Defrost temp warmer than any other position

[C-ya]

I noticed this when I first got the truck, but it hasn't been enough of a bother to worry about. But since we are all about sharing things here, I thought I'd throw this out and see what the group thinks.

When using the heat (still a little chilly here) with the temp set at 70 and any outlet setting other than defrost, it is nice and comfy. Should the need arise to hit the defrost button, the temp immediately rises coming out of the vent even though I haven't touched the dial to adjust it. I have to turn the temp down to 62 for it to be bearable. When I switch back to any other mode, I have to change the temp back to 70 or it is too cold/cool.

Thoughts? This is the auto HVAC system with dual temp controls.

Hmm, next up will be to see if the rear vents get hotter like the front or if it is limited to the front.

 

[The_Roadie]

There are like seven air plenum temp sensors in the system. Only a Tech II tool can get at the detailed readings for each of the sensors to see what the control module is basing its decisions on.

The defrost part of the plenum is the worst to get to, for sure. Sounds like paying for a Tech II diagnosis is in order if it annoys you enough to get troubleshot, then fix yourself.

Or it could be a flaky control module - got any local buddies to away with?

 

[AtlWrk]

I've actually been reading a lot about the Auto HVAC operation today for a planned conversion in the near future. I would guess (and it's only that: a guess) that the control module is intentionally kicking up the temperature for more effective defrosting. According to AllDataDIY, defrost mode will automatically shut off the auxiliary HVAC so also overriding the temp doesn't seem unreasonable. There are 4 plenum temperature sensors: dash left, floor left, dash right, floor right. With no defrost plenum sensor the control unit may have to make an educated guess for how to set the temp actuator.

Others with the auto system should be able to confirm if this is normal behavior or indicative of a problem.

 

[Mooseman]

I have the auto HVAC and when I put on defrost only, right away the fan starts blowing full speed no matter how warm the engine is or how cold the temperature but I've never noticed it getting overly warm. I rarely use it, opting for the heater/defrost mode which remains fully automatic. And it also spreads the heat more evenly.

You also have to remember that when using defrost or heater/defrost mode, the A/C compressor will run to remove moisture if the temperature outside is above freezing so if you don't really need any defrosting at all, you should keep it on heater only as you can't cancel the A/C while on defrost.

One oddity I have found that no matter what temp setting or mode I use, except defrost, it never puts the fan on full speed when on auto. I have to set the fan speed manually if I want full speed for quicker warm up or cool down.

 

[IllogicTC]

Auto appears to do that with the fan, yes. On full-up or full-down, it generally goes one notch below full but recently I've had it pick two below full. Obviously this is part of its algorithm for deciding how to best condition the cabin.

A/C has no cancellation in defrost, HOWEVER it can be cancelled in Auto. You can turn the thing down to 60 F, and obviously you'll get A/C air. hit the snowflake if it's not on. Hit the snowflake again and it appears to stop giving A/C air.

 

[C-ya]

I tried it in mix mode (def/floor) and the temp increased as it does in defrost only. Then I turned on the rear heat, with everything on vent which means the rear is coming from the roof vents. When I hit defrost, the rear changed to the floor, but the temp did not increase like the front did.

It's annoying enough that I notice it since I have had trouble breathing when the air in an enclosed space is very warm since I was kid with asthma. Being the driver (or the passenger for whatever reason which doesn't happen very often!), since it is my truck, I reach over and adjust the temp. If my wife was driving by herself, she would probably never notice since she drives her car that way - full hot or full cold. I can't get her to use a midpoint setting for anything. When I drive the car, I'll pick a point that is comfy and leave it there. Next time I drive it after her, it's back at the top or the bottom, full hot or full cold. She has a manual dual climate system in her car. At least both levers are at the same end.

 

[IllogicTC]

Well, guess what, gentlemen? I'm feeling quite curious now. I'll take my thermocouple with me and see if I can get readings at the different vents.

 

[C-ya]

Good idea. I can do the same as we have a couple of thermocouple units here at work.

 

[IllogicTC]

Ambient temperature: 76 fahrenheit (verified via dash display and thermocouple)
Engine coolant temperature: 201-203 fahrenheit, as reported via OBD-II.

HVAC settings:
Highest fan setting
Highest temperature setting (on both zones)

Floor: ~110 fahrenheit
Vent: ~140 fahrenheit (!)
Defrost: ~125 fahrenheit.

Measurements taken at the opening of the vent, literally in the midst of the grille. The thermocouple was not allowed to touch the plastic or any other thing, and was held for at least 60 seconds to allow temperature to stabilize.