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Graham

Thermostatic switch

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Doing some advance planing/pondering about the cooling system for BUO and I was going to have a radiator with provision for a thermostatic switch for the cooling fan but am I correct in thinking that I will need a relay to switch the fan its self as the thermostat won't be able to handle the current.

 

ATB Graham

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Hi Graham

 

I believe that a relay is recommended - I fitted a 'Revotec' fan and thermostat to the 5 a few years ago - that was supplied with a relay...

 

Cheers

Ian

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+1 for a relay

 

Bob.

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+1 for relay.

I have my thermo switch on the 3a in the metal down tube.

Mine has a manual switch and a seperate dash light telling me it's on. Just a note of caution I Found the light came on when the thermo switch was on. This didn't indicate if the fan worked !!!

All sorted now - the light is on when the fan is on either manually or on auto temp demand.

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Excuse me highjacking the thread just to put this relay thing in context - there is nothing special about them. The contact size in a relay and in a normal switch will be the same size for a given current rating, so if the switch is man enough there would be no advantage in having a relay as it adds complication - just another thing to go wrong. It is necessary to use one where the switch has insufficient current rating for the load, or to minimise the length of wire which carries high current i.e. putting a relay close to the load may allow shorter wire runs than looping back via a dashboard switch, and hence there is less voltage drop.

 

To do it properly and to minimise contact wear there also should be spark-quenching on all high-current-carrying contacts but no-one seems to bother with that in cars.

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3236_AppNote&DocType=CS&DocLang=EN

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I vote for a relay, and for the thermo switch in a stainless tube in the coolant line from the radiator to the water pump.

 

The relay allows you to wire a new , fused, positive feed aside from the existing ignition circuit.

If you choose this also allows the rad fan to run after shutting off the engine.

 

The thermo switch is the hose to the block monitors the output of the cooling process, which for me is better than monitoring the input at the top pf the rad, a switch in the bottom of the rad would be as good.

 

Steve

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Hi Rob,

as usual you are quite right about the spark-quenching - I suppose the added expense is too much.

 

Another contributor to using a relay.

If the thermostatic switch is quite expensive then it may be cheaper and quicker to replace a relay that takes the load than the thermoswitch that could simply control the relay control current. It is not always a good idea though put a very small through big contacts.

 

Use a relay with the built in quenching diode.

 

Roger

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Hi Roger - hope you are feeling better now?

 

The bit about not using big contacts for small current is true and is due to the need for contact 'wetting'. Too small a current can fail to properly break through the minute oxidised film which forms on contacts made from base metal, hence the need for precious-metal or even mercury in relays where the current to be carried is very small. To overcome that, things are sometimes arranged so that the contacts scrape together slightly to mechanically remove the oxide, rather than to just touch. It isn't likely to be a problem with the sort of currents found in car circuitry though - more in signal and audio applications although in most cases that is now handled by semiconductor switches.

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I only have a basic understanding of electrickery but I have a Thermo Switch, 88°C on, 79°C off, in a new stainless down tube on the LHS of the vehicle. It is wired through a relay to control the fan with a separate dashboard mounted switch which allows me to override the thermo switch and turn the fan on in traffic. There is a separate indicator light which shows when the fan is operating. In other words the same set up as Steve and Hamish.

 

It works well.

 

Rgds Ian

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If the contacts of the thermostatic switch fail as a result of switching quite high currents, one then has to open up the coolant passage in order to replace the device. So, a secondary advantage in using a relay is that the relay, which is switching the high current, is the more likely to fail, but is changed easily and without the need to open the coolant passage.

The overdrive relay is subject to the same sort of current surge as that required to start a cooling fan, and I have had to replace my overdrive relay twice in the last 23 years.

Ian Cornish

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Hi Ian,

it isn't the current surge in turning on that causes the arc and the damage to the contacts.

It is the switching off where the arc is present (something to do with inductive loads i think).

The OD relay has an inductive load (coil) but is quite small. The rad fan motor is much bigger,

 

But I agree about the relay taking the pressure of the thermoswitch.

 

Roger

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The main point of using a relay is that it enables a low current device, thermoswitch to switch a high current device, a fan motor.

 

Arcing across contacts produced from inductive loads can be stopped by using a resistor and capacitor across the switching contacts to absorb the unwanted energy.

 

Help I now have a headache.

Dave

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Modern cars tend to use relays for their fans and as far as I know the the thermostatic switch is usually on the low current side of the relay.

I'm sure there are exeptions.

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i would use the relay so you can switch a separate feed my fan often consumes 25 amps on start up, a lot for the original loom to take.

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It depends of course on exactly how it is wired, but an indicator lamp may not indicate that the fan is actually rotating, merely that the switch or sensor has operated.  This is the system I have.  I learnt the lesson the hard way when the engine got very hot in very demanding circumstances and operating the switch did not have the desired result  - because the inline fuse had dropped out of its holder at some time beforehand!  The light was on, though!

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I have just fitted a revotec switch with the temp sensor fitted into the stainless tube, Prior to this my engine rain very hot and thus I had previously removed the thermostat from the  water pump housing system. Having fitted the new kit my engine then ran cold. So I have now fitted a new thermostat back into the pump housing expectating that when the water gets to the right point the fan would kick in, however this is not the case and the needle on the temp dial gets up to the red line.

How is that possible? 

Turning the engine off to let it all cool down I have checked that there is power to the relay etc. Whilst having a cuppa and a think the fan then came to life which I assume is the water circulating the heat.  

Any ideas on what it going on?

I am wondering if I should reconnect the manual override switch I still have in situ. That said I am not very good with the electrics so would I wire this directly to the motor or via the relay?

Thanks, Richard

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Richard,

My first action would be getting air out of the system. That is a common issue, and happened to my car too. The thermostat is mounted at the highest point, this does not help. You can check if it is opening by feeling the radiator and engine.

Radiator should get hot when engine is above 80-90 degr.C.

Waldi

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What's the best way to get the air out of the system?

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I need to get it all sorted as I am driving the car to a friends wedding in Den Hagg in May and have promised my wife that it will not break down!!!

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A lot of people raise the front of the car when filling.

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The Revotech sensor has an adjuster on it, which you need to set so the fan comes on at the temperature you wish it to.  I suspect yours is set too high at present.

Mine comes on when the temperature gauge gets approx one third of way between normal, & the top end.

Bob.

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My switch is in the down tube but it isn’t adjustable, it operates between 82 and 89 deg C (I think, it was a while ago I fitted it). If your fan isn’t coming on, the switch may be one that operates at too high a temp. 

Rgds Ian

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Is the temp gauge working correctly, i.e. is the engine really getting too hot?

Pete

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