A/C System - Performance Test

REF. NO. MBNA 83/30

TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS

DATE: July 1991

SUBJECT:
A/C PERFORMANCE TEST
MODEL 107 STARTING MODEL YEAR 1982
MODELS 123 AND 126 (EXCEPT .024/025) STARTING MODEL YEAR 1981 MODELS 124.026/030 STARTING MODEL YEAR 1986
MODEL 201 (EXCEPT .029) STARTING MODEL YEAR 1984

The A/C Performance Test described below supersedes the test described in the Education for Excellence manual and also in the service microfiche.

For further information regarding working on the A/C system, please refer to S.I. MBNA 83/25, October 1987.

Test Conditions

1. Do not allow vehicle to sit in direct sunlight prior to and during test.

2. Check condition of compressor V-belt.

3. Check function of auxiliary fan by turning ignition key to "ON" and bridge both pins on the refrigerant high pressure switch, auxiliary fan (S32). The fan should engage in low fan speed.

Effective Serial Number

3.1. Check function of auxiliary fan in high speed. Depending on model and model year, bridge switch (see table shown) with ignition "ON". Fan should engage in high fan speed. Also observe for correct direction of rotation of fan (clockwise as viewed from front of vehicle).

4. Check fluid level of receiver-drier.

4.1. Disconnect one wire from refrigerant pressure switch (S31/1).

4.2. Run engine at idle and engage A/C system at push-button control panel.

4.3. Observe sight glass and reattach wire to pressure switch (S31/1). Shortly after compressor clutch switches on, the fluid level must rise and the refrigerant should flow past the sight glass, free of bubbles (the refrigerant will not be visible). If bubbles are visible, check system for leaks.

5. Measure and record shop temperature six feet from driver's door (ambient temperature).

5.1. Determine if relative humidity is closer to 40% (normal) or 90% (hot, humid or raining).

6. MODEL 201 ONLY: Pull connector from S31 and connect to ground so that auxiliary fan is running during entire test.

7. Remove cap from service valve on suction and discharge line. Connect pressure gauge to service valves. Assure that the hose connection fittings are equipped with a pressure pin in the center of the fitting. Close engine hood against rear seal.

8. Open center and left/right air outlets. On model 126 vehicles, close the side air outlets and open air outlet in rear passenger compartment console.

9. Place thermometer in center air outlet.

10. Open all windows and close all doors.

The following test is designed to confirm the operating efficiency of the air conditioning system. Be certain to have performed all the necessary test conditions before actually performing the test.

Compare the temperature and pressure values obtained from the test to the values listed in the charts. Find the measured shop temperature (ambient temperature) in the left most column and read the values across, according to the approximate relative humidity (40% or 90%).

Possible causes for differences in values

Pressure
^ Suction pressure too low, discharge pressure too high: expansion valve defective, fluid reservoir, condenser or evaporator clogged.

^ Suction pressure too high, discharge pressure too low: expansion valve defective.

^ Suction pressure and discharge pressure identical: refrigerant compressor defective.

Temperature
^ Outlet temperature too low or high: temperature sensor evaporator faulty, supply cables are shorted to ground or control unit defective.