This question must come up monthly. Simply using a switched power source, especially one that is hot in the ACC position, can cause the choke to open prematurely if the key is on but the engine is not running.
GM used two different methods, depending on the vehicle. Most used an oil pressure switch in the power line to the choke. This way, the choke doesn't start to heat up until the engine is running and there is oil pressure. I ran a fused wire (20 A fuse) from the BATT terminal on the alternator to the switch, and from the switch to the choke. Very simple to wire and very clean. I like to use the Standard P/N PS64 switch. The middle terminal works the OIL light, the other two are a normally open switch that closes with oil pressure. Use these to control the choke. If you have an oil pressure gauge instead of a light, simply use a brass tee fitting to connect both this switch and your oil pressure sender.
The other way is to use a relay that is triggered by the field wire on the alternator (brown #1 wire on SI-family alternators, for example). The field wire is only hot when the alternator is turning. This is how GM wired the stock G-body cars with electric choke when the gauge package was installed. Of course the down side with that is that the choke will close if you break the alternator belt. A typical wiring diagram for the second method is here:
Edit: added schematic for oil pressure switch option
For my electric choke, I tapped into the 12V wire into the pink wire coming off the distributor. Good info with that oil pressure switch, might just have to do mine that way with a tee when i hook up my oil psi gauge.
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