Jump to content

AC blows warm air for a few seconds randomly


EricCR
 Share

Recommended Posts

Hey folks,

 

My 2003 sometimes does this weird thing of blowing warm air for a second or two when the AC is on (it's the digital HVAC). I don't know if it's a rudimentary way of keeping the right temperature but this is the only car I've seen in which is so noticeable. Since the car is relatively new to me, perhaps it's a non issue?

 

Otherwise the AC blows cold and doesn't seem to be getting warmer over time. Is that something I should go and have it checked?

Link to comment
Share on other sites

Thanks. Coolant as in freon, aye? The weird thing is that other than that, it's ice cold 99% of the times. I'll still go and have it tested regardless. Just knowing that it's not normal in these cars is reason enough.

 

I'm still learning a lot from this Pathy. I took the intake manifold apart and all 6 butterfly valves along with their respective screws were still there, so I'm way less concerned about the car now. I loctited all the screws anyway (they were not loose but didn't take much force to unscrew them either). I'm going to make a routine of checking them every 50k miles or so. I also have an oil leak which sounds like a PITA to repair so I'm still deciding if it's worth fixing. Other than that and a rattling rear hatch, the car is perfect so I'm very happy with it.

 

With the lockdown I haven't been able to really use it other than for short trips and such. It's a shame.

 

 

  • Like 1
Link to comment
Share on other sites

  • 1 month later...

So I took the car to have the AC system checked and everything was OK. It's charged, there are no leaks and there should be no apparent reason why it randomly blows warm air. At the shop they mentioned that a faulty heater core might be the culprit but I can't see how that could be related.

 

I have found two things in the past weeks too:

  1. I normally set recirculation mode and leave it like that. As with any car, air resistance causes fans to be louder in that mode, but for the few seconds that I feel warm air, I also hear as if the recirculation flaps open and let outside air in (maybe blend door actuator?).
  2. If I set the AC temperature to 60 F (lowest setting) then there's no more warm air. This is my workaround for now but I do want to get to the bottom of this.

The above leads me to think that the HVAC has problems measuring cabin temp and adjusting itself. I cleaned the sensor under the windshield and I'm considering buying a used HVAC panel to test it out (they're relatively cheap anyway).

 

 

Link to comment
Share on other sites

I’m not sure if the 03 is the same as a 98 but the cabin temp sensor on my 98 is to the right of the steering wheel and to the left of the radio, right about where my keys hang from the ignition, but on the dash.

 

I do also have a sunlight sensor on the top of the dash, under the windshield.

  • Like 1
Link to comment
Share on other sites

Yup. I downloaded the factory repair/maintenance manual and it's the only place where that minuscule grille is referenced. Thanks for confirming it. I will take that lower dash panel off and see if the sensor is dirty. There is also an air intake temp sensor (for the AC, not the engine) behind the glovebox. I will see if that one is easy to reach and clean.

 

I spent a few hours last night reading and the automatic HVAC takes the readings from the internal temp sensor, the intake temp sensor, the sun load sensor and the desired temperature to calculate how to adjust the internal blend door. It's a fairly complicated system. What the AC shop told me now makes some sense. According to the service manual, as soon as the system reaches the desired temp, an actuator moves the blend door to allow some air to pass through the heater core so the air warms gradually and temp doesn't drop further.

 

Since I can get both cold and hot air I don't think my door actuators are bad, but probably the HVAC panel itself, which houses the logic board and is supposed to take all those readings and make adjustments, might be acting up and it's opening the blend door too much, exposing the heater core for a few seconds and then it quickly figures that the temp is increasing a lot and goes back to closing it.

 

 

Link to comment
Share on other sites

Ok, I removed the lower dash panel under the steering wheel and no wonder why my AC is acting up:

 

Iteduf-EERSu4-FX-DGc-UHfg.jpg

 

Interestingly, in my E39 there is a little electric fan that pulls cabin air through the sensor. In this Pathy it's vacuum driven and there is a hose that goes all the way to the blower behind the stereo. The hose is as dirty as the sensor is.

 

Next thing is finding the intake air temp sensor and also cleaning it up.

  • Like 1
Link to comment
Share on other sites

Wow! Yeah, no surprise that's making it unhappy.

 

The auto climate in the WD21 has a hose off the back of the climate control head unit and a little fan drawing air constantly. The service manual says to check if it's working by holding a lit cigarette in front of the grille, which is next to the buttons and sliders. Speaking of smoking, the PO did, so I'll bet mine looks like yours inside! Something else to check the next time I've got the dash apart.

Edited by Slartibartfast
Link to comment
Share on other sites

On 12/10/2020 at 9:52 PM, Slartibartfast said:

Wow! Yeah, no surprise that's making it unhappy.

