Cooling Fan Wiring (including Optional Dash Switch)

[DrSarty 90 1 Cars]

As many people post topics relating to cooling fans not working, or ask about how to wire a new push or pull cooling fan onto their new radiator, or even a dash mounted fan (over-ride) switch into their car, I thought I'd offer up this very basic diagram.

 

This diagram assumes the use of a 3-pin thermoswitch in the radiator, but would work similarly with a 2-pin thermoswitch; you just ignore the loop including the resistor if that's the case for you.

 

DOWNLOAD FILE BELOW

 

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Extra info:

 

1) The 3 pin thermoswitch

 

There are several part numbers, based on their opening and closing temps. For example one switch may have a 88-92 degC {A} and then a 93-97 degC {B} switching range.

 

Just to explain this and how it works (as this is often the component that fails!), it has 3 pins, all of which are live biased, i.e. none of the pins is ever a ground/earth.

 

One pin (typically the horizontal pin - the other 2 are vertical and parallel), is permanent 12v live. It should be connected to the battery positive terminal, perhaps via the shunt box.

 

The other 2 pins are the two switching pins for temp ranges A and B (above). The 12v live is fed into the thermoswitch all of the time, so this is not an area to go waving your screwdriver around if there is not protective rubber boot over the connections.

 

Inside the thermoswitch, once the coolant reaches the lower temp in range {A}, pin 2 becomes live, i.e. giving you a 12v live output (which powers the fan). The fan has a permanent ground/earth by the way. So when pin 2 is live, the fan comes on.

 

But that 12v power out of pin 2 goes to the fan via a resistor, the size of 2 fingers, usually fastened under the headlight somewhere. This resistor, resists the current (and gets warm in the process!) so the fan only runs at a slower speed.

 

If the temp falls again, pin 2 goes neutral again, and the fan stops.

 

If the temp continues to rise, at some point pin 2 will either go neutral again, and/or pin 3 on the thermoswitch will become live, effectively taking over from pin 2 and sending direct 12v live (which is coming in through pin 1 remember) to the fan. This pin 3 route does not go via a resistor, so the fan runs at full speed.

 

Pin 1's live feed should be permanent, such that it's live even with the engine off. Hence why engine fans are often still running on hot days when you park and switch off.

 

Basically if the thermoswitch wears out, or you lose the 12v feed in (on pin 1) your fan won't cut in. If you lose pin 2 out to fan then you'll still get high speed fan, which is not a drama. But if you lose pin 3 out to fan you could be in trouble, as low speed may not be enough to cool the engine.

 

 

Extra info cont/d:

 

2) Manual fan cut-in

 

By adding another permanent live feed via a relay, you can have the fan turn on manually via a dash switch when you want to, or (also) via a suitable ECU which has a programmable earth switched output. Most have, and some are even designated as fan control.

 

All you're doing with this, is energising a £2 relay* (from Halfrauds or VWP etc) by grounding it (aka giving it an earth). As shown in the diagram: main fan power in one side and out the other, with a switched ignition live on the switching circuit to mean the fan can't be left on when you walk away with the keys.

 

*Wiring in a relay is easy if you look at the little diagram on the relay itself.

 

So with your dash switch, just put a ring terminal on a short wire and ground it to the steering column from its pin on the 2 or 3 pin switch (3 pin switches usually have a bulb/LED in, which can be useful to show you the fan is on).

 

The other pin on the switch goes to the relay, on the opposite side of the switched live you've given it. This means, when you flick the dash switch you complete the circuit, which energises the relay and allows the 12v live to go to the fan.

 

This is how this set-up has 3 live wires going into the fan itself:

i) 1 direct from pin 3 on the thermoswitch

ii) from pin 2 on the thermoswitch via the resistor (to make the fan run slower)

iii) from the relay being used for the manual and/or ECU cut-in system

 

 

Extra info cont/d:

 

3) ECU controlled fan cut-in

 

I use this as well as the thermoswitch in the rad. This way I am sure something will turn the fan on.

 

It is not pictured in the diagram, but as it's just a ground from the ECU (when the ECU grounds it when the coolant reaches a certain temp) all you do is join it to the wire going between the relay and the dash switch.

 

What that means is that it doesn't matter whether the ECU grounds that wire or you do (by flicking the dash switch), the relay will energise sending 12v to the fan directly, i.e the fan runs at full speed in either of these circumstances.

 

 

Extra info cont/d

 

4) Fuse and cable ratings

 

Fans don't seem to draw more than 15amps, but the relay should be rated to at least 20amps and the same for the wiring. In fact 30amps seems to be more common for the cabling, relay and any fuses you include going from the battery to the fan and/or relay.

 

 

Extra info cont/d

 

5) Trouble-shooting thermoswitches and fans

 

Thermoswitches get old and fail. Fans can fail but I'd say it's rarer.

 

It's easier to test the fan really, by bridging the contacts on the thermoswitch. With the above explanation of how the switch itself works, it should be easy to see that by using a rubber/plastic/wooden handled screwdriver between pin 1 (12v live in) and pin 2 or 3, the fan should work. This DOES NOT test the switch itself.

