HVAC stuck in heat mode

Joined
Jul 23, 2008
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408
I need help figuring out a problem with my HVAC system. Here are the symptoms:

Up until a couple days ago the AC worked great. Starting today, it is stuck in "heat" mode and blows out very hot air regardless of the thermostat setting. I can move the temperature slider from it's coldest to it's hottest setting and I can't tell a material difference in the temperature of the air coming out. Also when I push the "Auto" button, it chooses the floor vents for the air supply rather than the dash vents. While I have not checked the refrigerant pressures, the low side lines on the compressor are very cold and the H side very hot, which coupled with normal looking and sounding compressor / clutch operation suggest the compressor is functioning just fine.

I'm working on two theories right now:
1) something is wrong in the HVAC controls/ buttons such that the controls think they are set for max heat regardless of where I slide the temperature controls.
or
2) the heater valve is stuck open so that even when the system is pushing refrigerant through the evaporator behind the dash, it's also pushing coolant through the heater and heating the air after the AC cools it (I do this often on purpose when defrosting the windows).

Any other ideas? Or ideas on how to most quickly diagnose which of these is the culprit? I might be able to find HVAC controls from another car that I know is working and swap that part in so I can see if new controls correct the problem, but I'm hoping for an easier solution (and a cheaper one since buying new controls won't be cheap).

If it matters, it's a 1997 HDJ81, but I think the HVAC controls are the same and the system certainly functions the same way, though I'm sure a few part numbers differ.
 
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blows out very hot air regardless of the thermostat setting

Heater control valve on the firewall. Check if the little arm is connected. If so, disconnect it and try moving it to closed.

Your auto climate controls are putting it on recirculate because that's the fastest way to drop temperatures.
 
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This ^ in addition, can you hear the mechanism running behind the dash (forgetting the name at the moment) that moves control valve’s arm, when you slide the heat setting from one side to the other?

Another thought that comes to mind is the potentiometer on back of the HVAC box. Sometimes the little solder nubs crack and thus the connection is wonky. Rather than stuck on one setting, seems like it usually leaves you with an “on or off” function, or erratic behavior - this is the case with mine, changes to full heat at the 85° mark, and below that = heater off. If you’re stuck on hot, control valve makes sense, just something to keep in mind if that‘s not the culprit.

 
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Joined
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Heater control valve on the firewall. Check if the little arm is connected. If so, disconnect it and try moving it to closed.

Your auto climate controls are putting it on recirculate because that's the fastest way to drop temperatures.
Good thought. I just had the hood repainted so it's possible that valve got smacked when they were removing or reinstalling the hood. I don't think there would be any reason to disconnect it intentionally, but they were working near the valve.

Your auto climate controls are putting it on recirculate because that's the fastest way to drop temperatures.
It's not recirculate it's switching to (actually it does do that also, but my recollection is all my LCs were defaulted to recirculating air regardless of the temperature), it's that the system is choosing the floor vents to output the conditioned air. Usually if I choose the coldest setting, the system picks the dash vents.
 
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I performed some experiments. Here's what I found:
  • The heater valve on the firewall did not move regardless what I did with the controls
  • when I disconnected the cable from the valve lever and rotated the valve all the way clockwise (while facing the firewall from the engine side), the AC would blow cold again. The cable attached to the valve was "fixed" - i.e., I couldn't move the actuator end of the cable in and out by hand.
  • Some of the buttons on the controls weren't functioning correctly. When I'd push the dash vents button, for instance, it wouldn't engage, but if I held the button in, the valves / doors behind the dash would move around to direct the air to the dash outlets, but as soon as I released the button, it would return to blowing out the floor vents. Some jiggling and effort could cause the button to stick in the correct spot and the "sticky" buttons were happening in many locations, not just the one button.
  • When buttons did work, I could hear actuators, doors, and stuff moving around behind the dash. I could hear nothing, however, when I moved the temperature slider.
I suspect something in the controls themselves is bad. If I have time tomorrow, I'll try to pull the HVAC control panel out and see if anything looks obviously wrong. I doubt anything would be visually apparent if it's buried in the buttons, but maybe there's a loose connection or similar issue that's fixable.

heater valve with the actuator cable detached and valve rotated full clockwise (must be the closed position)
IMG_5164.jpeg
 
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When I was chasing down my own "heat only" issue I found a connector behind the radio that the PO had disconnected. That doesnt sound like your solution... but just FYI.
I also found that the automatic AC amplifier (white box under dash above right knee) had circuit board corrosion likely from a leaky windshield. I cleaned it up and drilled a drain hole in the bottom of the box. This wasnt causing my problem but it does control all of the AC/heat switching and it has caused weird issues for others.
 
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I think I found the problem: Gunk in the buttons.

I removed the HVAC control module from the dash. At first I followed the above thread about the cracked solder points and removed the back of the module, but everything looked ok. I heard a noise when I turned the module over and noticed there is a removable face plate surrounding the buttons, so I pulled that off. When I did, black stuff started falling out - not a ton, but a handful of hardened pieces of sizes varying from grains of sand to 2mm x 10mm long strips.

It appears there is a thin layer of foam or rubber underneath the buttons. I'm guessing it's purpose was to make the buttons feel more luxurious / less harsh when depressed instead of a hard plastic on plastic feel. Apparently 25 yrs took a toll on that material and it was brittle causing it to break apart. I think fragments of that foam layer were preventing some of the buttons from properly engaging or disengaging leaving some stuck in position. I took an air canister I use on my computer and blew around the buttons to clean that stuff out. I thought about removing each button face and cleaning those individually, but the buttons seemed fine without the extra effort.

I reinstalled the HVAC controls, double checked the connections were solid, and re-connected the cable to the heater valve. Crisis averted - HVAC works again. It's possible it's a temporary solution if more of that rubber interferes with he buttons or it had some function beyond making the buttons sound and feel more luxurious, but I'll deal with that later.
 
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This ^ in addition, can you hear the mechanism running behind the dash (forgetting the name at the moment) that moves control valve’s arm, when you slide the heat setting from one side to the other?

Another thought that comes to mind is the potentiometer on back of the HVAC box. Sometimes the little solder nubs crack and thus the connection is wonky. Rather than stuck on one setting, seems like it usually leaves you with an “on or off” function, or erratic behavior - this is the case with mine, changes to full heat at the 85° mark, and below that = heater off. If you’re stuck on hot, control valve makes sense, just something to keep in mind if that‘s not the culprit.


I had the same issue last year and reflowing these solder joints fixed it.
 

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