There’s a big plastic (or is it rubber?) can connected by a large (1") tube to the bottom of the air cleaner on its top; on its bottom it’s connected to the EACV. I never thought much about it before; I had assumed it recovered vapors. Haynes and Chilton show it in the pictures but don’t mention it in the text.
Your title doesn’t match your post.
Are you asking what a resonator is or what that tube connects to
A resonator is a sound attenuation device.
It prevents noises from occurring at certain frequencies when a gas moves at certain speeds thru a system.
In your case, it’s connected to the intake duct to prevent noise
The electric air control valve is connected to the exhaust for air injection, the resonator quiets the induction.
In music a resonator is a sound amplification device; a sound attenuation device would be an anti-resonator, a muffler.
Haynes and Chilton don’t tell me what it does. Does it take air out of the air cleaner for injection into the exhaust to promote complete burning?
I see some models have an EACV but no resonator, some neither; it seems to be a kludge to fix up problems with the original design?
If it failed, what would be the symptom?
Air injection into the exhaust helps to reduce emissions.
The red hot exhaust gases that leave the cylinder still have some unburned fuel molecules left in them. They are unburned because there were no adjacent oxygen molecules next to them during the burning in the cylinder.
When fresh air, with its fresh oxygen, is injected into the red hot exhaust, those unburned molecules of fuel will now burn as they travel out the exhaust manifold.
They do more than attenuate the intake noise, they also flatten the power curve. A cylinder is basically an air pump. When the intake valve opens, it sucks air from the intake manifold creating a vacuum pulse in the manifold. When it closes, air flowing to the cylinder is suddenly stopped creating a pressure pulse.
These vacuum and pressure pulses travel from the intake valve toward the intake duct before the air cleaner. Pulses from other cylinders help smooth out the pulses as do restrictions like the throttle body and the air filter, but the pulses still exist. They will vary in intensity based on RPM and throttle position.
The pressure pulses oscillate between the valve and the intake horn. At certain RPMs, the vacuum pulse will hit the horn at just the right time to set up an intake pressure pulse that travels down to the valve and this pressure pulse hits the intake just as it is opening for the next cycle. When this happens, it rams air into the intake increasing power out. At other RPMs, the intake pressure pulse may hit the pulse created when the valve closes and reduces air flow down the intake port causing a reduction in power.
For racing applications, the engine will operate in a narrow band of RPMs, so the “tuners” will adjust the length of the intake runners for the intake pressure pulse to hit the valve at just the right time. But for street use, this isn’t practical. On the street, you use most of the RPM band so you would experience a very uneven power curve when accelerating.
The resonators are designed, and their are usually two of them to catch and store the pulses and release them slowly over a wider band of RPMs. This robs the engine of a few HP at the peak, where you rarely use on the street anyway, and releases it where the power would be cut by the pulses. It makes driving on the street much smoother, and quieter.