Fan Relay

[Joel]

Do you need a relay with a fan thermostat, or is that just for the fancy adjustable ones?

 

I got a thermostat off an escort I think, and just wired it in-line to the fan - it does work but only after you turn the engine off! That is to say, the engine runs for about 15-20 minutes before it boils over, at which point I turn the engine off and the fan cuts in. That's not to say it wouldn't have cut in anyway if I'd let the engine run longer, and me switching it off may be unrelated.

 

I just assumed the cut-in on the thermostat was too high, but could it be the missing relay that's stopping the fan cutting in sooner?

 

Hope that makes sense!

[Dave R]

shouldnt make a difference as to when the fan starts, but a relay is a good idea

most decent fans are pretty heavy on what current they pull, and the switch may not like it after a time

 

think my switch is off a p100 pickup, and has 2 switches.. cant remember what temp though

 

dave

[Guest robinj66]

You don't have to fit a relay and the circuit will work fine.

 

However fitting a relay will save putting high current through your dash and can (I am told) act as a "cut-out" should anything go wrong.

 

The thermostatic switch in your rad is just that - a switch. It's designed to close at a certain temp (hence allowing the fan to work) and to open again once a lower temp has been reached. In itself it should not determine whether your fan works with the ignition switched on or off. That will depend where you take your power source from (ie, switched ignition source or "direct" from the battery)

 

Hopefully Nigel et al will be along to correct any mistakes I've made (I'm still learning this electrickery lark )

[agent_zed]

Hi,

i dont currently have a relay in my fan setup, although i am planning on adding one soon. Your fan system should be independant of the ignition like in most production cars where the fan will continue to run after the engine is off until it is cool enough. I simply ran a heavy duty cable from my battery with an inline fuse to the thermostatic switch and then one to the fan. My only problem is that i put the switch into the end of the thermostat ally housing which means the fan cuts in a bit soon and takes ages to turn off as the engine cools slowly.

 

I am not sure but i think a relay is more likely to turn on and off cleanly when a current is reached where as i would think a thermostatic switch will turn on a bit more slowly as the temp gets close and may therefore wear the switch more as the heavy current arcs across. I am probably completely wrong on that one but thats my thinking ?!

 

hth

[Steve Walker]

I am not sure but i think a relay is more likely to turn on and off cleanly when a current is reached where as i would think a thermostatic switch will turn on a bit more slowly as the temp gets close and may therefore wear the switch more as the heavy current arcs across. I am probably completely wrong on that one but thats my thinking ?!

 

Thermostatic switches should not switch over gradually, they usually contain a belville spring (thin, slightly conical, spring washer) or a spring strip with a curve across its length and these are stable in two positions - as they are forced to move, forces build, until they flip to the opposite position (cone inverts or curve of strip reverses) causing the contacts to "click" straight over.

 

A relay should only be needed if the thermostatic switch is not rated for the current that the fan requires, but it'll do no harm to play safe and add one anyway.

[Joel]

Thanks chaps, some genuinely interesting info there

 

Incidentally my loom has specific output from the fuse box for the fan, so I think the fuse would blow before it puts too much current through the dash.

 

I guess the thermostat cut-in is just too high after all

[Guest Dan_Beeston]

Hi Mate,

 

Does the fan cut in immediately when you kill the engine or is it a short while after? After you stop the engine heat soak out of the head can cause the top hose temp to rise higher than it was when the engine (and thus water pump) was running, which might be causing the fan to come on..........

 

If it starts the second you kill the ignition it might be something wierd with the wiring, like the feed you are using only being live with the ignition off???

[Joel]

.... it might be something wierd with the wiring

 

Something weird in MY wiring Dan, surely not!

 

I wouldn't say it's an immediate thing, but it does only ever seem to come on after the engine's stopped running. I strongly suspect it's due to heat soak, as you say.

 

It may be a very rare occurance, but I'm confident the wiring's right on this one

[Andi]

Long time since the post, but if the problem is still there maybe

this rhocar link may help

[Andi]

http://nw.rhocar.org/fan%20wiring.htm

 

 

oops,