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Discussion Starter #1
On my 2008 Sienna LE with 134k, the AC does not blow cold when the front only is on. if you turn on the rear AC, the front will then become cold. I do not know about the front/rear system and how they interact to figure out the issue.

Are the front and rear freon systems connected?

My theory is that a valve is not opening with the front switch activates AC on the compressor. This is a half educated guess. My friend's VW compressor had a sticky valve that he was able to replace pretty easily.

Id like to hear from people with better understanding of our AC systems
 

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Best swing by your local harbor freight and pick up an AC gauge set to see whats going on with refrigerant pressures. Could be the pressure is low and turning rear on just moves the problem to the back. Even if you don't have a low pressure you'll still need it for properly refilling after breaking the system open to replace a part.
 

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Discussion Starter #3
thanks, that is a good suggestion. it will give a baseline idea of what the refrigerant level is and i can go from there. I would actually be happy that after 10 years of ownership, there is only a slight leak
 

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So, I'm having what appears to be the same problem with the same year van ('08) around the same mileage (140k) as the original poster.

Any thoughts on what it could be? Symptoms are that everything seems normal, but front AC doesn't seem to cool until the rear air is turned on.

Thanks in advance!
 

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Probably won’t help in this case but I had a 1990 prelude one time that the ac would come on and after a bit it would just shutoff. Isolated to ac control board. I took it out and resoldered every point on it. It worked from that day until I sold it.
 

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Thanks for your reply - that was a great catch on the control board. How did you diagnose it?

I was hoping Farmermac would be around, but it looks like he hasn't been here since April '19. I did send him a PM, so we'll see!
 

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It's all one freon system. I don't think there is any isolation valves regulating freon flow front to rear. Both evaporators should get cold. I believe you are just turning on the compressor and working flaps and fans front and rear.

So, when you just enable the front system and you are not getting cold air, is the compressor engaged? You should be able to see if the clutch is spinning or stationary.

If it is not engaged with just the front controls on, does that change when you turn on the rear controls? If so, then I would agree that you likely have an AC Amplifier issue.

On Gen-2 vans the AC Amp is right behind the front panel.
 
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Thanks for your reply - that was a great catch on the control board. How did you diagnose it?

I was hoping Farmermac would be around, but it looks like he hasn't been here since April '19. I did send him a PM, so we'll see!
Shop diagnosed it. But a buddy of mine who seems to know how to do everything suggested cold solder joints on the board. We took it out and saw cracks on the solder. That’s when we redid them. Got lucky.
 

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Discussion Starter #9
im so sorry i havent been on here in forever. I am the original poster. My van was simply low on freon. i used 2 cans to bring it up to the right pressure (!!) and its been fully functional since. Im assuming slow leak. The system was nowhere near empty, it was just low enough that i think the amount of freon was right for the smaller rear evaporator, but not enough to cool down the bigger front one.
 

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My guess as to why that happens - when the rear AC blower is off, front blower is on, and the system is low on refrigerant, a fair bit of liquid refrigerant builds up in the rear evaporator due to the very low evaporation rate there. That effectively starves the front evaporator of refrigerant.

- G
 

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Well, Farmermac saves the day. I did indeed buy a gauge set, 2 12oz cans of R134a (3 actually), and tonight added 2 cans. Brought the low side up to 33psi & high side up to 160psi @ 90F ambient, and low and behold, the AC started working and vents started blowing @ ~40F.

I suspect those pressures are still low, so if anyone reading this can point me to a chart for Siennas, I'd be grateful.

Greenlight, that makes sense - the fans are off, the evaporator is really cold, and the liquid builds up b/c of the low temp.
 
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