Woozy

Hey Folks,

I did a search on this and really did not find an answer. When I select Auto on my HVAC in my 1991 LS400 the air coming out will vary from cold to warm coming out of the floor vents. I thought I had an air pocket in my system, but when i checked that out it seemed fine. I always wondered why the A/C button was on when it was trying to heat the car in Auto Mode. So I turned it off, which took it out of Auto mode, and then i got constant Warm/hot air from the floor vent, but of course from then on I had to adjust fan and air direction manually. My question is, when in Auto mode should the A/C button go out when it is trying to heat the car, and if not why do I get cold and warm air alternately from the floor vents when trying to heat the car? If it is supposed to go out why dosent it?

Thanks

FireEMT978

The "Auto" mode "knows" where the heat should be put. When the car is started and it is cold outside, you will notice that the a/c mode is on. The reason the a/c light is on for the first short while is to defrost the windshield, which the a/c compressor actually is on whenever in defrost mode. After a few minutes of the engine warming up, as it gets hotter, the fan will change to higher speeds incrementally, the a/c light will shut off, and it will direct the heat to the floor, because heat rises. As the cabin temperature rises to the setpoint, then it will switch to the vents and blow cooler air out of the vents as needed to keep the temperature at the proper setting, because cool air falls. So, sorry to be long winded , but it places the air where it is most effective.

Woozy

Thats not how mine works, the A/C light never turns off, and the air from the floor vents is quite cold for a bit, then gets only warm, it never gets hot as it should when its below freezing outside, and the A/C light is always on. If I turn it off manually, the air from the flor vents gets nice and hot and seems to work fine, never getting cold, but its not automic, based on the temp selected.. i have to adjust the fan and direction manually...

FireEMT978

Sorry, I misunderstood.
I wonder if a "blend" door is sticking, or if a duct temp/mixed air temp sensor is not reading properly. I actually am a residential/commercial HVAC tech.

KMinAF

Double check to make sure that there isn't an air bubble in the system as this will cause an intermittent or cool air temp. There is a specific procedure for bleeding the system that if not followed could exacerbate the problem.

Woozy

I tried to find the procedure to bleed the system and had no luck, can anyone provide a link?