Little Trucks, Big World

My '84 idles between 2000 ad 1500 rpm.  In the past, the auto choke relay was bad and the cause.

On the ACR that's in the truck now, the solenoid closes when the key is in the 'on' position, but opens once the engine is running.  
Is that the normal operation for that relay?

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Hmmm, it's been a while since I've reviewed the electronics for that circuit.  But if my memory serves me correctly, the relay should close once the key is turned on, and should remain on.  The bi-metal spring within the choke then heats up and slowly opens the choke which will then also affect the fast idle circuit.  The choke is an electro-mechanical device, where the fast idle circuit is purely mechanical which works off the functioning of the choke position.

However, once the fast idle is set (by fully depressing the accelerator pedel once before starting the engine) I believe it will step down in rpm a bit once it's warmed up, but you'll still have to tap the throttle pedal once in order to get it to drop it back down to normal idle.

It sounds as if your choke relay is opening too soon, and not allowing the choke to open as quickly as it should which is keeping the fast idle active.

Let me double check that in the manual and get back to you.

Ok, I was pretty close but there are some errors in what I said mainly because the choke relay is not a normally open relay.  The choke relay is a normally closed relay, meaning it's closed (passing 12V) to the choke heater when the relay is de-energized.  So when the relay is off, the heater is on.

The choke relay and heater both get 12V directly from the ignition switch when it's in the IGN position.  But the relay is also controlled on the negative side of the coil by the L terminal of the alternator, just like the charge warning light.

So when you turn on the ign. switch (but before the engine starts) the choke relay is energized, meaning it opens the contacts and is actually not passing 12V to the choke heater (12V on one side of the relay coil, L on the other which is ground at this point).  This is the same reason that the charge warning light is on.  Because one side of that lamp is 12V, and one side is ground (L terminal).

Once the engine starts, and the alternator starts producing voltage, the L terminal becomes 12V.  This de-energizes the choke relay, closes the relay contacts, which actually then supplies 12V to the choke heater and turns it on.  And your charge light is off as well for the same reason, it now has 12V on both sides of the lamp which turns it off.

So as long as the engine is running, and the alternator is charging, the choke relay is de-energized which is actually turning on the choke heater.

However, if for any reason your alternator is not charging the L terminal would remain low (ground), your charge warning light will stay on, the choke relay would remain energized, and your choke heater would remain off.  This could also happen if you simply have a bad connection at the alternator L terminal.

So, is your alternator light on by any chance?  If so, that could be your problem with the choke which could result in the fast idle circuit being active when it shouldn't be.

Thanks for explaining all that.  It's counter-intuitive that an open relay sends power to the auto choke, but when you explain it, it makes sense, and that's how mine is operating.  We backed the auto choke down a little at the carb, runs great now.
I appreciate your help!

That is good to hear!  Glad to help.  Yes it is counter-intuitive for sure.  Even after I sorted through the circuit, and had it all figured out in my head, I still got confused trying to explain it lol.


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