Is it true?

[MMercury]

Just read in a book that locking the fan clutch (which is the way I have it in my coupe) shortens the water pump life. Is that true?

Am I the only one that does not trust viscosity clutches and would rather have the fan spin all the time?

Not exactly sure if you are describing the original style fan clutch with the drum brake friction material or the newer viscous style clutch. I am assuming it is the former.

I am not sure that I completely accept the notion that leaving the early style fan permanently engaged will “certainly” reduce water pump life. It is a question of degree. (Does leaving wiper blades in a horizontal position promote wind resistance and definitely reduce blade life? Does leaving an air-conditioning compressor clutch permanently engaged, automatically lead to early compressor failure? Or, is it more advantageous to let the compressor constantly cycle on and off?) The bigger concern is usually the drag on the engine, and in particular, the drag on an engine with a narrow power band in the upper rev range. It is one reason racers will attempt to reduce the most possible rotating mass, e.g., lightened flywheels, non power assist steering and axle driven alternators.

I tend to think a drive belt that is too tight can accelerate water pump failure far quicker than a permanently engaged fan. Same for an out-of-balance fan/fan clutch assembly.

Consider how many M10 engines ran or run with clutch-less/direct drive cooling fans. I am unaware that their water pumps have been any more prone to failure than the easier-to-replace M30 water pumps. It is true, however, that the early M10 fan assemblies were lighter and smaller than their M30 counterparts, but except in racing applications, I would argue the difference in water pump life and reliability is negligible. That said, I think the advantages to using a viscous fan clutch or an electric fan outweigh the permanently engaged alternative - unless your car is used 100% of the time in stop-and-go traffic or there is plenty of available FW190 prop wash.8)

In the quest to reduce rotating mass on an M10 clutch-less fan, one could delete the fan in favor of an electric version or modify the fan. I believe I still have a twin-blade fan from a 1600 as well as a few that were cut down. Another alternative was to run a flexalite fan or similar that flattens out at high speed and reduces drag. But it is still additional rotating mass. (Another downside to fiberglass fans of yesteryear was their tendency to become brittle and fragile. The metal flex fans are heavier than the BMW plastic equivalent.) Nevertheless, I have seen a flexalite fan or similar adapted to an E3 and it probably worked as well if not slightly better than a permanently engaged fan clutch. I never knew or questioned the reason for its installation.

Regarding viscous fan clutches, whether or not you personally distrust them, they have been used on a lot of cars, including BMWs with inline 6s, for a lot of years. I have had at least one failure but I continue to use them. I have also experienced a few flat tires over the years and continue to use inflatable tubeless tires too.

Worth re-reading? http://e9coupe.com/forum/showthread.php?t=3653

Porsche says:

UK style /extra cooling mode?

 

[m5bb]

Mine has the more modern fan clutch like a 535i might have. Temp runs below normal.
I have had a few fail but they only failed because they weren't locking up when the heat is on. Not the heater just that it is hot outside.
If you car has AC and the fan clutch is acting up you can always turn on the AC (electric fan turns on) and turn the thermostat down so the compressor is not running.
This would get you home unless you got stuck in some horrible heat and traffic.

Gary