 

If the fan does work then we know the fan's OK, but it should work on pin 2 (slow) and pin 3 (high). If it doesn't then the fan is likely knackered. If it works on one and not the other, it's likely to be a fault with the wiring going from the thermoswitch to the fan; it maybe a failed resistor.

 

If bridging pin 1 to 2 or 3 doesn't make the fan work, then test the fan in isolation, by taking 12v directly to the positive feed on the fan.

 

It's very difficult and fiddly to test thermoswitches, so I wouldn't bother. If the fan works in the test(s), then I'd just get a replacement thermoswitch.

 

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I hope this proves useful and helps anyone understand how it all works, how to wire it in yourself when building a new or refreshing your existing loom, and how to fault find the thermoswitch.

Suggested_Cooling_Set_Up.pdf

Edited July 12, 2010 by DrSarty

[Anthony 1,003]

Whilst the wiring in that diagram would work, it's far from the best way of doing it in my opinion if you're going to the hassle of changing the wiring and adding a relay, as you're retaining the frankly poor 205 GTi system of running full current through the thermoswitch, which in turn seems to make them prone to failure or (more commonly) poor contacts and thus non-working fans.

 

My prefered method is to do a modified relay controlled fan setup from a 205 diesel, which only runs low current through the thermoswitch and seems much more reliable in my experience. Because it's relay based, it's very easy to run an additional switch inside the cabin should you see fit, although personally I've never really seen the point as it's not needed if you have a reliable fan setup in the first place.

[DrSarty 90 1 Cars]

Agreed. And I wouldn't be in the habit of trusting a 20 year old thermoswitch or wiring with a new engine build that's for sure.

 

This was perhaps geared towards those fault finding why their fan doesn't work, and those building a new loom using a new thermoswitch.

 

Naturally there are other ways as you've pointed out and as Peugeot and other manufacturers use(d), i.e. low current, relay-switching circuits going through the thermoswitch, which I would prefer too.

[welshpug 1,657]

IIRC my 1992 ZX runs as Anthony suggested (though it is a single speed unit) a low current running through the thermoswitch and a relay in the engine bay fusebox.

 

 

One thing not mentioned on the diagnostic side, is that the 205 has a diagnostic plug for the very purpose of making sure the fan works, its a 5 pin brown plug with three wires in it, bridge the centre wire to either of the outer wires will bring the low speed (via the resistor) or high speed (direct feed) on.

[S@m 156]

Well, cheers Sarty, most informative; it should help me with some upcoming troubleshooting on an 8v turbo.

 

Sam

[GLPoomobile 958]

the 205 has a diagnostic plug for the very purpose of making sure the fan works, its a 5 pin brown plug with three wires in it, bridge the centre wire to either of the outer wires will bring the low speed (via the resistor) or high speed (direct feed) on.

 

Or in my case, only 2 wires, with a central (live) wire that has long since corroded and snapped leaving just a taped up stump in it's place

[Simes 248 2 Cars]

Looks good...a couple of things I'd do.

Put in an appropriate fuse between the relay and shunt box.

Keep the relay as close to the shunt box as possible, keeping the higher current cable much shorter.

 

I've toyed with getting my Emerald to control the 'fan' but have decided to keep it simple so far!!

[DrSarty 90 1 Cars]

Put in an appropriate fuse between the relay and shunt box.

 

I used a fused VWP relay. The blade fuse slots into the relay itself.

 

And my shunt box wire to my fan is also fused. I have a cable from the battery to a ring terminal in the shunt box, and then 4 maxi fuse outputs. One of them is a 30A fuse going direct to the thermoswitch (pin 1).

 

Thanks for the kind comments. It's not meant to change the world, teach people to suck eggs or anything else; but what I've shown/clarified/described here has never let me down in 6 years of 205s.

[Tilou 1]

Very interesting reading as I have this problem - but I can only locate a 4 pin brown plug, which we have tried to bridge but nothing happened.

 

Car is a 1993 1.6 auto (XU5M) - so if anyone can point me in the right direction of testing the fan with my set up it will be much appreciated.

 

As the car is currently not running I cannot even get it to local Peugeot garage for their diagnosis!

[welshpug 1,657]

IIRC your XU5M will have a twin fan arrangement so the testing is a little different, the wiring very different.

[pug_ham 244 3 Cars]

IIRC your XU5M will have a twin fan arrangement so the testing is a little different, the wiring very different.

 

My mum's old 94 Mardi gras 1.6 XU5M auto didn't have, it had the TU style expansion tank but the same cooling fan set up as my GTI.

 

I thought only the diesels had the twin fan & different relay wiring set up?

 

g

[Tilou 1]

It has a double grill but only one fan inside. For what it's worth (if anything) this car is a French LHD job, so am noticing many differences from descriptions on forums.

[welshpug 1,657]

ah in that case then it "should" be the same as described.

[petert 586]

All A/C cars have twin fans, even XU5M models here in Aus. To answer your question however, later models switch the relay to earth, rather than positive, to make the fan(s) come on. Same as 405's.