 

The auto climate in the WD21 has a hose off the back of the climate control head unit and a little fan drawing air constantly. The service manual says to check if it's working by holding a lit cigarette in front of the grille, which is next to the buttons and sliders. Speaking of smoking, the PO did, so I'll bet mine looks like yours inside! Something else to check the next time I've got the dash apart.

 

The way it works in our R50s is simpler: A hose connects the cabin air sensor to the main duct behind the stereo. As you activate the HVAC and air moves through the duct, it creates low pressure on the hose and it draws air in. Funnily, for all this to work Nissan had to leave an open vent behind the stereo so you have some cooling there :)

 

Anyway, long story short, the AC is still doing its random hot air shenanigans after cleaning the cabin air temp sensor.

 

I took almost everything apart short of removing the evaporator and the heater core. I was still able to see the AC in action and what exactly happens when I get those seconds of warm air. In short, it's the blend door actuator which is letting some air pass through the heater core (it opens for a few seconds, then the sensors probably detect the temp increase and the actuator closes the door again). That happens even with the temp set to 60 and air directed just to the front vents, which in theory and according to the service manuals, should never cause the blend door to open.

 

My hunch is that the HVAC control panel itself is bad since it should be taking the readings from the cabin air sensor, the intake sensor, the sun load sensor and the temperature you set to signal the actuators how to move. So I'm going to get a used one off eBay to try it out. I doubt any of the temp sensors are bad cause they are simply miniature MAFs which heat up and then measure air passing through, but that's still a possibility.

 

My worst-case-scenario plan would be to disconnect the blend door actuator. I don't use the heater anyway. If you suffer from not-so-cold air, you might have a similar issue with the AC being fine but air warming up because it mixes with air from the heater core.

 

I'm sorry I didn't take other pictures besides that one above, but let me try to make a diagram of everything back there.

Link to comment
Share on other sites

If you need instructions on how to remove each panel, glovebox, etc. just let me know and I can take some pictures.

 

This is roughly where everything is located. Hope it makes some sense. Blue arrows indicate cool air, red = hot and yellow = warm or ambient.

The green arrows are meant to indicate that each sensor communicates with the HVAC panel. The sensors themselves are located exactly where the text is.

I forgot to label the white "T" in the center of the heater core. It's the vent where the vacuum hose from the "Tc" connects.

 

pathfinder-r50-hvac-diagram.jpg

Edited by EricCR
  • Like 1
Link to comment
Share on other sites

Interesting. Does the actuator itself look like it comes apart? I opened one of the actuators in mine just to see what was inside and it came apart pretty easily, and wasn't all that complicated inside. I'd be curious to pop the actuator open and see if there's obvious dirt or wear on the contacts and pads that report what position the door is in. If the blend door has spent most of its life in or near the one position, all the wear on the internals will be in that one spot. A faulty sensor could be making it telling it it's more open or closed than it is, so the computer is trying to correct it based on faulty data. I would also want to check that the door isn't hung up on something that's preventing it from closing all the way (making the servo push, realize it's stalling out, back off, try again). I dunno, something to check if the new HVAC control head doesn't sort it out.

 

I'm working on a neighbor's '66 C10, and two of the three climate control levers are busted off. He said he would just reach under the dash a few times a year and set it "for the season." Much easier to do on that truck!

Link to comment
Share on other sites

lol, yeah. Disconnecting the blend door wire "for the season" is my back up plan. Where I live "the season" is basically the whole year, so there's that.

 

It's hard to confirm from a distance but the actuators look like any other actuator I've seen in the past, they have plastic clips and do come apart (here is one for sale). As for how easy it is to unscrew it from where it's hanging, hard to say. The actuator is in plain sight behind and under the stereo but reaching for the screws may require other stuff to come out. When I set the AC to full hot an full cold while looking straight down at the actuator and it looks like it's moving all the way. You may be right in that the position it is reporting is wrong so I'm not ruling it out completely. As for the blend doors not fully closing, I think in order for me to see them I would need to remove the whole lower dash. Since 95% of the time I do get sufficiently cold air, I suppose they do close so I'm not going to tear the whole thing down.

 

My next plan is to test the AC while unplugging things. Unplugging both temp sensors is easy enough, same as with the blend door actuator. The sun load sensor and the fresh air/recirc actuator are out of reach but I don't need to test those. This is going to be my troubleshooting guide (from cheap to expensive-ish):

  1. Disconnect cabin air temp sensor. Test AC. If there's still hot air coming out randomly, move on. Otherwise, replace that sensor.
  2. Disconnect intake air temp sensor. Test AC. If there's still hot air coming out randomly, move on. Otherwise, replace that sensor (this one seems impossible to find).
  3. Disconnect blend door actuator. Test AC. If the problem disappears it means either the actuator or HVAC panel are bad. Buy used HVAC panel and test again (~$60).
  4. If there's still hot air with a different HVAC panel (and everything else connected), replace the blend door actuator (~$80).

I feel like #3 is the jackpot but we'll see.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...