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There is no relationship between the actual temperature of the engine and the operation of an electric choke. The choke operates as a function of the time that current has been passing through the internal element. This means that you want to connect the lead to a wire that is energized only when the car is running. In a rough sense, this means a wire that is connected to the IGN terminal of the ignition switch. Note that this isn't completely correct; if your car is stalled, the choke will continue to open because the IGN terminal is still energized even when the car is not actually running. As a practical matter, however, I think it's a moot point.

You should also note that an electric choke is nominally calibrated to be used with a 12V system. This would imply that you shouldn't tap your coil wire, as it's only ~9V when the car is running. That would be unfortunate, as it allows a neat, out-of-the-way tap. However, there are two possible exceptions to this:

  • If you have bypassed the resistance wire to your coil (e.g. after installing a Pertronix setup) so that you always have 12 V at the coil.
  • If the choke has enough adjustment in it to compensate for the slower operation at 9V.

In addition to tapping the coil wire, there are several other options for providing power to the choke:

  • You can tap the yellow wire to the transmission shift switch.
  • You can also tap the blue (stator) wire from the alternator harness. This wire is energized only when the car is running. Given the routing of the alternator harness and the size of the Riviera air cleaner, you can make this tap very neatly as well. I don't believe that this will have any detrimental effect on the operation of your voltage regulator. The stator wire doesn't do anything except energize the field relay in the regulator. These relays typically don't require much current, so I'm guessing that you *should* be able to safely tap the stator wire. If I'm wrong, it will obvious, as the field relay won't close, which means the alternator light will stay on while the engine's running.
  • You can run a wire from terminal 4 (brown wire) of the regulator. Although this is the most direct connection to the IGN terminal, you'll have to route the wire clear around the engine compartment. Cosmetically, it's the least desirable option.

If you want to dispense with all of this confusion about which wire to tap, try this*:

  • Buy a combination oil pressure sensor/switch from your local GM dealer (part # 25036378; about $15).
  • Buy a 1/4" FPT-18 MPT adapter so that it will install in the engine block (about $2 from your local jobber).
  • Replace the original oil pressure sensor switch with the new switch.
  • Connect the original oil pressure lead to the silver terminal on the new switch.
  • Tap the power lead to the blower relay (red wire that runs from the power junction on the inner fender) and run the lead to one of the gold terminals on the switch.
  • Run a lead from the other gold lead on the switch to the choke unit.
  • If you want to fuse this circuit (always a good idea), the electric choke that I used has a resistance of 12 ohms.  A quick application of Ohm's law gives a nominal operating current of 1 amp; a 2-amp fuse should do nicely.

As far as the taps go, I recommend Radio Shack part # 64-3052 (Tap-In Squeeze Connectors for 18-14-gauge wire).

And because the question was asked, you can get an electric choke for an AFB for $26.95 from Carbs Unlimited (http://www.quadrajet.com/Carter4.htm) or about $16 from Allstate Carburetor and Fuel Injection (http://www.allcarbs.com).

* A special thanks to Bob Embrey for doing the legwork on the proper GM switch for this mod.
