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Fan wiring - Direct + Off + via Thermostat


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For some time I have ran successfully without my belt driven fan - using a Kenlow wired with a direct feed from Solenoid to inline fuse to Thermostat to Fan.

I'd like to create a system that allows me to switch between auto ( via thermostat ), all Off, and a direct constant On.

Having installed a system using a three way toggle switch that:-
a) tries to utilise a warning light in series only on when thermostat circuit is in use.
b) a warning light in parallel when direct on is in use

... I'm unable to get it to work without shorting the fuse upstream of the power supply to the switch.

You'll have guessed from this that I'm very much a novice electrician. Can anyone help?

Stage two would be to have a light that shone when the toggle was in the OFF OFF position.

Basic Auto electrical references seem to assume a level of knowledge I've not yet achieved.

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Firstly - a couple of points about what you've got.  I'm not sure wiring the warning light in series with the fan is a good plan - this will drop the voltage to the fan.  Also I think the bulb is quite likely to blow and then your fan will not work.

I'm guessing your Kenlowe has a draw of about 6 amps.  If your toggle switch is beefy enough what you have got should be safe enough.  However I think it would be better practice to switch the fan using a relay - plus it makes it easier to incorporate the 3 warning lights you are wanting.

Here's my suggestion for one way of doing what you want.  I would recommend 17amp cable from the 12v feed to the fuse, up to the toggle switch and up to 30 on the relay; then from 87 up to the fan.  All the other wires could be 5amp.  

The minor complication is you would need to include a couple of diodes (which are just electrical one-way valves) so the lights work properly.  If you don't have access to buy these you could always salvage some from a scrap bit of electronics.  The silver stripe would go nearest the relay in this case.

If you want to keep it simple, perhaps you could post a drawing of what you have and people could suggest more minor changes?  All the best!

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Attached my propsed circuit.
It utilises the constant live feed coming from the thermo switch.

Can anyone provide a link or part # of an appropriate diode.

Can I join the two fan suppy wires to create a single wire feed to the fan without creating a short.

Any thoughts??

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I think that looks pretty good and will work.  It just depends what you want the warning bulbs to do exactly.

I think you would have to join the 2 feeds to the fan (or does it have 2 seperate contacts for the supply?) but it shouldn't be a major problem with regards to feedback as the relays are never both energized at the same time.  A minor problem would be the auto light would also glow when switched to direct.

LED would actually be much better for my suggested circuit - simply substitute the 2 diodes with LEDs (and obviously wire 87a direct to the third LED).  These lights only give you a visual indication of toggle switch position - they don't reflect when the fan is running.  I wasn't sure what you wanted exactly.  I opted for one relay to make it cheaper but of course the second relay does away with the need for diodes.

A disadvantage of my circuit would be the fan couldn't continue its cooling when you switch the engine off (don't think that would matter much the water pump is no longer going anyway).

Your circuit would need the driver to have the dicipline to remember to switch the toggle switch off when leaving the car or else one of the relays will be energised all the time.  Ok not a major current draw, but I think it would still be undesirable on an older car with a marginal charging system.

Also IMHO I would use a 5amp relay for the relay circuit and 10amps for the fan but I'd be happy to be corrected by someone with more experience.

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That all seems a little complex.

Here are the requirements I had when fitting my electric fan
1) there should be a pilot light to tell me that the fan is running
2) the fan should run after the engine has been turned off if the coolant is hot enough to warrant it
3) there should be a manual over-ride for the fan
4) the override must NOT operate when the ignition is off (to prevent draining the battery)

I acheived these requirements with the following circuit, involving 1 relay, and 1 diode.

For the switch, I used a modified fog-light switch which I ground-off the decal and then
polished the end so that it matches the fog-light switch.  These switches are mounted
on the centre console and, although quite low down, are clearly visible.

The pull switch glows whenever the fan relay is energised, whether by the over-ride or
by the Kenlowe sensor.


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I've got a thermostatic switch and an overide switch in mine.
Never worry about leaving the overide on after turning the engine off as the is a modern unit and is pretty loud.

I used a LED to show when the fan runs, however once the car is over 40mph, the fan and motor act like a little alternator and light the led.  


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Bodder's system looks very neat and is very nicely thought out (not a scribbled mess done while half watching the telly like mine).  I particularly like the way it can run for a little while after the ignition is switched off just like a modern car.  I think I'm going to use that on my own car.

In practice do people find it is important to be able to overide the thermostatic switch?  Is this something that is frequently needed?

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Rust_Spot wrote:
In practice do people find it is important to be able to overide the thermostatic switch?  Is this something that is frequently needed?

Most of the time I forget about the Kenlowe.  The only time I used the override is when sitting in a traffic queue, just to keep some air moving under the bonnet.  This was especially necessary when I had SU carbs (with the dreaded waxstat jets).  With no fan at all, they soon leaned out the mixture to a point where the car wouldn't idle cleanly - with the kenlowe running, even though it is blowing hot air, it seemed to help.  

Now I have Dell'ortos fitted and things pretty much look after themselves - until I blew my cylinder head gasket  - but that's another story (see the thread on the car running on mineral water!)

glaramaraman wrote:
Can you remember the part number, spec, and supplier of the diode you used.

Sorry, I just pulled a reasonable sized diode out of my stock.  You should be able to get a suitable diode in Maplins.  It just needs to be rated for the combined current draw of the relay and pilot lamp.  The relay will draw something like 100mA, and the bulb something like 50mA, so a 0.5A diode (which I used) is perfectly adequate.

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