Mechanics

AUTO TEMPERATURE CONTROL MALFUNCTION

1992 Lexus SC 300

Heater problem
1992 Lexus SC 300 6 cyl Two Wheel Drive Automatic 186000 miles

My aircondition will work and turnoff as it should but the heater will not. In fact the heater and fan will not turn on at all. Is this the heater relay or air conditionor relay? I know the location of the heater relay but where is the location of the air conditioning relay?
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Dagon Garris
December 24, 2009.




Hi Dagon Garris,

Thank you for the donation.

Your question is a little confusing so I need to clarify this.

When AC is turned ON, is the blower fan working? Ie. Are there any air flow from the vents?

KHLow2008
Dec 25, 2009.
Yes the blower works fine in both the manual and automatic mode. In the climate control mode and when it become too warm for the temperature set, the air conditioner and blower works. But when it become to cool/cold the blower shuts off and no aiir, heat or blower run. If you put it on manual at any temperature, only cold air blows through the upper or lower vents unless you turn the airconditioner swicth off. Hopefully this helps.
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Tiny
Dagon Garris
Dec 25, 2009.
This does not seem to be any simple malfunction. See if you can retrieve any trouble codes.

RETRIEVING CODES

Indicator Check
1. Press and hold AUTO and recirculated air switches. See Fig. 2 . Turn ignition on. All indicators will flash 4 times at one-second intervals. Tone will sound when indicators flash.

2. A/C system will automatically enter diagnostic code check mode. To end indicator check, press and release OFF switch. Read codes at temperature display. See Fig. 2 . If a normal code (Code 00) is displayed but problem persists, go to symptom chart. Trouble shoot symptoms in numeric order from lowest to highest.

3. If trouble code is displayed, proceed to appropriate trouble shooting procedure. See DIAGNOSTIC CODE IDENTIFICATION table. Codes are displayed in ascending order. To slow rate at which codes are displayed, press fresh air button to change display to step operation. Each time fresh air button is pressed, display changes by one step.

4. If tone sounds as code is displayed, problem currently exists. If tone does not sound as code is displayed, problem occurred in past and does not presently exist. If ambient temperature is -4 °F (-20 °C) or less, code may be displayed even if system is functioning normally.


http://www.2carpros.com/forum/automotive_pictures/192750_HAVCdtc92LexusSC300_1.jpg




http://www.2carpros.com/forum/automotive_pictures/192750_ACControl92LexusSC300Fig02_1.jpg




KHLow2008
Dec 26, 2009.
Based on your testing guide, two codes came up. They are: 42 and 21. Only 21 beeps and therefore it this is what is causing the problem. Where will I find the Solar Sensor Circuit to replace it?

Tiny
Dagon Garris
Dec 28, 2009.
The solar sensor is a dealer part and can only be purchased from them. Alternatively you can try the junkyard to see if they have it. Before replacement of the sensor, testing should be carried out to determine if it is the sensor or other faults.

A control panel fault can also cause the code to be stored.

Do you have a DVOM and can you perform resistance tests?

KHLow2008
Dec 28, 2009.
Hopefully you didn't shell anythjing out for the solar sensor before being made aware of the FM note stating that the solar sensor may throw a code if the test is done in a dark area. You can simply shine a light on it to eliminate it as a candidate (no code if normal). I'm wondering if the Online Mitchel manuals contain these important notes?

Tiny
RocketmAn
Apr 4, 2012.
Thanks for the input, this is something that most would miss out.

KHLow2008
Apr 8, 2012.

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