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Old 06-22-2017, 11:31 PM   #1
tibra396
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Default L200 Automatic HVAC

We have a heatwave going here in California. When it hit about 99F my daughters 2003 L200 with automatic HVAC would only blow through the defrost and heater vents so we never felt cool. I was afraid the mode control motor had failed which is deep in the dash. The odd part was once it cooled down to 90F or so it worked great again.

I did some research to figure out what part I needed and came across the fact the system uses a solar sensor to determine which side of the car the sun is hitting. I covered the sensor and the AC worked great again. The permanent fix involved using some black tape. Much easier then tearing into the dash.

This episode reminds me that the best HVAC control involves three nobs for temp, fan, and mode. I do not need a computer to control my comfort.

I hope this helps someone in the future.

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Old 06-23-2017, 04:00 AM   #2
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Default Re: L200 Automatic HVAC

I totally agree with you on having just 3 knobs. My wife on the other hand has an 03 with the automatic controls and loves it. If I remember correctly even tho the 03 has the automatic hvac the system can still be controlled manually. The mode button cycles thru vent settings and there are 2 dials, one controls temp and one fan speed. When I drive her car I have the heat or ac blow on my feet and keep one or two windows open.

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Old 06-23-2017, 10:05 AM   #3
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Default Re: L200 Automatic HVAC

Some food for thought as the solar cell is described in service manuals as a dual use sensor. For the hvac auto climate control system;
Ambient Light Sensor
The ambient light sensor is a 2-wire photo diode. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor conductance increases. The sensor signal decreases as the conductance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts.

The ambient light sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted solar load value. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. A resistance check of the ambient light sensor should not be performed as it will damage the sensor.


The daytime running lights use the ambient light sensor; Daytime Running Lamps (DRL)

The ambient light sensor is a light sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes to the outside (ambient) light level. When the BCM receives this signal it will either turn on the daytime running lights (DRL) or the headlights. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. With the headlight switch in the OFF position, the headlights will either be turned ON or OFF, after an approximate 30 second delay depending on whether daylight or low light conditions are sensed.


Here's where it gets interesting (only for auto climate control systems) and confusing; Purge Operation
If the ambient air temperature is less than 2C (35F) and the coolant temperature is less than 10C (50F), then a 75-second purge will occur. The purge will begin as soon as the coolant temperature reaches 4C (40F). The mode door will move to the defrost position and the blower motor will be near half speed.
This purge mode only applies to hvac systems having auto climate controls where the AUTO button is pressed, temperature is dialed to desired comfort and the doors are automatically controlled to distribute cooled air. When I use AUTO mode, I notice the display missing the floor and face symbols, implying air is directed somewhere in between with less air flow felt at floor and face vents. I usually press the mode button and select face and floor with distinctive air flow coming thru.

What I find confusing is the PURGE MODE. It seems to describe what may be occurring as a possible culprit yet opposite to what was found interfering with ac use. My guess is the sun load/ambient light sensor is faulty and replacing it may correct the ac determined to blow cold air thru the defrost vents. Since its a dual function sensor to help determine when daytime running lights are needed or switch (automatically) to night time headlights, blocking it with black tape may trigger night time driving lights to turn on (headlights, tail lights and instrument panel lights). I notice my DRLs switch automatically to full night lighting when driving thru overpasses in slow traffic or in tunnels, switch back to DRL mode when reappearing in daylight.

I too was comfortable with a semi automated ac system (sports car) with the temperature selector adjusting from zero heat to any heat desired when ac was on to regulate interior comfort without computer control. I fiddle with my L300 auto climate control almost all the time, perhaps not counting on its 'automated' feature when its not fulfilling my needs to direct air where I want it to.

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Old 06-24-2017, 12:32 PM   #4
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Default Re: L200 Automatic HVAC

I do not know if this new problem is related to my black tape work around but now sometimes, seemingly randomly the high beams come on when the car is off. It has never drained the battery so they must not be on that long but the neighbors have called a couple of times. It is not the DRLs that come on which I would have expected. Note: on these occasions the parking break is not on.

I am going to guess the solar sensor is somehow failed and the software is mis interpreting it and causing the gremlins. I don't want to take it to a Chevy dealer for a SW update that they may or may not have. I am going to change the solar sensor and see what happens. It will take me a while to get around to it, when I do I will report back.

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Old 06-24-2017, 01:38 PM   #5
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Default Re: L200 Automatic HVAC

Access to the light sensor is by prying off the plastic dome cover and pulling out the sensor to disconnect it for replacement. Hopefully a replacement (or broken wire) solves the problem.

According to service manual info and personal experience, our daytime running lights use low beams so its a mystery how your high beams turn on. This shouldn't occur from taping off the light sensor but stranger things occur that defy explanation service manuals don't cover.

In the meantime, can windows be left cracked open to let hot air out? Maybe this can alleviate the 'hothouse' syndrome of an errant ac system refusing to allow manual control of airflow? Maybe aluminum foil over the sensor?

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Old 07-16-2017, 04:23 PM   #6
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Default Re: L200 Automatic HVAC

Another update. The HVAC & headlights are separate problems. The headlight problem is due to a failing high beam switch, which also has a flash to pass function. Even when the car is off the high beam comes on when you pull on the switch. Normally it won't stay on but sometimes on this one it will fail to pop back off when released. I will need to get in there and swap out the switch. I figured this out when going over some bumps the DRL light flashed off indicating the headlights were on momentarily.

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Old 07-17-2017, 12:18 AM   #7
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Default Re: L200 Automatic HVAC

^Good work, tibra396!! Please keep us informed of your progress.

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Gov't's grown bigger, but a chance exists that it will be reduced. I'm cautiously hopeful.

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Old 08-02-2017, 11:24 PM   #8
tibra396
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Default Re: L200 Automatic HVAC

Final update: The solar sensor was a wild goose. I chased it because of the random nature of the failure seemingly related to how bright and hot it was. I decided to go straight to the source and dig deep into the dash to look at the mode flap actuator. It was not that hard to get out. I had to pull the glove box and a few other brackets.

I opened the actuator to find a cracked drive gear that sometimes moved the flap, but not always. I wanted to get it fixed fast so I went to Pick & Pull to find a used one. Had to pull apart 3 Saturns to find one not cracked. There is a difference between an auto HVAC car and a manual one in the encoder that senses position. Manual cars use a potentiometer. But the gears are the same so I swapped it out. Seems to work now. This fix is very satisfying!

Replaced the headlight switch too, life is good now.

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Old 08-03-2017, 05:43 AM   #9
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Default Re: L200 Automatic HVAC

Good to hear you got it fixed. Thanks for the updates. Very often we never hear back on what the final outcome was. That is how we all learn.

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