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ACC servo modification

Started by 1980sdga, 13 September 2011, 01:08 PM


I have a new servo on the way but while waiting I've been working on my old ones with leaky/bad water valves and cracked housings.  I took one and cut the "tank" and water valve part of the servo off.

So far it works fine for switching the air flaps and controlling the blower speeds.  Just no water control. I'm working on that one...

I found some info on calibrating the servo but it's barely readable so I transcribed the part about servo calibration:

The electrical connection pins are numbered 1 to 10 from left to right looking at the "connection" side of the servo.

1  Connect ohm meter set at 2000 ohms to pins 1 and 2 (Feedback Pot). Meter should read 1400 ohms +/- 50 if the servo was in PARK            when disconnected.

2  Connect 12 volts positive to pin 4, negative to pin 5. The servo should run smoothly to the HOT pisition. The resistance of the feedback pot should decrease slowly and stop between 200 and 300 ohms.

3  Reverse power. Connect 12 volts positive to pin 5, negative to pin 4. The servo should run smoothly to the COLD position. The resistance of the feedback pot should increase slowly and stop between 1800 and 2100 ohms.

4  If all is OK connect 12 volt positive to pin 3 negative to pin 5. Servo should move partway toward HOT and stop in the PARK position. At this point the feedback pot resistance should read 1400 ohms +/- 50.

5  If the servo appears to operate properly and the feedback pot has no "open spots", servo is probably OK.  If the feedback pot does not read 1400 ohms +/- 50 at PARK from COLD loosen the pot retaining screws and adjust the pot position to read 1400 ohms +/- 50 at PARK coming from COLD.

At no time should the feedback pot resistance read "infinity", open, or drift up or down. If it does the feedback pot is dirty and needs to be cleaned.

6  Repeat steps 2,3 and 4 above until the feedback pot resistance always reads 1400 ohms +/- 50 when it stops in the PARK position.


Ambient Air Sensor- Should measure between 200 and 400 ohms depending on temp. (170 ohms at 104 F.  415 ohms at 64 F)  This can be checked at the MALE pins 2 and 3 at the test connector.

In-Car Temperature Sensor- Should measure between 1000 ohms and 2300 ohms depending on temp. (700 ohms at 105 F. 2500 ohms at 64 F.) This can be checked at the MALE pins 4 and 5 at the test connector.

Check of thumbwheel control-

1 It should read 900 ohms with the wheel on "75". If not insert a thin pointed knife blade along the RIGHT SIDE of the wheel. A light is handy for seeing the gears.

2 Position the pot at 900 ohms and hold it with the knife while rotating the wheel to center "75". The wheel should be centered at "75" with the pot at 900 ohms.  Make sure there is no play between the wheel and the potentiometer gear. If so dissasemble the panel and tighten the wheel-pot shaft.

I've been going over these test procedures with my servo and I'll move on to the sensors and check them.