MERCEDES-BENZ ML350 1997 Complete Repair Manual

TESTS.
Normal Ventilation In Regulating Mode
To check normal ventilation in regulating mode, set temperature selector knob to White range (vertical). Set
mode control knob to 4-o'clock position. Ensure A/C button indicator is illuminated. Air will vent from upper
and lower outlets and A/C compressor will engage. Ambient air will flow from center outlet and coolant
circulation pump will operate. If operation is not as specified, perform the following in order:
Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.
Check A/C pressure switch. See A/C REFRIGERANT PRESSURE SWITCH under ELECTRICAL
TESTS.
Economy Setting Not In Heating Mode
To check economy setting without heating mode activated, set temperature selector knob to Blue range. Set
mode control knob to the 9-o'clock position. Ensure A/C button indicator is not illuminated. Ambient air will
flow from center outlet and A/C compressor will NOT be engaged. If operation is not as specified, perform the
following:
Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.
Economy Setting In Heating Mode
To check economy setting with heating mode activated, set temperature selector knob to Red range. Set mode
control knob to the 4-o'clock position. Ensure A/C button indicator is not illuminated. Heated air will flow from
lower, upper and center outlets and A/C compressor will NOT be engaged. If operation is not as specified,
perform the following:
Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.
Recirculation Mode
To check recirculation mode, press fresh/recirculation button. Ensure fresh/recirculation indicator is
illuminated. Set blower speed knob to position No. 4. Blower fan noise will increase noticeably. If operation is
not as specified, check wiring between recirculated air flap element, heater-A/C switch and No. 16 fuse (15-
amp).
ELECTRICAL TESTS
Voltage Supply Circuit Z50/2
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Turn ignition on, with engine off. Connect voltmeter positive lead to socket box terminal
No. 7 and negative lead to terminal No. 9.
2. Voltage should be 11-14 volts. If voltage is not as specified, check voltage supply circuit Z50/2 between
No. 16 fuse (15-amp) and connected components. See WIRING DIAGRAMS
. Repair as necessary.

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Ground Circuit Z50/4
1. Connect socket box to A/C push button control unit wiring harness connector. See CONNECTING
TEST EQUIPMENT . Turn ignition off. Ensure A/C push button control unit is not connected to socket
box.
2. Connect ohmmeter positive lead to socket box terminal No. 9 and negative lead to body ground.
Resistance should be zero. If resistance is not as specified, check control unit ground circuit Z50/4. See
WIRING DIAGRAMS
. Repair as necessary.
In-Vehicle Temperature Sensor
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 12 and negative lead to
terminal No. 9.
2. Turn ignition on. Turn temperature control knob to Red range detent. Voltage should be 1.9 volts at 68°F
(20°C). If voltage is as specified, go to next step. If voltage is not as specified, check in-vehicle
temperature sensor circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay,
replace in-vehicle temperature sensor.
3. Turn ignition off. Disconnect test cable from A/C push button control unit. Connect ohmmeter positive
lead to socket box terminal No. 2 and negative lead to terminal No. 12. Measure temperature sensor
resistance at specified temperatures. See IN
-VEHICLE TEMPERATURE SENSOR RESISTANCE
table.
4. If resistance is not as specified, check wiring harness and connectors between A/C push button control
unit, in-vehicle temperature sensor, blend air flap actuator position sensor and temperature regulator
switch. Repair as necessary.
IN-VEHICLE TEMPERATURE SENSOR RESISTANCE
Icing Protection Temperature Sensor
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 11 and negative lead to
terminal No. 9.
2. Turn ignition on. Voltage should be 2.0-2.4 volts at 32°F (0°C) and 1.4-1.8 volts at 59°F (15°C). If
voltage is as specified, go to next step. If voltage is not as specified, check icing protection temperature
sensor circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay, replace icing
protection temperature sensor.
3. Turn ignition off. Disconnect test cable from A/C push button control unit. Connect ohmmeter positive
lead to socket box terminal No. 3 and negative lead to terminal No. 11. Measure temperature sensor
resistance at specified temperatures. See ICING PROTECTION TEMPERATURE SENSOR
RESISTANCE table.
Sensor Temperature - °F (°C)Ohms
68 (20)2100
77 (25)1700
104 (40)900

