Hot on one side, Cold on the other

[ringoP38]

Had a new issue pop up today. Felt like it was just not cooling the interior well and it's not hot out yet. Was a bright sunny day so the interior was a little warm put the A/C on and took off. When it felt like it wasn't cooling well I turned the temp down and felt cool air coming out my vents this was all till I put my hand over the right side center vent and felt HOT air coming out. So turned the temp down all the way on both controls. Left vent cold air, left center vent cool air, right center vent hot air, right vent hot air. Adjusting right side temp control cold to hot does not change the temp of those vents, hot all the time. Change the temp on the left side control to cold and cold air comes out of left side, turn it to hot and hot air comes out left side. FSR, maybe?

[1star2]

I have the same problem too. I replaced the FSR and that didn't fix the problem.

[ringoP38]

That's what I was afraid of. I know the P38s used to have blend motor issues though mine never did. Totally different vehicle but I wonder if there could be something there. Hopefully someone else has had this issue and worked through it.

[ZUNDFOLGE!]

Probably a blend flap failure. It's different from the P38 but same principle. Could also be the heater valve itself having a fault or restriction. Feel the coolant hoses coming out of the valve (valve is over on pass. side near inner wheel well I think) and see if it's hot. If the hoses are hot, your problem then most likely lies in the under-dash components.

[ringoP38]

Doubt it's the heater valve. I have cold air coming out the driver side vents but very hot air coming out the passenger side vents. A/C control? maybe? I sure hate to deal with probably's on these vehicles, just end up replacing unnecessary parts, oh wait, that's the stealership's approach.... and they are "trained" on these things....

[ringoP38]

Hooked up the All Comms this evening and ran a check on the HEVAC system. The only error it pulled was "Air Quality Sensor Line Fault". I cleared the system with no change in the issue and read the faults again and the same error came up. Could not find anything online relating to this error. Also does not even seem to relate to the issue? Has no one had this problem on a MKIII but me and 1Star2 ??? Sure hope to fix this before the hot weather starts coming which seems to be right around the corner. Going to be in the mid 80s tomorrow.

[G8HAV]

Hi,
Had the same problem and nothing shows on the computer set up.
I took off the valve on the passenger side of engine bay (basically a 3 port valve switched by demand) and dismantled and cleaned it out. Bits of crud had gathered and made the valves stick. Had no problems since.

Probably a blend flap failure. It's different from the P38 but same principle. Could also be the heater valve itself having a fault or restriction. Feel the coolant hoses coming out of the valve (valve is over on pass. side near inner wheel well I think) and see if it's hot. If the hoses are hot, your problem then most likely lies in the under-dash components.

[ringoP38]

Thanks G8HAV. So you had cold A/C air coming out the vents on one side of the vehicle and hot air coming out the other side? Think I know the valves you are talking about but they are on the driver side for me. I would have thought those valves just supplied hot air to the whole system or blocked it as needed. Trying to figure out how they would allow hot air to come out of only the passenger side vents (driver side for you).

When you say you dismantled and cleaned it out are you talking about taking the 3 port valve assembly itself apart and cleaning the parts inside valve or just the connector? Can you describe this process a little more and possibly what you used to clean it. Just trying to visualize it from the office before I start taking things apart and make sure I will have what I need ahead of time. Thanks again for the input.

[ringoP38]

I have the same problem too. I replaced the FSR and that didn't fix the problem.

You had any luck tracking down your issue?

[1star2]

You had any luck tracking down your issue?

I haven't gotten around to further troubleshooting yet. Probably won't be using AC here for a couple months.

[1star2]

Has anyone found a solution to this problem?

[LEBoyd]

The HEVAC system on the 03-05 is pretty complicated, with 53 pages (10%) of the RAVE Workshop manual devoted to this system.

Since "hot" and "cold" are relative terms, have you actually measured the air coming from the vents? Is "hot" actually just non-cooled air or is "hot" heated air, and vice-a-versa depending on where you are and what the temp is outside. For example, here in Houston right now "hot" area would have to be somewhere above 80 or 85 degrees since that's the high temps for the day.

Assuming you have the "highline" version (which that's all I've ever seen in these, but apparently there is a less sophisticated "lowline" version) you might consider looking at the two temp sensors and swapping them from one side to the other. If the "hot" and "cold" sides don't change, then it's not the sensors.

Do you get any change if you enable the Max AC switch? This should override most of the sensors to cool the truck as quickly as possible and doesn't shut off until you shut it off.

[ringoP38]

Still no resolution here.

The right side is only HOT air, meaning turn the right side temp control knob to any temperature and the right center and far right vent only blow the hottest air possible no matter temp outside, inside or whatever temp setting you have it on the right side control knob. The right side vents only blow one temp, as hot as the engine/system allows the heater to blow. Set your temp knob all the way clockwise to full heat and that is the temp my right side is stuck at. Maximum heat setting is the only temp that comes out of those vents. Turn the temp control knob on the left side and the temp adjusts as normal in the full range of cold to hot according to what you set the control temp knob to. Pressing Max A/C button works for left side vents but the right side vents only blow higher fan speed but still max heated air, no drop in temp on right side vents from full heated air.

[LEBoyd]

Since there is no change when you set the Max AC (I assume you have it on level two which manually overrides basically all of the valves, sensors and flap settings), it seems to point to the coolant valve.

When you set the ATC ECU to a temp, the ATC ECU reads the temp from the appropriate temp sensor and closes the appropriate coolant valve as necessary to that side of the truck. Enabling the ATC ECU Max AC on level 2, the temp sensors are bypassed and the ATC ECU closes both coolant valves. Since the Max AC switch is separate from the ATC ECU LH/RF switches, it appears the problem is related to the coolant valve.

I believe if the temp sensor was bad, grounded or shorted, it would default to blowing super cold (thinking it's 131dF).

This is just an educated guess considering the symptoms.

[ringoP38]

Thanks LEBoyd. Now we are getting somewhere. That is making a little more sense. I thought the valve under the hood on inner fender, I believe it is on the drivers side for US models, just turned the flow of hot air to the HVAC on or off. I did not realize that that was where the left and right sides are separated where temperatures are concerned. G8HAV mentioned something earlier but I did not follow that as I thought it was one in and one out for hot water flow to the fan/blower coils to supply heat. Tried to get clarification but did not hear back. He also mentioned about cleaning this valve system but no real detail as to what he cleaned. I am assuming he took the valves apart and was not referring to cleaning just the electrical connectors. I tried that and did not make a difference. Guess I'll pull the valves off and see how they come apart. If anybody has any experience with these valves and the inner workings I am all ears. Thanks for the help.