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RE: One more electric fan conversion question

To: "'Trevor Jordan'" <tjordan@bigpond.net.au>
Subject: RE: One more electric fan conversion question
From: Chris De Wet <DeWetC1@sapo.co.za>
Date: Thu, 7 Sep 2000 16:28:01 +0200
Thanks Trevor , will do so .

Dave , I hope you receive this .

Kind regards
Chris de Wet
P O Box 472 , Ifafi  , 0260  ,  South Africa  .
Telephone 012 - 3398071 ( office ) 012 - 2591129 ( home ) Cell 0839989744
Fax 012 - 3398118
One '56 TR 3 with o/d and one '64 Spitfire 4 ( Mk 1 )
Two Heralds and three early Spitfires without o/d for sale in order to
finance the restoration of the TR 3 .

-----Original Message-----
From: Trevor Jordan [mailto:tjordan@bigpond.net.au]
Sent: Thursday, September 07, 2000 2:21 PM
To: Chris De Wet
Subject: RE: One more electric fan conversion question


Chris

I am not on the Spit list, so you might want to forward this reply to Dave
Chu.

An adjustable switch and a manual override are not essential 
(although I have an override switch and indicator light on my car), 
but it is necessary to make sure that the thermostat and the thermal 
switch operate properly together.

When you install an electric fan, you end up with two temperature 
controls.  The thermostat controls the temperature at speed (the fan 
should be off) and the thermal switch controls the temperature when 
stationary.  In stop/start traffic the temperature will move between 
those two values.

The thermal switch temperature has to be higher than the thermostat 
temperature for it to work properly, otherwise the fan will be on 
most of the time as Chris DeStaffany mentioned below and the whole 
exercise is self-defeating.

(The other complicating factor is hysteresis; the thermal switch (and 
to some extent the thermostat) will exhibit hysteresis which is a 
band of temperatures in which a switch will be stable in either 
state.  For example, the thermal switch may not close until 85C but 
not open again until the temperature drops to 80C.  The switch will 
be open as the temperature rises to 85C and remain closed as the 
temperature drops back to 80C, so it can be either open or closed 
between 80 and 85C.)

I have found that the best practical method of setting up an electric 
fan is to put both the thermostat and the thermal switch in a pot of 
water and bring it to the boil.  A buzzer should be connected to the 
thermal switch, but the operation of the thermostat is easily 
visible.  Make sure that the thermostat is fully open before the 
thermal switch closes.  Then turn off the heat and make sure as it 
cools that the thermal switch opens before the thermostat closes.  If 
not, then you will need to change one or the other until they operate 
correctly.  You do not need to measure the actual temperatures unless 
you are really interested.

If you are using an adjustable thermal switch, you can use this 
technique to set the adjustment.  Alternatively, you can do it in the 
car by adjusting the thermal switch so that there is a clear increase 
in the temperature between highway running and stationary.

Regards, Trevor Jordan
74 TR6 CF29281U


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