viscous fan

g4zur · Postby **g4zur** » 03 Nov 2013 19:39

Hi All
Can someone please explain to me how the viscous fan should work.
I have no idea, sorry for the Ignorance.
My fan spins from engine start up, Is this correct.
Many thanks
Regards
Gareth

G.P.Albrighton

Beans · Postby **Beans** » 03 Nov 2013 19:52

Quite correct.

<center>

1976 TR7 FHC (needs some TLC ...)
1980 TR7 DHC (my first car, a.k.a. Kermette)
1981 TR7 FHC (Sprint engined a.k.a. 't Kreng
[url="http://www.tr7beans.blogspot.com/"]My full Weblog[/url]</center>

bmcecosse · Postby **bmcecosse** » 03 Nov 2013 20:28

The idea is that at higher revs it 'slips' ...

g4zur · Postby **g4zur** » 03 Nov 2013 21:10

Thanks very much to you both.
Gareth

G.P.Albrighton

Hasbeen · Postby **Hasbeen** » 04 Nov 2013 00:22

What ever you do, don't fit a new one, unless the car is overheating badly, & you've done everything else.

My new one is spinning much faster, & pumping so much air it is really noisy. I have to run the radio about 6 positions higher, 22 as against 16 on the readout, just to hear it.

Hasbeen

supercass · Postby **supercass** » 04 Nov 2013 12:04

[quote]Originally posted by Hasbeen

What ever you do, don't fit a new one, unless the car is overheating badly, 

 If it is noisy though, but not necessarily overheating doesn't that suggest wear and that it is at risk of flying off and causing damage? supercass

Cobber · Postby **Cobber** » 04 Nov 2013 13:05

I've never trusted clutch fans, had too many seize up at speed and shearing their mounting bolt then screwing their way into the radiator.

I say bin the damned things, and replace with thermatic fans.

"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"

80'Triumph TR7, 73'Land Rover (Ford 351. V8),
'89 Ford Fairlane
'98 MG-F, 69'Ford F250.
76' Ford F100

FI Spyder · Postby **FI Spyder** » 04 Nov 2013 13:18

You can tell a bad fan clutch by carefully turning it. Like a bearing it should be silky smooth. Like my Spider one, spare used one and new spare one. The one that was on the engine Ron gave me was not good and you could feel it was bad when you carefully turned it.

The idea behind the clutch is a fan will pull/push more air the faster it turns. There is a limit to this as it can only pull (push) so much air as at a certain speed you will get air cavitation. This happens in any fluid be it air or water and it happens with airplane propellers and motor boats. It happens around 3,000 rpm. (this varies on design as well). The amount of air (fluid) you can move can vary depending on the shape of the fan, numbers of blades, shrouding etc. but they all have limits. As car engines can go faster than 3,000 rpm there is no point in the fan turning faster than that as when the air/water starts to cavitate you will get less being moved. When the resistance of the air being moved equals the drag on the clutch, the fan will have reached maximum rpm. This limits the amount of draw (stealing of hp) on the engine. Not as efficient as an electric fan (why it's no longer used) but better than a solid fan of older cars.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

Mark H · Postby **Mark H** » 04 Nov 2013 13:30

[quote]Originally posted by g4zur

Hi All
Can someone please explain to me how the viscous fan should work.
I have no idea, sorry for the Ignorance.
My fan spins from engine start up, Is this correct.
Many thanks
Regards
Gareth

G.P.Albrighton
<hr height="1" noshade id="quote"></blockquote id="quote">

Seems there is some confusion about how the fans should work.

Fundamentally, it is a THERMO viscous fan. That means there is a fluid inside the clutch that gets more viscous as it gets hotter, thus making the clutch stiffer.

They are supposed to move a little air when cold at lower rpm, and slip at higher revs as others have said. When cold the threshold for the slip is quite low, maybe around idle. So the fan shouldn't really spin any faster when you rev up the engine.

When hot though, the clutch should get much stiffer. Revving up the engine should produce a roar from the fan. This only happens when the clutch gets really hot, which it might only do in standing traffic on a hot day.

The best way to check a clutch is to spin the fan when cold, and it should only spin about one revolution. If it spins for longer, the clutch is shot. If it doesn't spin at all, it is also shot.

They seem to have two failure modes : spinning too freely all the time, so the fan does nothing at idle, and locking up all the time.

1980 TR8, coming back to life

Hasbeen · Postby **Hasbeen** » 04 Nov 2013 13:46

As some may remember, there was nothing wrong with my old fan. However the new water pump had the impeller slipping to some extent on the shaft, in my new engine, causing overheating, as you can imagine. This was hard to diagnose when I expected the new pump to work.

Assuming the thing was getting hot because the new engine was tight, I fitted a new radiator & fan. Where the old fan would spin for one revolution when spun, the new one will stop almost as soon as the spinning hand is removed. it obviously spins more at higher revs, as the wind noise is much higher.

I have not refitted the old fan, as we are now heading into 35 C days, when I will want the air conditioning, so we will see how hot it gets then.

Hasbeen

Beans · Postby **Beans** » 04 Nov 2013 16:11

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Hasbeen

... My new one is spinning much faster, & pumping so much air it is really noisy ...<hr height="1" noshade id="quote"></blockquote id="quote">
I have the same symptoms with the new coupling on the DHC.
But as the temperature remains very constant under all conditions I can live with it.
And with the top down the noise is drowned by other sounds [:D]

<center>

1976 TR7 FHC (needs some TLC ...)
1980 TR7 DHC (my first car, a.k.a. Kermette)
1981 TR7 FHC (Sprint engined a.k.a. 't Kreng
[url="http://www.tr7beans.blogspot.com/"]My full Weblog[/url]</center>

Beans · Postby **Beans** » 04 Nov 2013 16:14

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by bmcecosse

The idea is that at higher revs it 'slips' ...<hr height="1" noshade id="quote"></blockquote id="quote">
Also at higher speeds the increased airflow through the radiator will stop the fan.

<center>

1976 TR7 FHC (needs some TLC ...)
1980 TR7 DHC (my first car, a.k.a. Kermette)
1981 TR7 FHC (Sprint engined a.k.a. 't Kreng
[url="http://www.tr7beans.blogspot.com/"]My full Weblog[/url]</center>

FI Spyder · Postby **FI Spyder** » 04 Nov 2013 18:08

There is no difference in the slip resistance between when it's cold or when it's hot on my car. It doesn't overheat with the difference in gauge reading between all types of driving including stop and go at 80 degrees. At really cool temps it might be another needle width lower which would be regulated by the thermostat opening rather than the fan.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

Mark H · Postby **Mark H** » 04 Nov 2013 19:24

Can be pretty hard to test the hot mode, it has to get really hot. The only time I heard the one on my XJS really kick in, was a hot day standing in traffic for a while. Then you can really hear the roar from the fan.

1980 TR8, coming back to life

FI Spyder · Postby **FI Spyder** » 05 Nov 2013 18:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mark H

The only time I heard the one on my XJS really kick in, was a hot day standing in traffic for a while. Then you can really hear the roar from the fan.
<hr height="1" noshade id="quote"></blockquote id="quote">

Are you sure is was from the viscous fan or was it from the A/C electric fans. Since the clutch is the same I'm assuming it would be a similar system where when it gets hot the thermal switch in the rad turns on the electric A/C fans to help cool things down (they make a racket).

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

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