Hi. Auto electrics are not my forte. My A12 has a scorcher HEI dizzy and coil. When I got the car the thermo fan was running the power wire from the positive terminal of the coil. When I swapped the scorcher set up over I left i this way. Everything is working but I'm not sure if this is the best set up with all that load off the coil. Would running the thermo power straight off the back of the alternator be a better option. Any help greatly appreciated..

is your battery still in the engine bay? why not just run the power from that?.. but yes power from the alternator is fine

It depends if you want a switched feed or not.

off the coil would give you power only if your ignition is on.
the back of the alt would be constant power... similarly the starter motor usually offers a source for constant power.

the best would be to get power thru a relay.

Generally fans draw a fair amount of load.

I would set up a relay which could be set up (wtih fuse) back to a constant power source. this ensures the load requirements can be met.

the switch can be then set up on the low power side of the relay.

your alt suggestion is fine.. just make sure it's fused.

Hope this helps.

Cheers

Ed

Oh yeh I was assuming you were using a realay and you were just asking where the best constant power source would be lol. If not using a relay wire one in stat!
Then you just have to determine how you want it to switch on eg. Ignition/temp switch etc.
If you want it to switch with ignition you could use you existing wire at the coil for the switched power

The power lead does have a fuse. There is also a relay with the wire's taped up next to the radiator. I don't have a switch for it just yet. I have just rebuilt my 32/36 weber and I'm playing around with jetting so just want to be sure that its getting the the right amount of spark. It would be easiest to run off the back of the alternator I think. Thanks for your replies. This web site and its members are awesome.

So do relays have a positive, negative and switching wire?? Like I said electrics aren't my forte..

Think of a relay as a powered switch.

there are 2 sides to the relay 1 being low current (swiching circuit) and 1the other being high current (load bearing power supply)

A relay has 2 power inputs. (1 low current and 1 high current).

it has 2 outputs 1 earth and 1 high current power.
(I cant remember the numbered poles but they are 35, 37, 85 87??)

when you operate a relay, what happens is this.

your low current circuit when it is powered up and earthed moves a switch which causes the high current output to power up.

your ON/OFF switch can be on either the positive or earth side of this circuit. this is on t the low current side.

now your "high current" side gets wired to a source of power and the output side gets wired to your device to be powered (ie fan).

so as wardy said earlier, in your case you are just moving your source of power from one place to another.

Hope this makes sense... Im not a sparky I've just had a bit of experience re-wiring my projects over the years.

Cheers

Ed

Yep Ed is spot on there, some good info. They can be a bit daunting at first but once you nut it out you will realize how easy they are.

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

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