I have 1998 Cadillac sts. Presently te control panel reads the outside temperature of 10 deg. C whcih is not correct. I had recently cnaged the engine so I want ot check if the temperature sensor is connected or not. Please advise me where the Outside temperature sensor is located.

You may want to have it check for DTC'S diagnostic trouble codes . A code may point to the problem , these HVAC systems are very complex .
DTC B0248, B0268, B0408, B0418, B0428, or B3531
The HVAC door actuator is an electronic motor with a feedback potentiometer. The HVAC control module supplies power and ground to the actuator. The HVAC control module controls the direction of the actuator by changing the polarity of the control circuit. When the actuator reaches the desired position, voltage is removed from the control circuit. The HVAC control module determines the current position of the actuator by monitoring the feedback voltage of the door position signal circuit. The feedback potentiometer position is a function of the actuator position.
• Air flow control 3 circuit refers to the front mode actuator
• Air flow control 7 circuit refers to the recirc actuator.
• Temperature control 1 circuit refers to the driver air temperature actuator
• Temperature control 2 circuit refers to the passenger air temperature actuator
• Temperature control 3 circuit refers to the rear air temperature actuator
• Air flow control 10 circuit refers to the rear mode actuator
DTC Descriptors
This diagnostic procedure supports the following DTCs:
• DTC B0248 Air Flow Control 3 Circuit
• DTC B0268 Air Flow Control 7 Circuit
• DTC B0408 Temperature Control 1 Circuit
• DTC B0418 Temperature Control 2 Circuit
• DTC B0428 Temperature Control 3 Circuit
• DTC B3531 Air Flow Control 10 Circuit

Your having air flow issues and temp. control !
DTC B0413, B0423, B0433, B3761, B3773, or B3782
This diagnostic procedure supports the following DTCs:
• DTC B0413 Temperature Control 1 Feedback Circuit
• DTC B0423 Temperature Control 2 Feedback Circuit
• DTC B0433 Temperature Control 3 Feedback Circuit
• DTC B3761 Air Flow Control 3 Feedback Circuit
• DTC B3773 Air Flow Control 7 Feedback Circuit
• DTC B3782 Air Flow Control 10 Feedback Circuit

Do you know what a scan tool is ?
Install a scan tool.
Turn ON the ignition, with the engine OFF.
With a scan tool, command the appropriate actuator in both directions.
Does the actual parameter of the appropriate actuator change?

Here's a link on your brakes. ABS Light Due to ABS Control Module Failure Cadillac Seville STS Common...

many cars automatically kick on the air compressor in defrost, to dehumidify the cabin air

most likely a bad temp switch

depending on the outside temperature , my 92 el dorado temperature operates in the range of 195 deg F to 207 deg F with a 50 / 50 coolant mix

go to a repair shop and pay them 10 bucks to print it out for you. most shops l will do this for you

YOU HAVE ADD THE WRONG FREON ALL CARS MADE AFTER 1996 USES R/134. FLUSH THE SYSTEM, ALSO CHECK THE BLOWER FAN FOR DEBRIS

I would guess the outside thermometer has lost its calibration.
Does it seem consistently 5, 10 or 20 degrees off all the time?
Let me know.
Regards,

When you change the heat system to read deg C, the outside air temp will also read deg C. To change between F & C press and hold the recirc button on the heat control and then press the + on the drivers side.

The temperature blend door on the driver's side might be broken or stuck. Also, if this vehicle does not have the correct amount of refrigerant, it will do what you are describing.