LX470 - No AC...but only in the front (1 Viewer)

[dubdromic]

Hey folks! Long time lurker, first time caller.

I have a new to me 2006 LX470 with around 180k on it. It's exhibiting a strange issue: the front HVAC will only kick out very hot air (once the engine is heated up). The rear HVAC system will blow hot and cold (depending on the ceiling control setting).

Here's what I have tried:

- Using the Auto button. This does alter the fan speed and vent combination depending on the desired temperature. It has no effect on the actual temperature of the air.
- Using the...not...Auto button. Manual fan and vent control as well as the AC button (it's a nav model, so the virtual buttons). Also no effect.
- Using the recirculating or external air button. No effect.
- Setting max AC (below 65 degrees) and just about every cold temperature on the temp gauge. No effect.

Additionally:

- The AC compressor is coming on. I'm not sure how to check coolant level...but I figured there would be no AC in the back if there wasn't coolant. (?)
- I think the air temperature coming out of the vents roughly increases with engine warmup. (Up to what I would say is the standard "hottest setting" temperature. I'm guessing.)
- I haven't ruled out that the rear is just blowing in external air (vs actually cooling it). It's been a steady 30 degrees here...tough to tell.

Anyway, any ideas on why I can't get cold air (of any kind) out of the front vents? Am I just an idiot and there's some button I'm missing? I was also thinking perhaps the cabin temp sensor could be bad or disconnected, but I'm not sure why it would default to "surface of the sun" hot.

Thanks all for the input! And for the literally years worth of info I've read here. Glad to finally have one of my own.

 

[Rebus Knebus]

Maybe the air mix door/servomotor?

 

[saucebox]

Check the easy thing first—right near where your keys hang, there are a few slots in the lower trim panel. There should be a hose connected behind that—it senses the air temperature and adjusts. Clean everything around that sensor as well. You'll have to remove that trim panel, but it's easy.

For whatever reason, when it's disconnected it's all-hot, all the time.

 

[Tres Toys]

If the back is cooling, then the compressor is working and you have at least some charge in the system. The front and rear AC system use separate evaporators and expansion valves. The evaporator and expansion valve that cool air for the front zone are located behind the glove box.
Starting with the most simple, some things to determine are:
1. Are the evaporator coils blocked with a bunch of debris or pieces of and old air filter that limits the air flow across them?(remove the air filter door behind the glove box and check to see if you have filters and if so pull them both out to inspect and then look at the evaporator coils for possible blockage).
2. Is air being directed across the evaporator in the cooling mode by the damper system. (can be easily checked with the air filter door off.)
3. Is refrigerant being metered through the expansion valve/evaporator. (temperature change between the coolant lines on the firewall going into the front evaporator should be noticeable when everything is working properly)
Let us know what you find out and best of luck.

 

[dubdromic]

Thanks for all the replies!

I think I have it nailed down, if not fixed. I read somewhere that there's an AC self diagnostic mode you can initiate by holding the auto and recirc/fresh air buttons when turning the ignition to the on position. It did its thing and kicked out a "41" code, which Google indicates is an issue with the air mix servomotor. I think that servomotor is in the passenger side footwell on the left side (next to the foot air vent). I pulled the few screws I could see and jiggled the servo and the air started coming out cold.

I don't think it's fixed - perhaps there's a contact or wire loose in there and I just happened to get it right - but it's nice to know where the problem is. The service manual indicates you an just pull it right out after removing three screws...but I can't see a third screw, so who knows. I'll probably have my mechanic just replace the thing next time it's in the shop.

In any case, thanks again!