HAC: Analog or Digital?

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Mar 12, 2016
Jocko Valley, MT
Maybe this was discussed before but I couldn't find it. Are the HAC valves strictly on or off, or is there a gray area where they are partially open, then gradually opening fully with elevation?

I ordered one to replace the unit missing from my truck. When I got it I sucked on one of the ports to" test" it (an activity more accurately described as idly goofing with a new part), and it gave a loud 'pop' which I hoped was normal, but kinda figured wasn't. I've driven the truck from down around 2500' up to near 7000' and there is always some vacuum at the distributor advance. It runs better without it on a daily basis in the low-mid 3000's, so currently disconnected.
It's a mechanical device.
The FSM explains how it works.

Its the hysteresis in the device (dead zone between on & off) that's messing with you at 3000'. It doesn't turn off till around 2500 ft. Then turns on again around 4000 ft.

If you're driving up from sea level, at 3000 ft, the HAC is still off.

But if you're driving down from elevation (above 4000 ft) at 3000 ft, the HAC is still on.

So how the engine runs while at 3000 ft depends on if you're coming down from elevation or driving up from lower elevation.
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Aha OS, thanks. I had read the emissions FSM front to back when I did my desmog but your explanation is much clearer on how the valve actually behaves. Never thought of the lag.
You could say there was some hysteresis in reading vs. understanding...

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