Lotus Elan

Wiring a thermistor for temp gauge and radiator fan...

PostPost by: Frogelan » Sat Feb 22, 2020 10:08 pm

I'm building a 65 Elan S2 race car and I'm enjoying working up the electrical system.

The Smiths water temp gauge I have opted for uses a thermistor (as capillary tube versions are fairly fragile).

It seems fairly obvious to me that this could also be used to activate the electric fan for the radiator, but can a temperature thermistor have a "measuring" and "switching" function (without unnecessary electronic complexity) ?

Has anyone tried this and can offer any advice on the wiring?

Andrew
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PostPost by: Craven » Sun Feb 23, 2020 12:39 am

Thermistor is simply a resistor that changes value with temperature, when used with temperature gauge the change in value varies the current in the circuit in an analogue way. To activate a switch on/off function you would need to find out the thermistor resistance at the selected temperature, probably by measuring the voltage across it to provide an input to an electronic switch. Something like this.
https://quasarelectronics.co.uk/Item/ve ... mostat-kit
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PostPost by: Frogelan » Sun Feb 23, 2020 5:45 am

Thank you for that info...what a fascinating kit.

I was rather hoping to find a way to "simplicate" but will look into the applications of this. It would need to be packaged carefully for the engine bay.

It certainly gives me other ideas!
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PostPost by: nmauduit » Sun Feb 23, 2020 9:27 am

radiator fans are often controlled by a thermally actuated switch (typically with about 10°C between on and off, e.g. 90°C on 80°C off) one can usually chose within a range from ~80°C on to ~110°C on , a relay is needed to avoid pulling too much current through it.

41eNca+gwuL._AC_SL1260_.jpg and
rad fan thermal switch


M14 (1.5mm pitch)

It also enables to keep the rad fan circuit autonomous and remain still operationnal in case the thermistor stops working.
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PostPost by: Frogelan » Sun Feb 23, 2020 10:21 am

Thank you Nicolas. I was hoping that a combined screw in sender / switch might exist. I have already planned the relay. The next job is to order a 225mm fan.

Ideally I'll locate them in the expansion tank to keep the wires away from the manifold/headers.

I'll opt för a thermistor and switch sensor like the one shown after working out the ideal fan operating temp.

Does 90C seem to be optimal (aluminium radiator with very close to stand size with shrouding) ?

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PostPost by: Craven » Sun Feb 23, 2020 12:16 pm

Frogelan wrote,
The Smiths water temp gauge I have opted for uses a thermistor (as capillary tube versions are fairly fragile).
You will still need a voltage stabilizer (10v) for this type of gauge.
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PostPost by: mbell » Sun Feb 23, 2020 2:12 pm

Frogelan wrote:Ideally I'll locate them in the expansion tank to keep the wires away from the manifold/headers.


Locate the temps senor in the expansion tank? That wouldn't be a good idea as there will be a temperature difference to the coolant circulating in the cooling system.

You might get away with a dual sensor in the normal position but the sensor is normal located on the radiator side of the thermostat. I think a lot of modern cars use a sensor in the radiator outlet and control the fan based on temperature of coolant returning to the engine. If that's too hot then it will cause the engine temp to increase.
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PostPost by: billwill » Sun Feb 23, 2020 2:19 pm

Frogelan wrote:I'm building a 65 Elan S2 race car and I'm enjoying working up the electrical system.

The Smiths water temp gauge I have opted for uses a thermistor (as capillary tube versions are fairly fragile).

It seems fairly obvious to me that this could also be used to activate the electric fan for the radiator, but can a temperature thermistor have a "measuring" and "switching" function (without unnecessary electronic complexity) ?

Has anyone tried this and can offer any advice on the wiring?

Andrew



Why bother, you are only more likely to reduce reliability and come up with something that the AA/RAC patrol cannot fix, when it fails on a long journey.

Using a thermistor will almost certainly require an electronic circuit and a relay whereas the technology of thermal switches is well established and reliable and available with high current ratings., and if you really require it, small enough to keep a spare in your travel toolbox.

Thermostat housings are available, threaded to take a fan switch, or you can insert a 'straight alloy' tube into the upper hose with a threaded hole on the underside for such a switch.

Image

https://www.carbuilder.com/uk/aluminium ... using-32mm
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PostPost by: Frogelan » Thu Feb 27, 2020 2:32 pm

Thank you everyone all for these ideas.

BTW when I mentioned placing the sensors in the "expansion tank" I meant the tank between the cylinder head and the radiator.

I'm still rather surprised that there is not a combined sensor that can handle water level, water temperature and the fan.
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PostPost by: billwill » Fri Feb 28, 2020 12:37 pm

Frogelan wrote:Thank you everyone all for these ideas.

BTW when I mentioned placing the sensors in the "expansion tank" I meant the tank between the cylinder head and the radiator.

I'm still rather surprised that there is not a combined sensor that can handle water level, water temperature and the fan.


I think that your tank there is an extra item, they are not there on elans ex factory. just a rubber pipe with a kink in it.
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