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Discussion Starter · #1 · (Edited)
Recently purchased a clean 2011 SV that has a couple of minor issues. One of them is the behavior of the vents and the selected temperature.

Symptoms:

1. If I start the truck in the morning and let it warm up in the driveway with vent set to defrost and full heat, the vents will still be blowing cool air until I either rev the engine to 2K RPM for several seconds, or start driving. I have read that this is likely due to an air bubble in the cooling system (makes sense, the heater core is the highest point) and I plan to try to purge it tomorrow. Coolant does not appear to be low otherwise, and while running the temperature never rises above the mid mark on the gauge, so I don't suspect there is any issue with the thermostat or water pump.
2. When the truck is restarted with a hot engine, the vents will often blow maximum heat regardless of the temperature setting. Sometimes setting the temperature setting to cool, back to hot, and back to cool again slowly will work. Sometimes it doesn't, but after driving for five minutes or so, the vents will start blowing to the selected temperature.
3. Usually after 1 or 2 are cleared up, after driving for a few minutes, the temperature selector knob acts normally and the vents react in a timely manner to temperature selection changes.
4. One time I parked the truck, shut it off, and was reading my phone. About 1 minute after shutoff, I could hear what sounded like the mixer motor actuating for several seconds, then it stopped - almost like it was "parking" the mix door to a default setting of hot. I flipped the key to ON and changed the knob around, and confirmed that this was the sound I had heard, as each position change on the knob resulted in the same vent mix motor noise from inside the dash. I have NOT yet checked to see if the motor is correctly actuating in cold or hot (heat soak) conditions, and I have not heard this post-shutoff noise again since, but honestly I haven't remembered to check for it.

I have downloaded the FSM and there are several things I plan to check:

1. Unplug, clean, and reseat connector to the vent mixer motor
2. Possibly check connection to temperature control panel, as the truck does have an aftermarket radio/fascia and the previous owner may have damaged the harness
3. Perform continuity and resistance tests on the vent mixer motor and temperature control harness as recommended by the FSM
4. Possibly pull the vent mixer motor and door actuator to check for any physical blockage or seizing
5. YOUR SUGGESTION GOES HERE ::smile::

I will be spending some time in central Montana this Christmas so having the HVAC system working properly to warm up the truck before entering is very important.

Thanks!
 

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Very well written post with initial research and thought!

I can add a few very minor points:

#1 are exactly the same symptoms I had, so I'd be pretty certain that adding coolant (and purging air properly) will resolve that condition.
#4 On my truck, this parking of the blend door occurs every time (I think its about 5 minutes after ignition off). I do not know how to check if it is correctly parked.
 

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I believe that an electronic glitch on my 16 SV-VTP caused the blend door to open while the truck was in operation on the interstate. The AC simply went from putting out cold air to putting out tepid air and nothing I tried could change that. Later on that night full function returned. Left the darn truck at the shop for a month they drove it for 250 miles and never happened again...
 

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I have an 06 that would blow cold air at idle and hot at even slightly increased revs and the overflow tank showed proper levels. I added about half a gallon of coolant, ended up about 3-4 inches over the max line. Ran it a bit and the system must have purged the air in the heater core. Levels now read normal. Coolant didn?t leak anywhere as I had done the radiator bypass. Works great now. Also my AC wasn?t working, would flicker on and off. Switched out the blower motor resistor for 30 bucks and the ac and fan speeds are back to normal.
 

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Discussion Starter · #5 · (Edited)
Thanks for the responses

I was able to mess with the controls for a while today, with the truck parked and not running, so that I could hear and observe the behavior of the vent mix door motor as I changed the temperature settings. I noticed no real consistency in how the control behaves. At times the vent mix motor responds immediately as you move the temperature knob to a different position, whether that be a couple of steps, or a whole sweep of the knob from one side to the other. Then the next thing you know, you set it to a new position, and the motor stops responding.

The only thing that was consistent was that, when it did stop responding, the vent mix motor would eventually come to life and move the mix door to the desired position - but it might happen 30 seconds or so later. I didn't time the exact delay to find out. You might think that this could happen with a poor electrical connection in a moving truck, but I was sitting perfectly still, not even running the engine. And tapping the dash/knob cluster or the vent mix motor housing didn't bring either of these players back to life, either. It just happened as I was sitting motionless in the seat.

I didn't really have time to pull the vent mix motor connector (it seems the upper glove box needs to come out to get it) but it felt solid and jiggling it or lightly tugging it had no effect on the behavior.

The motor sounds normal when turning and does not sound stressed, loaded, or otherwise like it is experiencing mechanical resistance.

I have looked over the HVAC system several times in the FSM, and frankly I have no idea why Nissan needed to put a system this electrically complicated into what is otherwise a pretty simple truck. I can understand it for auto-controlled dual zones, which I don't have, but (as far as I know) there is no auto control in this particular system.
 

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Discussion Starter · #6 ·
Well I feel pretty dumb for not having checked it earlier, but I pulled the main radiator cap and found that it was low on coolant - about 1/2 gal worth. That would definitely be an air bubble. Oddly enough the reservoir showed a level just on Max so I assumed that the radiator itself was full. Never assume anything I suppose!

I did not have any of the Nissan blue coolant, so after checking online to verify that I could add regular ethylene glycol with a 50/50 distilled water mix, I went ahead and filled up the radiator, filled up the reservoir, and started the truck to let it idle in the driveway on an incline. About 5 minutes later when it had warmed up, hot air was blowing through the vents on the defrost setting, and system had good pressure - appears that the air bubble has been eliminated.

I do not know that the Nissan dealer checked or topped off the radiator when processing this vehicle as a trade-in, so while I don't believe the truck is losing coolant, it does make me suspicious. Their techs may have just made the same assumption that I did, which was that if the reservoir shows fluid, the main tank is probably full. I will have to keep an eye on it.

I pulled the transmission dipstick and it was not contaminated, though I was not expecting to see what turns out to be the mostly-clear Matic J fluid. That's a first for me.

However the issue of the delayed vent mix door actuation still remains.
 
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