automatic choke

[mckpaul]

Ok - thought I had the Q-jet automatic choke thing figured out, but too many years have gone by since I messed with one. And down here it so rarely gets cold enough to be an issue.

Tell me about that bi-metalic spring in the little housing that closes and opens the choke based on temperature. I know it as it gets warmer it moves/rotates enough to allow the choke to open completely, and when it's cold it moves the other way to close the choke, and I know about the choke vacuum pull off valve and all that when the engine starts etc. Guess I always viewed the choke spring assembly as "electrically assisted", the one wire connector bringing power to the unit, using voltage to heat the coil and make it move more, have more travel, other than just depending on heat from the engine. The wire on mine was disconnected when I got it, and if you set it where it closes when cold it never opens completely when the engine is warm, and if you set it where it is open when warm, it never closes enough to give you choke to start when it's cold out. I read +12 volts on the wire, but when I connect it the voltage drops to 0 volts, and there is an indicator light on the dash that says "Choke" that comes on when connected. Guess I'm reading the voltage on the open wire back through the bulb. So this one wire connector is actually just a ground trigger for the dash light that goes open when it gets warm or what? Does the bi-metalic spring strictly work off the heat from it's surrounding area? Used to be that way years back, but those were bolted directly to the intake.

Can someone tell me how this really works?

[Zilverado]

Great question.

Sorry, I don't have an answer, but I've had these same questions for a while, ever since my choke "difficulties". I sorta faked my way through it and made it work again, but I'm looking forward to the responses to your question.

[J-Spec C10]

the choke light comes on when its all hooked up, and some part of the system is not functioning. The connector on the wire is a problem around here. they even sell replacement pigtails at most parts stores. The wire is supposed to be "hot" or "live" when the ignition is on. This heats up an element in the black plate that the spring is on. If the element is broken inside, the choke light will come on with the wire connected. The element is used to heat the spring, which in turn opens the choke plate slowly. The primary and secondary choke pulloff's are used during the warm up process.

[swervin ervin]

The choke light will come on if the choke coil is bad, as it seems to be for you, since it won't work right either way. Replace the choke coil and it should work fine once again.

[ElGracho]

Ahh the age old question of the choke light. This light on the dash of our trucks is certainly misleading. It doesn't come one when the choke has power or is working as you might expect, its really more of a low oil pressure light labeled "choke."

I found out this information the hard way, but made this drawing from my wiring diagrams book.



As you see here, if the engine is off (ie no oil pressure) but the ignition is on, the choke light will glow. When the engine builds enough oil pressure after starting to shut the pressure switch, the choke light will go off, but the choke will get voltage. The resistance of the wiring and the bulb prevent the choke heater from ever heating the bimetal strip enough to move the choke on the carb. If this oil pressure switch breaks, is disconnected, or the engine actually loses oil pressure, the choke will eventually shut (or never open) causing the truck to run really crappy when it gets warm enough to not need the choke. The choke light (if the bulb is good) will also stay on all the time.

When the engine is running, the oil pressure switch shuts. This puts 12v on both sides of the choke bulb and it goes out. The choke heater coil has more current and will slowly open the choke as it heats up. When it gets hot enough, the resistance is high enough to limit the current to some fairly low level allowing the choke heater coil not to burn up and lead a relatively long life.

Now for your specific scenario. When you have the wire removed from the choke and the engine off, the voltage you see is from the inst light circuit. When you hook this wire up, you see zero volts meaning the resistance of the coil on the choke is much lower than the light bulb (ye old voltage divider). Probably means the choke heater coil is bad (shorted).

If you were to start the truck (shutting the oil pressure switch) and measure voltage, it should be closer to 12v, but it might also just have blown the choke fuse (lowest fuse in the block (20A), next to the signal flasher) whenever the coil shorted in the first place and still read zero volts, check that fuse too.

Wow, long answer. Now I know what KIILEW must feel like!

[mckpaul]

EXCELLENT ! Thanks for the replies guys.

I'm a low voltage technician of sorts by trade, actually a planner/designer for a company that does systems in commercial buildings, fire, TV, paging/intercom etc. in schools, offices, etc. Deal with stuff like this all day, sometimes something that should be plain as day doesn't make sense. The schematic makes it all clear. Would have never thought to include the oil switch in the picture.

But the oil switch is not currently connected, the 84 motor was pulled out and replaced with an 88, the switch terminals are spaced differently, so the connector is left hanging. But that's just the tip of the iceberg. Ground wires from the body and ESC system were left hanging and not reconnected during the engine swap out, they didn't bother to reattach the wiring harness support rings to the engine to transmission bolts (half of which were only hand tight), still have some unidentified two pin connector hanging on the right side between the engine and transmission, one pan bolt missing on the tranny pan, dash trim was pulled off and half the screws missing... could go on... sorry... just venting...sloppy work like that bugs the crap out of me....

Anyway, thanks again guys !

[J-Spec C10]

sounds like you have a lot of wiring fun ahead of you, good luck!

[smitty62]

ElGracho;
I've tried to follow your description, but I'm not very technically oriented--does this mean that If I put an electric choke on my truck which didn't have one originally, that it work ok with just a hot wire from the ignition switch, or will it burn the choke coil???
Thanks

[ElGracho]

It won't burn up the choke coil, that will work. They don't recommend using the same wire that goes to your coil, since you don't want to limit the energy going to your ignition system. I would recommend tapping into a fused terminal on your fuse box the is on with the ignition only. Don't use an accessory terminal since that will heat the choke when the truck isn't necessarily running.

[smitty62]

Thanks El;
I guess they routed current thru the oil sender to prevent the choke from heating anytime the key is on. Thanks again