins freely. I've tried using the small bolts that hold the pulley on for extra purchase - two of the four came undone (great, I thought!), but the other two won't - because of the hexagonal shape of the big nut, I can't get a ring spanner on them, and an open-ended one just started to round them...

  • Do you have a picture by any chance? If it is anything like my BMW, there may be a bolt head you can use for extra purchase
    – Zaid
    Commented Jul 9, 2014 at 20:16
  • 1
    Is this the part you are trying to remove? I take it you cannot get the large nut off of where the pulley is supposed to be? If so, I can add this image to your post and you can describe it better. Commented Jul 10, 2014 at 1:46
  • @Paulster2 - that's the one, I've added the photo, thanks. My photos is a bit dark, but hopefully you can see how the two relate...
    – Nick C
    Commented Jul 10, 2014 at 8:11

3 Answers 3


I take it the above picture of the fan/pulley on the vehicle is not your vehicle, but a photo of the same thing from another vehicle?

If it is your vehicle, wrap a belt completely around either ribbed pulley part and hold it on one side to gain purchase (grab both sides of the belt in one place with one hand). Believe me when I say this will give you more than enough purchase to get it done.

EDIT: If you do not have enough purchase by just using your hand, try wrapping the belt around the pulley and grabbing the belt with a pair of Vise-Grips (or other locking pliers), then grab the Vice-Grips and have at it. This should give you enough purchase.

EDIT2: Something like this:

enter image description here

The arrows denote where the Vice-Grips should be placed. Get the Vice-Grips as close to the pulley as possible. Use enough gripping force to grab the belt so it won't slip, but not enough to damage or destroy the belt itself. You can roll the Vice-Grips back on the pulley for more purchase (will sinch up the belt for more grip on the pulley).

If this isn't your vehicle, how much of the original pully is left? If there is any bit of the jagged edge of the pulley, you should be able to get a screwdriver or other longer prying device in between the pulley and the water pump to stop the free motion of the nut you are trying to loosen. Only pry out enough on the pulley to gain purchase: prying out on it harder may damage the main bushing of the water pump underneath, which will cause premature failure.

If all else fails, take the water pump off entirely. This will give you better access to everything. You may even be able to hold the impeller of the water pump still while breaking the fan nut free. Buying a new water pump gasket would definitely be worth the cost compared to the amount of time you've spent trying to get this thing free.

  • That is my car. I've tried wrapping the belt around it, but still couldn't get enough purchase. Perhaps I just need to find a stronger helper... It's not the water pump - as far as I can tell that pulley only does the fan - and it's attached to the front cover of the engine, which I don't think I can get off with the fan in place...
    – Nick C
    Commented Jul 10, 2014 at 10:55
  • @NickC ... See above for EDIT: :D Commented Jul 10, 2014 at 12:20
  • @NickC ... See above for EDIT2: ... :D Commented Jul 10, 2014 at 12:36
  • hmm, looks worth a try! I'll have another go tonight... Not bothered about wrecking the belt as I have a new one to replace it - the old one was damaged when the PAS pump failed
    – Nick C
    Commented Jul 10, 2014 at 13:06
  • @NickC ... Glad I could help! Commented Jul 10, 2014 at 22:21

I believe you can improvise over the approach shown in this diagram.

E39 M5 fan clutch removal diagram
(source: m5board.com)

Instead of using a spanner to 'lock' just one of the pulley bolts, try to use a long implement similar to the "special tool" depicted in the diagram to lock two pulley bolts in place. I have found great success with water-heater mounting brackets since they are thin enough and do not flex under load (plus the grooves were already cut in to wrap around the pulley bolt).

Before attempting to undo the fan clutch nut, I'd advise to let it soak in WD-40/ PB-blaster for a few hours to make for easier removal.

  • The diagram doesn't work, but your concept is sound, I'll try that if I can figure out a way of rigging something up - I tried it with a screwdriver between two bolts, but gave up when that started to bend!
    – Nick C
    Commented Jul 10, 2014 at 10:57
  • @NickC : a screwdriver won't do, as you've discovered :) It might be worth it to modify an angle bracket.
    – Zaid
    Commented Jul 10, 2014 at 13:52

Using the new pump, you are able to determine the size of its large nut. It is either 34 or 36mm across the flats. Using a flat piece of metal, make a flat spanner with a length long enough to extend out of fan blades to fit the nut. With the 'spanner' on the nut and with a engineers hammer, strike the improvised spanner to undo the nut. The inertia of the assembly allows it to come undone. Viscous couple spanners to do exactly the same are available here in the UK for around £5 ($10 ?). Use a dab of Locktite when fitting the new pump, impacting it in the other direction to tighten it. In a workshop with an air chisel, they can be undone by a couple of short bursts aimed at an angle to the nut.

  • I've got a spanner (though they have gone up, it was nearer £15, I was expecting around £5), the problem was stopping the pulley from turning while undoing the nut.
    – Nick C
    Commented Jul 11, 2014 at 13:46
  • The threads on the coupling have a wide smooth pitch, the thread is a left hand thread. They do not need to be tightened to a high torgue as the rotation of the engine will always be too tighten the coupling. The idea is to strike the spanner with the hammer to undo or tighten it. The small amount or inertia of the coupling together with the impact of the hammer on the spanner will loosen the nut. It is then a case of just pushing the nut undone with your finger, whilst wiggling the fan assembly to loose its weight. Commented Jul 12, 2014 at 12:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .