1991 S4 Auto temperature not functioning
#1
Instructor
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On my 1991 S4, my automatic temperature control has stopped working and has reverted to all hot or all cool. This condition started a few das ago - I first noticed it was not maintaining temperature correctly in the morning (slight heat on), and going too hot in the cabin. Now it's all or nothing.
I have verified that the heater control valve is working correctly, doesn't leak when closed, and closes when a vacuum is put to the valve.
It closes when the temperature slider is moved all the way to the left - any other position it opens fully.
The AC compressor comes on as normal when the switch is selected. If the slider is all the way left I get cold air, anything else the heater overpowers the AC.
I inspected the outside temperature sensor and all the wiring looks OK. I disconnected the plug and used contact cleaner on the male and female connectors, then replaced it.
I removed the side kick panel on the drivers side to measure the resistance on the setting motor terminals (using Wally's troubleshooting guide), but I must be dumb, I can't see the setting motor plug in connections.
Can the outside temperature sensor fail and can it be tested in place?
Thanks for the help.
I have verified that the heater control valve is working correctly, doesn't leak when closed, and closes when a vacuum is put to the valve.
It closes when the temperature slider is moved all the way to the left - any other position it opens fully.
The AC compressor comes on as normal when the switch is selected. If the slider is all the way left I get cold air, anything else the heater overpowers the AC.
I inspected the outside temperature sensor and all the wiring looks OK. I disconnected the plug and used contact cleaner on the male and female connectors, then replaced it.
I removed the side kick panel on the drivers side to measure the resistance on the setting motor terminals (using Wally's troubleshooting guide), but I must be dumb, I can't see the setting motor plug in connections.
Can the outside temperature sensor fail and can it be tested in place?
Thanks for the help.
#2
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The all-or-nothing action is often due to a problem in the temp sensing string.
Per the wiring diagram on Page 97-377:
A yellow wire runs for Terminal 4 on the "CONTROL UNIT AIR CONDITIONING SYSTEM" thru Terminal 3 on 4-pole Connector T29 to the connector for the OUTSIDE TEMP SENSOR in the left front fender. The yellow wire then runs from the sensor back thru Terminal 4 in the connector, thru Terminal 2 in 4-pole connector T31 to the INSIDE TEMP SENSOR , behind the lettle grille on the dash. From the sensor, the yellow wire returns thru Terminal 1 of T31, and back to Terminal 14 of the CONTROL SWITCH.
You might start by checking continuity thru the string from Terminal 4 to Terminal 14. If the string is open, see if you can find the break.
Per the wiring diagram on Page 97-377:
A yellow wire runs for Terminal 4 on the "CONTROL UNIT AIR CONDITIONING SYSTEM" thru Terminal 3 on 4-pole Connector T29 to the connector for the OUTSIDE TEMP SENSOR in the left front fender. The yellow wire then runs from the sensor back thru Terminal 4 in the connector, thru Terminal 2 in 4-pole connector T31 to the INSIDE TEMP SENSOR , behind the lettle grille on the dash. From the sensor, the yellow wire returns thru Terminal 1 of T31, and back to Terminal 14 of the CONTROL SWITCH.
You might start by checking continuity thru the string from Terminal 4 to Terminal 14. If the string is open, see if you can find the break.
#3
Team Owner