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Date: Thu Jul 11 07:51:21 1991   
To: (z-car list)
Subject: Re:  Cooling problems

<I've had my '77 280 for four years now, with no cooling problems
<until recently.  Over the last month, I've noticed it's been
<running hotter and hotter.  Last night the temp gauge was close
<to redline after about 10 minutes of driving, and it overheated
<shortly afterward.  I let it cool down and drove the rest of the
<way home successfully with the heater on.
<It looks to me like my fan clutch may be wearing out.  I used
<to be able to hear the fan when idling while the engine was hot,
<but now I don't think I hear it.  I also found that the fan 
<rotates freely when the engine is stopped (and still hot).

Funny you'd mention that.  I just replaced my fan clutch.  I'd been
chasing an overheating problem for months.  Even mounted an electric
fan up front.  What finally clued me in was that I noticed that the 
fan did not rev up much even when the gauge was near red.  Replaced
the old one and the problem disappeared.

I took the old one apart and discovered why.  There are actually
2 clutches inside the unit.  One is a disk that spins in close 
proximity to a couple of others and and uses oil drag to 
provides the residual drive even when the clutch is cold.  The other one is 
a similar assembly with a thermostatic valve that varies the amount of oil 
in it according to temperature.  This unit is responsible for high drive 
during hot weather.

In my case, most of the oil had leaked out over the 17 years it was in
service.  The oil looks like 120 weight gear lube and has that 
wonderful japanese fish oil smell.  I suspect I can refill mine with 120
weight gearlube and reuse it.  Great incentive to, as the aftermarket 
part cost $80.


Subject: Re: Cooling problems
Date: 13 Jul 91 00:03:11 EDT (Sat)
From: John De Armond

		Normally, the fan does nothing traveling down the road at
		over 30 mph. (thus the clutch to save horse power). The air
		going thru the front of the radiator is more than enough

Not exactly, as I found out on my car.  Even before the high 
capacity radiator, the temperature would rise a bit unless the 
speed was above about 50.  After I replaced the clutch, the temp
meter stays straight up at anything over about 10 mph.  I've
observed the same effect on my 260 which has a stock radiator.  
Zs without hood louvres just don't cool very well at moderate

As an aside, I've been doing some experimenting over the last
couple of weeks during our heat wave here in Atl and have 
finally figured out that louvres will help my car's cooling
quite a bit.  A set is shortly going to be cut in the hood over
the turbocharger and if that is not enough, a set of shark gills
will be added to the quarterpanels.  Ultimately I plan to have
an oil cooler on one side and an intercooler on the other where
the battery and windshield bottle are now.
		Silicon is used inside the clutch fan.  As it heats up, 
		it causes the fan clutch to conduct, turning the fan. 
		As it cools, the clutch drops out, and the fan free wheels,
		and the engine is not required to pull it.  Great gas saver
		and hp saver. 

Not in Z clutches.  The oil is the distinctly fishy oil that the
japanese are so fond of.  As I mentioned in another note, this oil
has an amazing resemblence to 90 wt gear lube.


Date: Mon Jun 28 21:57:19 1993   
Subject: Re: Fan Clutch Diag

 Is it a bad idea to replace the stock fan with an
 electric one?  It seems that on hot days (most any
 day here in Dallas :-) the fan causes significant
 drag and makes a bundle of noise...

| Lasse Ohlsson         Ericsson Network Systems Inc.         (EXU Dallas) |
| E-mail:          Internet Z-car club member # 48  |
| My company does not trust me to have my own disclaimer... So I won't.    | 

[An electric fan won't move anywhere near the amount of air the engine
fan will when the clutch is fully engaged.  I know, I've tried it.
I *hate* the sound of a Z fan winding up.  At one time I had two pusher
fans and two puller fans on there at once.  Sucked up just about all the 
alternator could put out and still the engine got hot in traffic.

Date: Mon Sep 13 01:33:26 1993   
From: John De Armond
Subject: Rebuilding the fan clutch
To: (z car list)

Well I searched my archive and couldn't find my clutch rebuild article
so I decided to just write it again.  This article applies to any
Z-car clutch that can be disassembled.

The clutch works by shearing a viscous fluid, the drag from which couples
force from the driving shaft to the driven hub.  The clutch consists of
two sections, a fixed drag section that provides a minimum level of
torque and a variable drag section controlled by a bimetal thermostatic
coil visible from the outside.  

