When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Hi Louis, had the same problem sometime ago. i was able to resolve it by opening the HVAC hot water valves beneath the engine ECU in the bonnet, i cleaned the valves with stainless steel cleaner greased it and coupled it. Its been fine since then.hope this helps.
Hello Dewsoetz,
Where is this valve exactly? Removing means coolant will come out? I'll do it next after the full discharge and refill by the AC shop as everyone here suggested.
The car is still so nice to change to a newer one. A few hundreds sure would be much less than $36k and have to leave this wonderful forum
Wish I have a 3 car garage since I just bought a new eGolf for commuting. Q7 is still like new to be parked on the driveway if the D4 is acquired.
Hello Dewsoetz,
Where is this valve exactly? Removing means coolant will come out? I'll do it next after the full discharge and refill by the AC shop as everyone here suggested.
The car is still so nice to change to a newer one. A few hundreds sure would be much less than $36k and have to leave this wonderful forum
Wish I have a 3 car garage since I just bought a new eGolf for commuting. Q7 is still like new to be parked on the driveway if the D4 is acquired.
Thanks all,
Louis
Louis, the coolant wouldn't come out. its just the pins in the hvac hot water mixer you need to clean, you dont need to discharge or release the coolant. and please do this first before discharging and refilling the gas. I'm very sure its the valve and not the gas in the system. If you can locate the ECU, the valve is located to the right of the ECU in the bonnet. Wishing you luck.
Louis, the coolant wouldn't come out. its just the pins in the hvac hot water mixer you need to clean, you dont need to discharge or release the coolant. and please do this first before discharging and refilling the gas. I'm very sure its the valve and not the gas in the system. If you can locate the ECU, the valve is located to the right of the ECU in the bonnet. Wishing you luck.
You experts made a believer out of me... Still don't get it
OK, 2 days ago I went and had the AC system evacuated to -30psi, make sure no air in the system. Filled it up with 620g = 21.7 oz of R134a Freon, since it's +/- 20g, I pushed for 640g = 22.5oz. Yesterday it was 63F, I didn't feel anything, I thought it was a waste of time and money, but today, 80F outside, set the temp to 63F and the air came out everywhere, all vents cold... I read, the vents do have memories, so until it's actually erased, the new memories will be in place? Anyhow, it's working and I don't know how and why low Freon caused all these vents problems.
The one thing I've learned is that when the vents blow hot air on the driver side (front and rear and pillar), you can't add more freon, you have to evacuate and recharge the system.
I found the valves per Dewsoetz but never opened it.
One evaporator feeding two side-by-side heater cores
This diagram from SSP 271 for the Phaeton HVAC system which it 90% shares with the A8 seems to explain the lop-sided temperature failure mode.
The way I understand it, there is one evaporator, which feeds 2 side-by-side heater cores. The air is cooled (and dried) before it’s heated back up to the desired temperature. When the refrigerant is low, the evaporator is not able to perform in such a way that it’s uniformly cold across its entire area. So this means a side-to-side temperature gradient of air coming from it. So air coming through the left portion is warmer than that which has passed through the right portion. That in turn means that the left and right side heater cores are receiving different temperature inlet air, which in turn translates to different temperature outlet air into the cabin, side-to-side. There is only one evaporator outlet air temp sensor, and one outlet sensor per heater core, and they are positioned in the center of each so can only read the temperature coming out of that one small part of the evaporator/heater core. As the system assumes that the heater cores are receiving equal inlet air temps across their surface area, it sets the heater water valves to suit. When in actuality the heater cores are receiving different temperature air across their areas, so it ends up delivering over “re-heated” air to one side, and under “re-heated” air to the other side.
Thanks Dave, so Dewsoetz did hit the right solution also, the pump valves.
I'll open the valves up to see if it's dirty or sticky, may be I did waste time and money on evacuation and recharge
Cheers,
Louis
Thanks Dave, so Dewsoetz did hit the right solution also, the pump valves.
I'll open the valves up to see if it's dirty or sticky, may be I did waste time and money on evacuation and recharge
Cheers,
Louis
No, I think in your instance it was low refrigerant level which causes the evaporator to output uneven temperature air into each heater core. Evidenced by the evac and recharge charge fixing it. Sticky heater valves could also produce funky side to side temp variations, but that would most likely be sensed and modulated out by the heater core output temp sensor until it got really bad/gummed up so it’d be harder to diagnose.
04-15-2018, 11:38 AM
