A/C - ACC Servo Unit Testing
83mercedes03TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS
DATE: September, 1983 REF. NO. 83/37
SUBJECT: TESTING OF AUTOMATIC CLIMATE CONTROL (ACC) SERVO UNIT
MODELS 107,116,123
Complaints that the ACC system is not functioning properly (temperature setting can not be obtained) may now be investigated using a simplified test program and special test harness.. By measuring the resistance changes of the feedback potentiometer with an ohmmeter when the servo motor is operating, the function of the servo unit itself can be determined. It is expected that with the use of this test harness (previously by using the ACC tester) the servo will be verified as either faulty or functional and help eliminate unnecessary servo unit replacements.
FIGURE 1 - ACC SERVO UNIT:
Preliminary Information
Requirements for a properly functioning ACC system are a clean heater core and a fully charged air conditioning system.
If necessary, the heat output (Refer to S.I. 83/8, Part B) and the A/C output (Refer to applicable microfiche, Group 83) should be checked.
FIGURE 2 - SERVO TEST HARNESS:
A. Servo Isolation Test
1. Remove 5-pole plug from servo unit and connect test harness plug (Fig. 2) to servo.
NOTE: Notch in plug must be matched with tab in servo.
Figure 3 - "Max. Heat" test connection:
Connect red and blue clamps to battery and ohmmeter to white test leads as shown in Figure 3.
2. With red clamp connected to positive (+) and blue clamp to negative (-) terminal of battery, resistance value should already indicate Max Heat or servo should audibly move to Max Heat (100-400 ohms) within 3 minutes.
NOTE: The special test harness contains a resistor and diodes to supply a constant approx. 1.5 volts. Full battery voltage should never be applied.
Figure 4 - "Max. Cool" test connection:
3. Change polarity of harness connections to Blue (+), Red (-) (Figure 4). Servo motor should run in reverse and resistance should increase to Max Cool (1600-2000 ohms) within 3 minutes.
HINT: Harness Hook-Up
(+) Red clamp to positive = Max Heat (+) Blue clamp to positive = Max Cool
NOTE: If either of the tolerance limits are not met, the servo unit is defective and requires replacement. After replacement, verify that the ACC system functions correctly.
If measurements fall within tolerance limits SERVO UNIT IS FUNCTIONAL and testing must be continued.
B. Systems Isolation Test
4. Turn ignition key on again and press "AUTO-LO".
Figure 5 - Ambient air temperature sensor:
5. Remove plug (8) from ambient air temperature sensor at firewall (Figure 5). Wait 3 minutes.
FIGURE 6:
6. Remove 5-pole plug from servo unit and connect test harness plug to servo and ohmmeter to white test leads as shown in Figure 6.
NOTE: Do not connect red and blue clamps.
7. Resistance value should indicate Max Heat (100-400 ohms).
8. Remove test harness plug and reconnect 5-pole servo plug.
FIGURE 7 - AMBIENT AIR TEMPERATURE SENSOR:
9. Bridge two terminals of ambient temperature plug (8) removed in step 5 (Figure 7).
10. Change temperature setting to maximum cooling (65~F).
NOTE: Servo motor should now be audibly turning and stop at Max Cool within 3 minutes. Remove 5-pole plug and connect test harness plug to servo and ohmmeter to white leads without connecting red and blue clamps as shown in Figure 6. Correct resistance value is 1600-2000 ohms.
FIGURE 8:
11. If measurements do not fall within resistance value limits, a part of the ACC system (excluding the already tested servo) IS FAULTY. The problem may be in the sensor chain, pushbutton switch diode (Figure 8, arrow), amplifier, or due to poor contacts (corrosion) of electrical connections at the servo or fuses.
Perform Function Test with tester beginning with Job 83-602. Note the following possibility for amplifier output depending on the supplier:
AMPLIFIER TEST 83-606 (Compressor Switch "ON")
Time Allowance
83-S37-01 Servo isolation test A only 0.3 hour
83-S37-02 Servo and system isolation test A and B 0.5 hour
Tool Information
Dealer Net
Tool Name Tool Number Group/Category Price*
ACC Servo Test Harness 916 589 00 63 00 83/B 21.60
*Price subject to change.