Triumph Repair: TR7 Viscous Coupling Question, vented hoods, viscous coupling

Question
QUESTION: Hi Howard:

Thanks for your assistance with my previous TR7 questions.  

I had a general question about the fan viscous coupling on the TR7.  When the motor is not running, should the fan freely turn or should it just turn a little bit (i.e., maybe 45 degrees) when spun by hand?

Thanks
Matt

ANSWER: Hi Matt,

The fan should rotate cold with just a small amount of hydraulic resistance, but have much more resistance when heated. I don't remember ever seeing a quote on number of degrees it would move with a spin and I guess that would depend on how hard one could spin it.

I'm sure the manufactures meant well with viscus clutches but I was never impressed with the cost vs results of a viscus clutch. I did read a report once that indicated that most cars tested showed that above 35 MPH most all fans were a liability and only useful from 35 MPH and lower. This was even more true with cars that used lower front air dams or vented hoods (like the TR-7# and vented side fenders like some of the more exotic cars.

Most cars without the vented engine compartment, suffer from the normal high pressure air under the car thus restricting the volume of air that can get through the radiator. Even though at speed there is a layer of high pressure on the front of the car the design of the hood and vents of the TR-7 does vent any pressure in the engine compartment and thus allows more air to be forced through the radiator by the ram air effect.

Check the resistance of the fan when cold and then run the car until heated up and check it the same way to see that the fan is stiffer. I worked on many TR-7's and owned several and even though while working in the dealerships I was required to replace defective clutches I replaced mine with electric fans and used a manual switch so I could turn that off at cruise speed. #A/C cars came with electric fans thermostatic controlled)

Howard

---------- FOLLOW-UP ----------

QUESTION: Hi Howard:

Thanks for the reply.

I will do the checks you have indicated.

One note, however.  When the car is cold the fan will only rotate a short distance and stop.  This makes me wonder if the coupling is bad.  However I will do the checks you have suggested.

Thanks
Matt

Answer
I have found some that someone had added the wrong oil to it. The oil is a silicone based oil that gets thicker as it heats up and some of the units have a refill screw and some can be taken apart and if regular oil was added it will work backwards and the fan will spin free when hot and have resistance when cold. Silicone is thin cold and thicker when hot. That fan clutch oil is sometime hard to find.

Howard