Viscous fan problems

[ian65]

The main cause of burning, slipping and snapping fanbelts is the viscous coupler going bad.... this is very prevelant on 1st gen Rx7 given their age and limited use.





Undo the 4 no 10mm bolts that hold on the coupler to the front of the engine and and remove it and then check and free off the pin that controls the viscous coupler.
On the front of the fan is a bi-metallic strip that heats up and bows outwards.





Underneath this strip is a small brass pin which is usually pressed in by the bi-metallic strip. As the strip bends, it relieves the pressure on the brass pin allowing it to move outwards and this in turn operates the valve inside the coupler to allow the silicon oil inside the coupler to flow and work the coupler.

Sometimes, due to lack of use / age, this pin seizes in its hole and so the whole unit fails to operate, either being stuck open, closed or somewhere inbetween.

To check the operation of the pin, carefully prise out the bi-metallic strip from it's cradle on the front of the coupler and check that the brass pin moves smoothly and easily.










If not, polish it up and lubricate it then carefully refit the bi-metallic strip by pushing it back into place.
The strip, when cold, must be in contact with the pin and push it into its hole.
If the strip is heated with a hot air gun / hairdryer, it should bend and allow the pin to pop up from its hole.

If the internal seals have gone in the coupler and allowed the oil to seep out, it's time to throw it in the bin.

[Casey]

A very helpful post above - thx.

I've just removed the viscous coupler form Kimi to check it's operation. The pin moves freely enough, but it does not touch the bi-metallic strip when cold - it only protudes about 2mm or so. I can push it in, and it springs back to the same point. As I understand the operation though, the pin needs to be in contact with the strip and needs to move out further as the strip heats up and bends outward? The fan is permanently running flat out when the engine is cold, I assume it shouldn't cut in until the engine is warm?

Any way to fix the coupler? If not I'll think about the electric route I think!

[ian65]

Casey,
viscous fans are the work of satan and aren't worth bothering with in my opinion.... much better to go electric! ...... but if you need to get it working in the interim, these diagrams help show how it works...







When the engine is cold, the coupler, while not quite freewheeling, has little resistance and the fan spins relatively slowly. At this time, the bi-metallic strip presses the pin in, which in turn closes the valve within the coupler and stops the silicon oil from flowing. You can see that at this time, it is essential for the bi-metallic strip to be in contact with the pin and to be pressing it in as far as it will possibly go. Bend the bi-metallic strip flatter to achieve this if neccessary.

When the engine gets hot, the bi-metallic strip bows outwards, allowing the pin to pop out and subsequently allowing the valve plate within the coupler to move and the silicon oil to flow. This will progessively stiffen up the coupler, according to how far the pin has moved out and the valve has opened.
With the valve fully open the coupler is practically locked and the fan will spin at engine speed.

The only diy repair you can do to these couplers is to check the operation of the bi-metallic strip and the pin. If the valve inside the coupler has seized or if the fuild has leaked out in the past, then it's knackered. They are mega expensive to replace and not worth the hassle quite honestly.
Loads of 1st gen RX7's have had problems with these things as they have aged, the first sign usually being a squealing or snapping fanbelt.
Better to go electric.... how many manufacturers of modern cars fit viscous couplers these days?

[codge]

A very helpful post above - thx.

I've just removed the viscous coupler form Kimi to check it's operation. The pin moves freely enough, but it does not touch the bi-metallic strip when cold - it only protudes about 2mm or so. I can push it in, and it springs back to the same point. As I understand the operation though, the pin needs to be in contact with the strip and needs to move out further as the strip heats up and bends outward? The fan is permanently running flat out when the engine is cold, I assume it shouldn't cut in until the engine is warm?

Any way to fix the coupler? If not I'll think about the electric route I think!

My fan is the same Malcolm. Doesn't quite touch the bi metal. A spare fan is again the same. Car runs fine up to 4000 rpm with (relatively) good fuel consumption so it's OK for me.

4000 rpm may be too slow for RPM though?

[codge]

Yeh, they do blow a lot. I can see electric is much better because the viscous must be moving a lot of air unnecessarily.

But as said, I get by and fuel is not disappearing too alarmingly....well rephrase......I've got used to the rate of fuel burn.....space shuttle rate...

23mpg average over last 4 years...better summer and worse winter.

[ian65]

Oddly, the incessant roar of the fan going round makes me think of the old volvos that used to make a similar noise. They've got viscous fans too. I wonder if they'll fit the FB?

probably not and viscous fans aren't worth fannying about with.... if it's knackered, bin it and go electric... the engine will be much more responsive

[myatt1972]

Oddly, the incessant roar of the fan going round makes me think of the old volvos that used to make a similar noise. They've got viscous fans too. I wonder if they'll fit the FB?

I worked at an independant Volvo garage for 7ish years, the early 244's had a solid fan (very noisey) the later ones had viscous.
Both were crap and wouldnt fit a 7.

ELECTRIC !!!!! its the only way

[RX7SA]

Can anybody suggest which electric fan or rather from which car would be best to source the electric fan for my SA? My car was last on the road in 93 and have no doubt I will have this problem when I come to it. WOuld be good to try and source the parts now.

Thanks
Simon

[A7RXY]

Read this
viewtopic.php?f=23&t=74

[RX7SA]

Yep sorry after I posted I saw this thread and meant to delete my post but got called away to do some work! Sorry about that