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4. If resistance is not as specified, check wiring harness between temperature sensor and control unit. Repair
as necessary. If wiring harness is okay, replace icing protection temperature sensor.
ICING PROTECTION TEMPERATURE SENSOR RESISTANCE
Activation Voltage Temperature
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 2 and negative lead to
terminal No. 9.
2. Turn ignition on. Turn temperature control knob to Blue range detent. Voltage should be more than 2.0
volts. Turn temperature control knob from Blue range detent to Red range detent. Voltage should be more
than 3.0 volts.
3. If voltage is as specified, go to next step. If voltage is not as specified, check recirculated air switch
circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay, replace recirculated air
switch.
4. Connect voltmeter positive lead to socket box terminal No. 5 and negative lead to terminal No. 9. Turn
ignition on. Turn temperature control knob to Blue range detent. Voltage should be more than 3.0 volts.
Turn temperature control knob from Blue range detent to Red range detent. Voltage should be more than
4.0 volts.
5. If voltage is not as specified, check recirculated air switch circuits. See WIRING DIAGRAMS
. Repair
as necessary. If circuits are okay, replace recirculated air switch.
Blend Air Flap Actuator Motor
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 1 and negative lead to
terminal No. 9.
2. Turn ignition on. Turn temperature control knob to Blue range detent. Voltage should be less than one
volt. Turn temperature control knob from Blue range detent to Red range detent. Voltage should be 11.0-
14.0 volts.
3. If voltage is as specified, go to next step. If voltage is not as specified, check blend air flap actuator motor
wiring harness circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay, replace
blend air flap actuator motor.
4. Connect voltmeter positive lead to socket box terminal No. 8 and negative lead to terminal No. 9. Turn
ignition on. Turn temperature control knob to Red range detent. Voltage should be less than one volt.
Turn temperature control knob from Red range detent to Blue range detent. Voltage should be 11.0-14.0
volts.
5. If voltage is as specified, go to next step. If voltage is not as specified, check blend air flap actuator motor
wiring harness circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay, replace
Sensor Temperature - °F (°C)Ohms
43 (6)3600
59 (15)2300
72 (22)1700
77 (25)1500

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blend air flap actuator motor.
6. Turn ignition off. Disconnect test cable from A/C push button control unit. Connect ohmmeter positive
lead to socket box terminal No. 1 and negative lead to terminal No. 8. Resistance should be 120 ohms.
7. If resistance is not as specified, check blend air flap actuator motor circuits. See WIRING DIAGRAMS
.
Repair as necessary. If circuits are okay, replace blend air flap actuator motor.
A/C Button
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 10 and negative lead to
terminal No. 9.
2. Turn ignition on. Depress A/C button switch to on position and set blower speed knob to position No. 4.
Ensure A/C button indicator is illuminated. Voltage should be less than one volt. Depress A/C button
switch to off position with A/C button indicator not illuminated or set blower speed knob to off position.
Voltage should be 11-14 volts.
3. If voltage is as specified, go to next step. If voltage is not as specified, check A/C button wiring harness
circuits. See WIRING DIAGRAMS
. Repair as necessary. If circuits are okay, replace A/C button
switch.
4. Turn ignition on. Depress A/C button switch to on position and set blower speed knob to position No. 4.
Ensure A/C button indicator is illuminated. Connect voltmeter positive lead to socket box terminal No. 4
and negative lead to terminal No. 9. Voltage should be more than 3.0 volts.
5. Connect Fused Jumper Wire (124 589 37 63 00) between socket box terminals No. 4 and 11. Depress A/C
button switch to on position and set blower speed knob to position No. 4. Ensure A/C button indicator is
illuminated. Measure voltage. Voltage should be less than one volt.
6. If voltage is not as specified, check A/C button wiring harness circuits. See WIRING DIAGRAMS
.
Repair as necessary. If circuits are okay, replace A/C button switch.
A/C Refrigerant Pressure Switch
1. Turn ignition off. Disconnect refrigerant pressure sensor connector. Turn ignition on. Connect voltmeter
positive lead to pressure sensor harness connector terminal "B" (Pink wire) and negative lead to terminal
"A" (Tan wire). Voltage should be 4.75-5.25 volts.
2. If voltage is as specified, replace A/C refrigerant pressure sensor. If voltage is not as specified, check
wiring harness between refrigerant pressure sensor and All Activity Module (AAM). See WIRING
DIAGRAMS . Repair as necessary. If circuits are okay, replace AAM and retest A/C system.
Outside Temperature Sensor
1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST
EQUIPMENT . Turn ignition off. Connect ohmmeter positive lead to socket box terminal No. 12 and
negative lead to terminal No. 3.
2. Disconnect test cable from A/C push button control unit. See OUTSIDE TEMPERATURE SENSOR
RESISTANCE table. If resistance is not as specified, check wiring harness between outside temperature
sensor and A/C push button control unit. See WIRING DIAGRAMS
. Repair as necessary. If wiring
harness is okay, replace outside temperature sensor.

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OUTSIDE TEMPERATURE SENSOR RESISTANCE
READING ACTUAL VALUES WITH HHT
Connect Hand-Held Tester (HHT) to DLC and follow displayed procedures. Actual values displayed on HHT
show normal operation value of A/C systems or components. See ACTUAL VALUES
table. If value is not as
specified, check system or components in order. Repair as necessary.
ACTUAL VALUES
REMOVAL & INSTALLATION
Sensor Temperature - °F (°C)Ohms
68 (20)2100
77 (25)1700
104 (40)900
122 (50)600
Step (1) Test ConditionValueSystem/Component
01.....11-14 VoltsVoltage Supply Wiring (Circuit
No. 30) & Battery
02Engine Operating At
Normal TemperatureAbout 176°F (80°C)Wiring, ECT Sensor & AAM
03Ignition ONAmbient Air Temperature
About 68°F (20°C)Wiring, Outside Temperature
Sensor & AAM
04Ignition ON, Blower At
Stage 4ON/OFFWiring, Blower Motor Relay,
Blower Motor & AAM
05Ignition ONON/OFFWiring, Engine Cooling Fan
Stage 1, Aux. Fan & AAM
08Ignition ONON/OFFWiring, Coolant Circulation Pump
Relay, Coolant Circulation Pump
& AAM
09Engine On, A/C Indicator
On, Blower Motor At
Stage 4Refrigerant Pressure At
174 psi (12 bar)(2) Refrigerant Pressure Sensor
10Engine On, A/C Indicator
On, Blower Motor At
Stage 4ON/OFF(3) AAM, A/C Compressor
11Ignition ON, Activate
A/C SystemYES/NOWiring, A/C Push Button Control
Module, AAM
(1)Step numbers not shown are not used.
(2)Check A/C refrigerant pressure switch. See A/C REFRIGERANT PRESSURE SWITCH under
ELECTRICAL TESTS.
(3)Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.

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A/C EVAPORATOR HOUSING UNIT
Removal & Installation
1. Drain engine coolant. Remove coolant expansion reservoir nuts, disconnect hoses and remove reservoir.
Discharge A/C system, using approved refrigerant recovery/recycling equipment.
2. Remove expansion valve self-locking nut and discard. See Fig. 5
. Disconnect refrigerant lines from
expansion valve. Plug exposed refrigerant lines to protect A/C system from moisture and debris.
3. Remove instrument panel. See INSTRUMENT PANEL
. Remove Electronic Transmission Control
(ETC) module. See Fig. 4
. Disconnect air mix flap actuator connector.
4. Disconnect recirculated air flap element. Disconnect evaporator temperature sensor. Disconnect blower
motor resistor and blower motor connectors. Remove A/C push button control module. Release wiring
harness.
5. Remove insulating mat. See Fig. 5
. Disconnect water drain hose from bottom of A/C housing. Release
A/C housing-to-air distributor clips and remove A/C housing upward. To install, reverse removal
procedure. Recode radio. See RECODING RADIO
under PROGRAMMING. WARNING:To avoid injury from accidental air bag deployment, read and carefully
follow all SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR
BAG SYSTEM procedures in AIR BAG SYSTEM SAFETY article in
GENERAL SERVICING.
CAUTION: Before disconnecting battery, obtain 5-digit radio security code from
vehicle owner. Radio will be disabled until properly recoded.

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Fig. 4: Removing Evaporator Housing Unit
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
A/C MIX FLAP ACTUATOR
Removal & Installation
Remove instrument panel. See INSTRUMENT PANEL . Disconnect air mix flap actuator connector. Remove
3 screws and remove air mix flap actuator. To install reverse removal procedure.
A/C PUSH BUTTON CONTROL MODULE
Removal & Installation
Disconnect negative battery cable. Remove A/C push button control panel. See A/C PUSH BUTTON
CONTROL PANEL . Remove 4 A/C push button control module screws and remove control module from

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A/C housing. Disconnect control module connector. To install reverse removal procedure. Recode 5-digit radio
security code. Using HHT, check for DTC and erase, and set clock.
A/C PUSH BUTTON CONTROL PANEL
Removal & Installation
1. Obtain 5-digit radio security code from vehicle owner. Disconnect negative battery cable. Using radio
removal plates, remove radio. Disconnect electrical connectors.
2. Remove instrument panel center section. Remove 4 A/C control panel screws and remove control panel.
Release pull cables and disconnect connectors. To install, reverse removal procedure. Recode radio. See
RECODING RADIO
under PROGRAMMING.
BLOWER MOTOR
Removal & Installation
Remove passenger-side instrument panel under cover. Remove blower motor mounting screws from A/C
housing unit. Disconnect blower motor connector and remove blower motor. To install, reverse removal
procedure.
COMPRESSOR
Removal & Installation
1. Discharge A/C system, using approved refrigerant recovery/recycling equipment. Remove accessory
drive belt. Disconnect low pressure refrigerant line from compressor. Discard "O" rings. Plug exposed
openings to protect A/C system from debris and moisture.
2. Disconnect high pressure refrigerant line from compressor. Discard "O" rings. Plug exposed openings to
protect A/C system from debris and moisture. Disconnect electrical connector. Remove compressor
mounting bolts.
3. Raise and support vehicle. Remove compressor from under vehicle without spilling refrigerant oil. To
install, reverse removal procedure. Ensure refrigerant oil quantity in compressor is correct. See
COMPRESSOR REFRIGERANT OIL CHECKING under GENERAL SERVICING. Tighten bolts to
specification. See TORQUE SPECIFICATIONS
.
CONDENSER
Removal & Installation
1. Discharge A/C system, using approved refrigerant recovery/recycling equipment. Remove cooling fans.
See COOLING FANS
. Disconnect refrigerant line from receiver-drier. See Fig. 5 . Discard "O" rings.
2. Plug exposed openings to protect A/C system from debris and moisture. Remove refrigerant line from
condenser. Remove radiator upper rubber seal. See Fig. 6
. Remove condenser upper mounting screws.
Lift condenser upward and remove. NOTE: Each time A/C compressor is removed and installed, receiver-drier must be
replaced.

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3. To install, reverse removal procedure. Use NEW "O" rings lubricated with refrigerant oil. If installing a
new or repaired condenser, add 0.68 ounce of NEW refrigerant oil to condenser. Tighten refrigerant line
bolts to specification. See TORQUE SPECIFICATIONS.

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