Went out shopping to Maplins armed with a big list and came back with a bagful of goodies  A 30A relay, an in-line fuse holder with 20A fuse, some 30A and 3A wire and a nice little panel-mount blue LED…

30A relay, an in-line fuse holder with 20A fuse, some 30A and 3A wire and a panel-mount blue LED from Maplins

The thermostatic controller I got from the Staffordshire Vehicle Components stall at the NEC Classic appears to be much the same as any other generic thermostatic switch kit…

Thermostatic fan controller from Staffordshire Vehicle Components

Before starting, I’d spent a bit of time using Turbosport’s lovely SEARCH feature, so reckoned I had a fairly good idea of what needed joining to what 

Now to try and put theory into practice in my fully equipped electronics laboratory (or the breakfast table as it’s also known ). The last time I waved a soldering iron about was probably in a Physics class at school – and I remembered it was awkward trying to get at stuff and not end up soldering the power lead too, so I also invested in a nifty gas powered soldering iron while I was out. Crackin’ little gadget it is too  – you fill it up from a fag lighter refill can, the little reservoir lets you see when it’s full and there’s no wires to get in the way 

Ended up with this…

Escort radiator fan wired up on the bench

In schematic form, it looked like this …

Brewed up before testing things  which also meant I had some boiling water to go in the mug that’s pretending to be the radiator. Connected the circuit up to the positive on the defibrillator, turned the thermostat dial down past 90 degrees, heard a click as the switch thunked and then … nothing  After a wee while of checking connections and Google, I found a how-to page that indicated I should supply power to both pins 30 and 85 on the relay, so thought it couldn’t hurt to try. The schematic then looked like this …

Another brew  and a top-up of the “radiator”, apply the power, the switch clicks, the cool ice-blue LED lights up, signifying that the Day After Tomorrow-style winds are approaching and then … nothing 

Another thinking session was obviously needed, involving a trip out to the gym for a swim, steam, cuppa and a cookie  Once I got back, I tried some deductive reasoning – if the LED lights up, but the relay doesn’t then switch the power to the fan, either the relay’s duff, or it’s not getting the trigger from the thermo switch. Not having a spare relay, I tested the second theory first by taking the LED out of the circuit and having a straight run from pin “2” on the switch to pin “86” on the relay.

Another brew  and another top-up of the “radiator”, apply the power, the switch clicks and then … whizz, whizz, whizz as the fan attempted to launch itself off the laboratory workbench and into the breadbin 

Now, my question for anyone who’s either got this far after reading all that waffle above, or if you just skipped straight to the end to see how it turned out, is this … why doesn’t the circuit work with the LED in-line?

Is it because the LED’s duff, or do LEDs not work like normal bulbs and you can’t put them in-line and expect the circuit to just “pass through”? If it’s the latter, should I keep the working circuit as is and run a second wire from pin “2” on the thermo switch into the positive on the LED and then connect the negative to earth?

Whatever the answer is, I’m still chuffed to bits  Thanks to everyone who’s helped so far… 


EDIT 03/02/08 :

Group4_Mark2 : Good to see that you got the fan going without the led. As you said the Led is not like a bulb in that it cannot cope with very high current and should have a current limiting resistor in series with it to prevent it blowing up. I would remove the Led from its current location and put it across the + and – of the fan with a 600ohm (or something close) resistor in series (in-line) with it. This will give you approximately 20mA through the Led which is more than enough to light it up. The Led may have been damaged by your earlier attempt so you should test it first by connecting it across the + and – of the battery with the 600ohm resistor in series.

Dave : I don’t know why it didn’t work Stu 
I assume the controller is under destined to be under the bonnet. I ran an extra wire from (on your diagram) the 87 pin, to the LED on the dash and an earth to somewhere near it. Reduces the wires needed and does exactly what you were after in the first place. It also means you know when feed is being sent to the fan, and not just when feed is sent to the relay. (you know if the relay is working)
If you earth pin 86, and and use the wire from pin 85-pin 30 to “feed” your controller you can cut down on even more long wire runs. (I think)

dangerousdave : If the led is a 12v one though it wont need a resistor in it? Whenever i have tried leds which needed resistors they popped straight away after connecting them 

Group4_Mark2 : I forgot about 12V leds. I agree that the Led should pop instantly if there was no resistor. If the Led has a built in resistor then it will limit the current so should still be connected across the fan and not in line with the relay. It takes up to 1 amp to switch on a relay and the led will only allow 20 or 30 mA to flow.