The most common failure, indeed the only failure I've ever seen, is
from the viscous fluid leaking out.  Typically this happens over
several years and is not noticed because it doesn't leave a nice wet spot.
If the unit goes completely dry, the clutch will sometimes seize.  
More common is that just enough fluid leaks out that the variable side 
no longer functions.  The fan still turns at idle but there is insufficient
drive to cool the engine.  

Generally the seals are still OK and don't need to be replaced.  Rebuilding
consists of opening the housing, cleaning it and refilling with fluid.
Opening the housing involves removing the clutch from the fan and removing 
the 4 6 mm bolts that hold the housing together. Then gently pry the 
housing open.  There is an O-ring so a little force is needed.  There is
nothing to fall out and no springs to surprise you :-)

Pour out any remaining oil and clean with brake cleaner or something
similar.  Then completely fill the shaft-side housing with 90 wt gear
oil and add about 5 cc of laquer thinner.  This slightly swells the
rubber in the seals and makes them more leak tite.  Make sure all air
bubbles are eliminated.  Then assemble the housing, replace the bolts
and the job is finished.

From: emory!!rick (Rick Kirchhof)
X-Source: The Hotrod Mailing list
Date: Jul 1993
Subject: Re: street comp. ratios

>  (Eric Y's engine runs >200F w/ 180F t'stat)
>  Sounds to me like more cooling is needed. If the cooling system won't hold
> its set point and you've got a good t'stat (maybe a high flow one is worth
> trying) then there's a problem with dumping the heat. More rows or more
> airflow, probably the latter since the A/C seems involved. Maybe a pusher
> fan tied to the A/C compressor clutch line would help you out.
> ----------
> Posted by: emory!!jws (James W. Swonger)

Several good ideas already mentioned, but I have had good luck in a
number of cars using one or both of the following ideas.  This assumes
that normal cooling system things like fan belt tension, slippage on
chrome pulleys and radiators that need to be rodded out have been 

You can adjust the engagement point of your fan clutch.  You can also use
a "extra heavy duty" fan clutch and/or a different fan blade.  The
highest strength units are found in diesels and  1 ton 7.4L P/U trucks.  
You can find out if changing the set point of the fan clutch will help 
the problem.  Reach around front of the fan clutch (not always easy to
do) and un-hook the spiral spring from it's anchor.  Putting the spring
end on one side of the anchor will fully engage the fan for all temps,
and the other side will totally lock it out.  Try it each way, and run
the engine above 2000 rpm's for at least 2 minutes.  The change doesn't
always occur instantly.  Leave it in the position that makes the most
noise/airflow.  If you can't make it really roar, buy a new, good
quality, THERMOSTATIC fan clutch.  The non-thermostatic ones are as bad
as flex fans.

Do a test drive creating all of the problem conditions and you will hear
constant roaring from the fan as well as finding the maximum cooling
limits of your current engine/cooling system combination.  If this solves
the problem, set the fan clutch to be more aggressive.  If not, AND you
heard LOUD fan noises at 50+ MPH, your radiator, airflow or coolant flow
is a problem.  

Airflow can be changed by increasing the action of the clutch or going 
to a steeper pitch fan and/or one with more blades.  GM uses 5 and 7 
blade asymmetric fans that DO NOT FLEX.  ANY flex fan is the wrong 
solution for real cooling above an idle.  To learn how to set the fan
clutch spring, find a junk one or remove yours.  Heat the spring 
briefly with a lighter keeping the flame moving.  Watch the direction
that the center shaft turns.  This is what happens when the air temp goes
up and the spring issues an engage order to the clutch.  Let it cool, and
un-hook the spring.  Rotate the spring/shaft to the end of travel.  You
will want to move the loose spring end by 1/8 to 1/4 inch for each
adjustment.  If you cause the shaft to be more in the same direction as
it went when heated, you will make the fan begin to lock up at lower
temperatures.  The other way will make it more lazy, and allow higher
water temps before the fan begins to help.

I have done this for years to help out lazy and worn fan clutch units 
that caused poor A/C performance, overheating, run on at shutdown, ping
when related to coolant temperature, and so on.  It is important that 
the spring be clean.  This makes for quick reactions as the air temp 
changes.  Brake clean is good for a quick spray off and grime remover.  
With practice you can make the adjustment installed, and need no tools.  
This allows you to save $ and correct for minor problems that even new 
units have with their set points.

