Is my AC broken? (1 Viewer)

[sbman]

I've got an issue with my A/C that's been driving me nuts for years. This picture sums it up.

No matter what I do, the adjustment is a hair trigger around a certain spot. The spot moves around depending on ambient temp, but a tiny, tiny bit of movement goes from full cold to full hot. In the tiny, tiny, almost impossible to find spot between full hot and full cold, there is some variability of temp. When it's in that zone, the temps between the floor vent and the dash vents are never the same. The floor gets hot air while I get cool air up top until I hit full hot both places.

No matter what other settings I use, this is how it always works. The recirc button works properly, I mean I get outside or inside air by selecting it. The buttons up top work fine, air comes out the places the pictures show when I press the button. The 'AUTO' button works and it takes over the system and blows air the way it wants to but the temp is still this hair trigger thing.

I've looked at the schematics, the FSM and there is no description of this type of problem. I can't understand how the bottom vents can have hugely different temp than the top vents, it looks like the mixer flaps are driven by a single servo. The dash and windshield defrost vents are always in sync with air temp, if one is cold so is the other.

In the summer I have to use full cold A/C all the time because if I mix just a bit of 'warm' air the floor blasts hot air or I have to disable the floor vents and the trans tunnel cooks the feet with no airflow down there.

So, is this working correctly, or is mine broken? I've never been sure if this is normal operation or not.

 

[SpenserAK]

There is a potentiometer on the back of the HVAC box that controls the temperature adjustment. Over time the solder joints fail and need to be reflowed, which seems to fix the issue for many. Your symptoms may be slightly different, but seems like a related issue. Here's a thread detailing the topic. Hopefully it's your solution

Erractic Heater Temp Control Fix

My heater was possessed. But after a simple exorcism, all is well. I've had this problem with my heater on my '94 FZJ80 for some time. I would select a temperature with the slider and randomly the heater would go to full heat. I could jiggle the temperature slider and it would return to...

forum.ih8mud.com

Mine is either heat on or off, but the tipping point is the 85° mark. No other weird symptoms like the odd blending you're getting. Haven't gotten around to attempting the fix yet.

 

[sbman]

That's a good thread. I'll check the solder joints on the pot. I'm not too hopeful though because jiggling it doesn't make any difference at all. A couple of people later in the thread described my exact issue but never reported back if they fixed it.

So I'm assuming this isn't how it is supposed to work.

 

[jpoole]

I don't know why you are blowing hot and cold at the same time and likely can't be helpful beyond what's shared above but can share that my CE has the auto button and it works in an intuitive, predictable manner across the full sweep of temperature adjustments. It's often set within a few degrees of 70 and will blow hot or cold based on the temp of the interior with no need for input from me. If the slider temp is way different than interior temps the fan will go to a much higher level and then drop to lower levels as the slider and interior temps approach each other.

I'd consider trying to figure out if your heater valve on the firewall is functioning correctly as it could be open when it's supposed to be closed, which could be a clue about what's wrong with the system.

While you are checking the circuit boards I'd also clean out the white hose (hit it with a vacuum) that is behind the dash plate in your photo and make sure it's clear and connected to the hvac control plate/cover so that cabin air can be circulated over the temp sensor which helps the system know how to behave. The temp sensor pipe being clogged or disconnected shouldn't cause the mix of hot and cold simultaneously but does contribute to overall auto hvac functionality and can get clogged up, dirty or knocked loose over time.

Hopefully it's something obvious on the circuit board once you get in there.