grant v

I had my aircon repaired and we've been having some pretty hot weather is SA over the last week or so. I've notice that the temp gauge starts to go up high in traffic, but drops to normal when moving, particularly when I switch the aircon off.
How do I check if the viscous fan is still ok?

GlenL

Move the fan by hand when the engine is hot and off. If it spins fairly freely then it's bad. It should be fairly sticky. Yes, "fairly" isn't tremendously helpful so you've got to judge for yourself.


BTW - replacing the clutch is expensive compared to re-filling the clutch or putting in an electric fan.

WallyP

With the engine at full operating temp and the bonnet raised, reach into the engine compartment and rev the engine to about 2000 rpm and hold it there. After a couple of seconds, there should be a noticeable increase in the air flow from the fan.

There are several threads on refilling the fan clutch...

NoVector

Grant - Did it run hot before the A/C was worked on? I wonder if it's dragging your engine.

BTW - Electric is the way to go... my $.02. You can adjust when they turn on, less drag on the engine, and gives a lot more room for working on stuff in the front of the engine. I have no idea where you shop for one there, but a couple of the Renlist sponsors sell them and they're also available on e-bay.

IcemanG17

If your viscous fan is no good, I have two I'm not using that have plenty of tension......PM if interested

Landseer

You can refill one.

Requires a syringe, a basketball needle and a tube of silicon fluid.

Sources for fluid are toyota parts dept. or else hobby shop (for filling differentials on RC cars)

Toyota is more expensive.
Go with the thicker or thickest fluids.

Really helped my cooling problems.

jpitman2

If 'fixing' the AC has exacerbated a cooling problem, check for debris between the condenser and the radiator core - its amazing what can get caught in the gap there. get some good light down there , and if much rubbish, Blow out with air or water.
jp 83 S 53k, all electric cooling.

Landseer

Good point.

Also, can backflush radiator using a hose and a 16gallon heavy duty shop vac.



How old is water pump? How is seal behind thermostat?

dr bob

Highlighted section is urban legend. It takes more effort from the engine to move the same air with an electric vs the original mechanical fan. The alternator is way less than 100% efficient at converting mechanical to electrical energy, and the fan motor is way less than 100% efficient at converting the discounted electric energy back to mechanical. Saving grace for electric is they offer no drag when they aren't turned on. Plus the space issue mentioned above.

Landseer

I have one with electric.

Looking to convert it back to viscous, hopefully before summer season.

GregBBRD

When hot, if the fan spins more that a revolution or two after turning the engine off, it's bad.

NoVector

Dr. Bob - You've forgotten more about these cars than I'll ever know, so I defer to you. I’ve seen it on stated on Renlist before, but honestly, how the fan’s pulley and resistance from the fan doesn’t increase drag on the engine more than an alternator that’s already there anyway is more than my brain can comprehend. Just seems counter-intuitive… But there must be something to it, as I don't think any auto maker is designing cars with mechanical fans anymore.

dprantl

Just because the alternator is there does not mean it is doing anything until you ask it to give power. For example, try putting down all windows at the same time and see the RPM dip from the alternator's extra load.

Dan
'91 928GT S/C 475hp/460lb.ft

dr bob

There's no doubt that a well-managed electric fan is generally a better option. Mechanical fans with thermo clutches like ours are better than with no clutches. They react loosely to the temperature of the air that's passed through the fins. But they do spin whenever the engine spins, albeit at a lower speed with the reduced coupling of the viscous clutch. So there's still some load on the engine even when the fan isn't needed. My comment is solely on the situation where either fan would be required. Under that specific circumstance, the electric fan takes more horsepower than the mechanical fan to move the same amount of air. Remember, the alternator needs to generate the electric power ultimately used to turn the fan. That mechanical and the resultant electric power isn't free, any more than the direct mechanical fan power is free. With the mechanical fan, there are friction losses from the belt that are different than those for the alternator, but for electric-fan cars the air pump is still driven, and the difference between belt friction losses for light load on the alternator aren't signifcantly less than for the full load case. Meanwhile, let's say that the alternator is 95% efficient at converting mechanical to electrical energy. That's a very generous number. Continue the illustration with 95% example efficiency for the fan motors. Under these most optimistic numbers, net efficiency is no better than 0.95 x 0.95, or about 90% net conversion efficiency. If you could put your hand on the alternator or the fan motor while they are running, all that heat would be an indication of wasted energy, lost in the conversion proceses.

NoVector

Dr. Bob - Thanks, I think I understand what you're saying--and I've read a few other web pages this evening on the topic too. Although ref your statement above, on my 84, there is no belt to the air pump (either). It too is removed. I'm using a 928 Motorsport kit...

Sorry Grant--didn't mean to highjack your thread...