yeah all good. Im still not sure if it would be affecting coil output a lot but I will wire it off my alternator this weekend to be sure and put a switch on at a later date. Thanks for all the advice so far.

I like that thermo switch... i cant work out how it fits? is it on a T piece?

sorry I just hit the 2nd link... all clear now.

heres a nice thermal switch you can get from jay car for $1 or $2.



and the links 1 2

This is a nice simple diagram I just Googled of how to set it up when you're ready. The opposite of how I said (it's switching the positive via thermal switch rather than earth but same result)-

This is what I plan to do as it doesn't need to be on all the time especially for cold weather starts.

The relay isnt actually connected to anything at the moment. Who ever put the fan in the car just left it screwed next to the radiator. I didn't realize it wasn't hooked up at first as the + and negative wires where running to the fan behind the relay so I thought it was ok. So there is a relay there but its just cosmetic at the moment. The positve lead of the fan is straight to the coil with a fuse and the negative is to the battery.

The other good reason to go the switched relay path is that if you decide to run a temp sensor to switch the fan on and off then it will be easy to wire in a "normally open" temp switch to the earth circuit and run the + signal wire straight to battery voltage.
This will then mean that the fan only kicks in when the temp sensor/switch reaches it's preset temp and earth. You can choose different temperature sensor/switches to match what temp you want to stay at.

a simple test light with an alligator clip to earth is very handy for working out which wires are hot and when they are ON/OFF.

if you havent got one, you can rig one up using a bulb, bulb holder and pigtail with wires.

just check something first...

trace/follow each of your relay wires and see where they go.

if the positive from coil is high current draw then it will probably affect your coil. I wouldn't put 2 high load items on one circuit.

if however, your fan switches on when your coil is active it means this is likely to be your low current switch. You can check by following the relay wires to see where the other power is obtained from... lazy sparkies will piggy back to anywhere but a good one will have taken a nice robust source like your alt, starter post or a big fat cable.

If it turns out it is on your switching circuit, I would leave it with no detrimental effect to coil operation.

sorry if I'm confusing the issue.

summary: first step, work out if this is your low or high current circuit attached to coil.

Cheers
Ed

Cheers. I will have a play later..... do you think it would be affecting spark atm?? ( off the coil)