Viscous Fans – Benefits?

I am trying to work out the pros and cons of replacing the viscous fan coupling on my Series1 XJ6. My coupling allows the fan to spin freely with the engine stopped, but as I understand should only allow a quarter turn before stopping.This fan is not thermally controlled in any way.But when my engine starts, the fan turns at the idle speed, so must be pulling air through the radiator.Now what else happens with the car underway is not so certain.

As I see it, the benefits of a viscous coupling as fitted to the series 1 are:

Fan works well up to 2500rpm, so ensuring good cooling in slow traffic.

Fan slips at speed, reducing engine power needed to drive the fan and also resulting in less noise. The fan does not free wheel at high speedso reducing drag. A free-wheeling fan or propeller has the same profile as a similar diameter flat plate at speed – hence whya multi–engine aircraft either feathers or locks the blades should an engine fail.

So the question is – should I replace my slipping coupling (cost $250 recon) or just run as is? I don’t see much reduction in drag at speed leaving it on as the engine and bonnet are creating much of that “drag” With the car travelling at say 75mph at 3000rpm, will the fan be spinning at that 3000 rpm speed?
I have just put the radiator in to replace the original double row core with a modern triple row.The shop reckon that it will have almost twice the cooling capacity of the old radiator, so do I really need a properly working viscous coupling?

 

Have you driven the car yet? If so, are you happy with the coolant temp under various conditions?

Cheers
DD

 

You seem to relate the fans function with speed and rpm. I thought the fan engaged in response to rising temp. It will be loosely coupled at start up or when testing with the engine cold. As temperature increases the valve controling the coupling fluid opens and the fan is driven more at engine speed , that may happen at any rpm/speed depending on ambient air. Is the fan in the S1 different?

 

Yes the fan on the Series 1 is not thermally controlled. It merely slips at an certain RPM - around 2500 I believe. My car has not suffered overheating, despite my fan coupling not supposedly working properly, which is why I am posing this question. The fan rotates at the idle speed, so pulling air through the radiator in a traffic jam. So my question is - Why do I need a properly working viscous coupling" - the answer would seem to be - I do not! With the new triple row radiator, there will be even less work for this fan to do. Yet over the years I have seen so many references to making sure that this coupling is not slipping!

 

Hi,
I think it is a case of 'suck it and see'. If you decide that you do have adequate cooling then no further work is required.
Over the years I have had considerable problems with viscous coupling fans, particularly in the Rover SD cars I drove for many years and as such are rather prejudiced against them (unfairly?). No doubt about it, if the fan continues to spin freely when the engine is stopped, I would say the fan's mechanism has failed and in the case of the Rover, overheating would be just around the corner, steep hill or traffic jam.
I certainly like the idea of the upgraded radiator, in my opinion too much cooling capacity is barely enough
My preference tends to be for electrical thermostatically controlled fans but that's a personal thing.
Cooling is always problematic in as much as there are numerous rather cheap and/or fragile components integral with the system, I'm thinking thermostats, hoses, vee belts, radiator cores and water pumps, the failure of any one can totally destroy your engine, should they fail and the rising temp go unnoticed. When they do fail there is often no warning.
This is not the case with many other engine failures which might be maddeningly inconvenient but does not ruin an otherwise perfect engine.
An acute sense of smell tuned to the sweet whiff of radiator coolant is often your first warning of a failure.
Al

 

All the fan clutches slip to some degree, even new ones. It's a matter of how much? and when? Yours, in combination with the upgraded radiator, is obviously getting the job done even if it's less-than-optimal in its operation.

The thermal clutches are more likely to cause or contribute to overheating as the typical failure mode is non-engagement or weak engagement....meaning the fan simply wont turn fast enough to pull enough air. Even this can go unnoticed if the car is driven in only in cooler temps and/or doesn't see a lot of stop-n-go city traffic.

Cheers
DD

 

You are spot on here Alan. My bottom radiator hose blew off once and the smell was the first thing I noticed!

Doug, thanks for your comments. I never intend to drive this car into the city whether in winter or summer, mostly a country car, but could get stuck in traffic for 5 minutes or so on a hot day.
I think with the new radiator in, the fan will be sufficient as it is.

John