Hot engine start - bypass fan while cranking

[aaronjb]

Brilliant! Glad to hear it worked out

[Bence]

Hi Aaron and thanks for this. I delayed responding as I wanted to tackle it and yesterday I did after sourcing a changeover relay.

Just seen you schematic below but worked off your description and it works a treat so thanks!

Cheers, Bence

[aaronjb]

Does this help any?

On the left is a "typical" fan circuit - it doesn't actually matter if the temperature switch (thermostat) is on the 'ground' side of the relay coil or the feed side (pin 86 or 85 here); in fact feed & ground are meaningless across the coil anyway. But what happens on the left is that when the thermostat 'closes' it makes the circuit between battery and ground through the relay coil, energising it and pulling the switch (pins 30 & 87) closed which connects the +12v through to the fan.

On the right, what you're doing is 'inserting' the changeover relay between the ignition live that was previously going to pin 85 (or it could be between the thermostat output and pin 85, or between pin 85 or 86 and ground, it doesn't matter as long as it is 'in circuit' on the coil side of the fan relay) and using the C/O relay to selectively disable the fan relay when the starter is engaged by connecting the coil between the output of the 'start' position on the key (or button) and ground (so pin 85 to start pos, pin 86 to ground). Pins 30 & 87a are normally closed and so when you aren't trying to start the car the new relay is effectively invisible, when you hit the starter the relay switches contacts (pins 30 & 87 are connected) which disconnects the fan relay coil from its circuit and turns the fan off.

Easy peasy, lemon squeezy

(I've omitted any fuses etc from the diagram, you could put a small fuse in-line with the starter feed to pin 85 of the new relay; as you're "injecting" pins 30 & 87a into the feed to the existing fan relay that will already have its own fuse)

Aside from furgling, probably upside down in the footwell, to get at all the wiring 

[KevinW]

You could do this with a standard changeover relay (which has NO and NC connections):

Pin 86 to ground
Pin 85 to the 'start' position of the ignition switch along side the starter solenoid

Then move the wire going to pin 85 of the fan relay to pin 30 of this relay
Wire pin 87a to pin 85 of the fan relay
Leave pin 87 unconnected

Now when you turn the engine over the changeover relay coil will be energised, connecting pin 87 (unconnected) to pin 30, effectively disconnecting the fan relay coil.
When you aren't turning the engine over the changeover relay coil will be de-energised, connecting pin 87a to pin 30, allowing current to flow to the fan relay coil and the fan to run.

That would have been easier to draw out than write, I think... and it makes some assumptions about how the AK fan is wired, like assuming it has a relay!

I have a similar problem to Mr Bence, and that's a great solution - but would def appreciate a little diagram Aaron.

[aaronjb]

You could do this with a standard changeover relay (which has NO and NC connections):

Pin 86 to ground
Pin 85 to the 'start' position of the ignition switch along side the starter solenoid

Then move the wire going to pin 85 of the fan relay to pin 30 of this relay
Wire pin 87a to pin 85 of the fan relay
Leave pin 87 unconnected

Now when you turn the engine over the changeover relay coil will be energised, connecting pin 87 (unconnected) to pin 30, effectively disconnecting the fan relay coil.
When you aren't turning the engine over the changeover relay coil will be de-energised, connecting pin 87a to pin 30, allowing current to flow to the fan relay coil and the fan to run.

That would have been easier to draw out than write, I think... and it makes some assumptions about how the AK fan is wired, like assuming it has a relay!

[johnnycobra]

Have a word with Jon at AK ..

[Bence]

Hi all, hope you can help?

When engine gets hot and fan kicks in, if I stop the engine then go to restart, the fan is still running and drawing enough current to prevent the starter from cranking.
I want to bypass the fan while the engine cranks and starts then after starting the fan resumes as normal.

I assume this can be done through an additional relay between ignition and fan relays but would greatly appreciate if anyone has a wiring diagram to help?

I'm not brilliant with wiring although I did all mine with no issues but can follow a simple wiring diagram Hopefully.

Car is AK with LS3

Cheers, Bence