Try it, it works.

From: emory!!rick (Rick Kirchhof)
X-Source: The Hotrod Mailing list
Date: Jul 1993
Subject: Re: Cooling (was Re: street comp. ratio)
X-Sequence: 5712

> This is probably true for GM cars but it isn't true for GM trucks.
> The light trucks use flexible fans and no fan clutch.
> I haven't had any cooling problems when using flexible fans but I
> have had big time cooling problems with the clutch type fans.  I'll
> stick with either flex or electric fans, thank you.
> Bob Hale
> ----------
> Posted by: emory!!hale (bob hale)

Are you referring to S trucks or full size C, K, & R models?  I have 
never seen flex fans on full size iron, or am I missing something?  
Refer to my original post for the reasons I "modify" them.  People who
buy the low dollar brands or those that are not true thermostatic fan
clutches, are disqualified for not paying attention.

Think of a truck pulling a heavy load up a hill.  When the transmission
(or driver) downshifts, the rpm's rise.  Flex fans move quite a lot of
air at low engine speeds, but BY DESIGN flatten out as the engine speeds
up.  Back to our truck climbing the hill with your car hauler, travel
trailer, boat, etc, in tow.  Just when the engine and A/C need max
airflow, the flex fan has laid down and died.  In reality, the faster you
rev it, the worse it works.  You also pay for the extra drag under normal
conditions because the low speed airflow is what the flex fan specializes
in.  There is just no way to turn it off, and the fuel economy suffers.
Fixed, agressive blades and a properly functioning fan clutch are hard to

I stand by my statement.

From: emory!!rick (Rick Kirchhof)
X-Source: The Hotrod Mailing list
Date: Jul 1993
Subject: Re: Cooling (was Re: street comp. ratio)

Bob Hale continues to reply with:

> To get adequate cooling you need air flow through the radiator.  Once
> you get to about 30-40 MPH there is plenty of air available just because
> of the motion of the vehicle through the air.  The fan becomes unnecessary
> at road speeds.
> Flex fans have excellent air flow capability at low engine speed and
> they flatten out and reduce their drag and power requirements at
> higher engine speed which normally corresponds with higher road speed,
> just where you don't need the fan any more.  I have fixed cooling problems
> by replacing cluth fans with solid drive flex fans.
> The flex fan does have a drawback - it's lots noisier than a non-flex
> fan, and the nature of the noise changes interestingly as the RPM
> gets into the range where the blades flatten out.  It's strange to
> hear what sounds like the engine RPM decreasing as you rev it up.  It's
> actually the fan sound changing as the blades flatten out.
> If you happen to have a compound low gear and you happen to need very
> high power output for long periods of time at low vehicle speeds then
> a flex fan might not work for you.  Normal operating conditions don't
> include these circumstances.
> BTW, don't try to run a rigid fan with a solid coupling.  It's
> guaranteed to smoke your fan belt when you rev it up.
> Bob Hale         
> ----------
> Posted by: emory!!hale (bob hale)

What do you pull?  I know of any number of RV owners who have done what
you say and find that there is NO airflow just when they need it.  Your
comment about airflow from road speed is true but optimistic.  Try more
like 45+ mph.  In a Class A motorhome (those that use a complete body made 
by the motorhome company), road airflow may never become sufficient due to
restrictive front end designs and lots of heat generated.  Even a lowly 6
cylinder P/U, when equipped to tow, will have a numerically high axle
ratio that would cause a flex fan to not work when the needed because of
high cruise rpm's or a downshift.

Compound gears would certainly cause poor flex fan performance, but many
other real world situations do as well.  Consult _Trailer Life_ or a
similar motorhome magazine for readers experiences with your idea.  Late
model Suburbans with 7.4L and dual factory air have the best fan clutch
you can buy AS WELL as an electric pusher fan to keep the A/C from
wimping out at idle.  A flex fan will not perform as well at any speed
under this kind of load.  BTW, the drag even on a flex fan that is fully
flat is quite a bit higher than a freewheeling clutch fan.

In most situations, cooling can be less than optimum and you will get away
with it.  When you really push the total drivetrain, lots of marginal
equipment introduces itself to you.

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