1991 S4 Auto temperature not functioning
06-14-2015, 11:32 AM
#1
Instructor
Thread Starter
1991 S4 Auto temperature not functioning
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.
06-14-2015, 01:29 PM
#2
Rennlist Member
Rennlist Site Sponsor
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.
06-14-2015, 01:38 PM
#3
Team Owner
