Electric choke wired to alternator
Yes, but picking a different power point will not change the varying voltage. The alternator B+ (output) terminal and the battery positive terminal are electrically common. Voltage varies across the entire electrical system with engine speed and load. If you need an always-live line-voltage feed, the alternator B+, the battery poz, the big terminal on the starter, and the biggest terminal on the starter relay are all electrically fine places to get it; base your choice on location, access, and other factors.
No good. The coil usually doesn't get line voltage. Remember, it's downstream of the ballast resistor. It sees roughly 7v. Also, the choke heater draws a fair amount of current; conceivably more than the coil primary circuit wants to handle efficiently. Moreover, hooking up accessories at the coil + will sap voltage from the ignition system, causing driveability and performance faults. Remember Gus Wilson!
The choke needs an ignition-switched line-voltage feed. The alternator B+ (and all the power points common with it, see above) are not ignition-switched, they're always-live, so none of them is a good choice. The customary power point for an electric choke is on the upstream/ignition switch side of the ballast resistor (compared to the downstream/coil side of the resistor). Easy to find which is which: unplug both wires from the ballast resistor and put a voltmeter across ground and one unplugged wire at a time. Turn the ignition switch on and watch the voltmeter. When you find the wire that goes to battery voltage when you turn on the ignition, that's the one you want to tap. I like to use Posi-Taps for this kind of operation.
Remember, the choke also wants a thermostatic ground modulator. If you hook the choke's ground wire directly to ground, the choke heater will come on as soon as the ignition is switched on. This means there's no adaptation of the choke opening time to engine temperature, and usually you'll have cold driveability problems (stalling). Also, if the engine takes awhile to start, or if for some reason you sit there with the ignition on but the engine not running, the choke heater will back the choke all the way off even though the engine's cold and needs the choke closed to start reliably. Some electric chokes have a thermal modulator built into the choke housing itself; these usually have only one wire connection (+feed). The 2-wire ones that contain only a thermostatic coil and a heating element need a thermal modulator. You can see the modulator, which is a little 1" black box with a copper mounting strap, in this photo of an electric choke retrofit kit:
Yes, but picking a different power point will not change the varying voltage. The alternator B+ (output) terminal and the battery positive terminal are electrically common. Voltage varies across the entire electrical system with engine speed and load. If you need an always-live line-voltage feed, the alternator B+, the battery poz, the big terminal on the starter, and the biggest terminal on the starter relay are all electrically fine places to get it; base your choice on location, access, and other factors.
No good. The coil usually doesn't get line voltage. Remember, it's downstream of the ballast resistor. It sees roughly 7v. Also, the choke heater draws a fair amount of current; conceivably more than the coil primary circuit wants to handle efficiently. Moreover, hooking up accessories at the coil + will sap voltage from the ignition system, causing driveability and performance faults. Remember Gus Wilson!
The choke needs an ignition-switched line-voltage feed. The alternator B+ (and all the power points common with it, see above) are not ignition-switched, they're always-live, so none of them is a good choice. The customary power point for an electric choke is on the upstream/ignition switch side of the ballast resistor (compared to the downstream/coil side of the resistor). Easy to find which is which: unplug both wires from the ballast resistor and put a voltmeter across ground and one unplugged wire at a time. Turn the ignition switch on and watch the voltmeter. When you find the wire that goes to battery voltage when you turn on the ignition, that's the one you want to tap. I like to use Posi-Taps for this kind of operation.
Remember, the choke also wants a thermostatic ground modulator. If you hook the choke's ground wire directly to ground, the choke heater will come on as soon as the ignition is switched on. This means there's no adaptation of the choke opening time to engine temperature, and usually you'll have cold driveability problems (stalling). Also, if the engine takes awhile to start, or if for some reason you sit there with the ignition on but the engine not running, the choke heater will back the choke all the way off even though the engine's cold and needs the choke closed to start reliably. Some electric chokes have a thermal modulator built into the choke housing itself; these usually have only one wire connection (+feed). The 2-wire ones that contain only a thermostatic coil and a heating element need a thermal modulator. You can see the modulator, which is a little 1" black box with a copper mounting strap, in this photo of an electric choke retrofit kit:
