Test Conditions and Functions
Function Test
Test Conditions, Observe:
A. Compressor cutout if electromagnetic clutch of refrigerant compressor fails to cut in:
- Test steps 1 to 6
B. Cutout of electromagnetic clutch of refrigerant compressor:
- Test via compressor cutout control Unit (N 6), test steps 1 to 5
Commercially Available Tester:
Multimeter e.g. Sun, DMM-5
Function of Compressor Cutout:
General:
To protect the simplex belt drive, the electromagnetic clutch is switched off via a control unit if the refrigerant compressor blocks up.
Components of the compressor cutout:
1. Speed sensors
2. Control unit
3. Microswitch (only in vehicles with diesel engine and automatic transmission)
1. Speed sensors:
The speed sensors measure the speeds of the engine and of the refrigerant compressor. They consist of a magnetic core and a coil. When the starter ring gear or the shaft of the refrigerant compressor rotate, an alternating voltage is induced in the coils of the speed sensor and this is passed on to the control unit (N 6).
L4 Speed sensor refrigerant compressor
The alternating voltage and thus the frequency rises and falls depending on the speed.
Note: In gasoline engines, the engine speed comes from terminal ID at the cable connector of the diagnosis plug.
L3 Speed sensor, starter ring gear (in diesel engines on the ring gear of the flywheel)
2. Control unit:
The control unit compares the two speeds of the engine and refrigerant compressor and switches off the refrigerant compressor if the speed difference is approx. 30 %.
N6 Control unit, compressor cutout
3. Microswitch (S 27/1) only in vehicles with diesel engine and automatic transmission:
The microswitch switches off the refrigerant compressor at full throttle below approx. 1050 rpm up to approx. 2150 rpm via the control unit (N8) (improves start-up characteristics).
S27/1 Microswitch
S27/1 Microswitch
Function, Description of Compressor Cutout:
In vehicles up to 01/88, the refrigerant compressor only cuts in approx. 10 seconds after the engine reaches a speed of approx. 600 rpm (to stabilize the engine speed).
In vehicles as of 02/88, the refrigerant compressor cuts in after 4 seconds in model 124.0 and immediately in model 124.1.
The control unit (N6) only compares the two speeds of the ring gear and refrigerant compressor 2 seconds after the refrigerant compressor cuts in (due to slip in the clutch during switching on). If there is a speed difference of more than 30 % when the refrigerant compressor cuts in, this condition is monitored for 200 milliseconds. If the difference no longer exists after 200 milliseconds, the refrigerant compressor remains switched on. If the speed difference continues to exist because the refrigerant compressor operates with difficulty, the control unit (N6) immediately switches the refrigerant compressor off.
This procedure is repeated only if the ignition is switched off and the engine restarted.
N16 (MAS-USA)
Emergency Cutout Of Refrigerant Compressor
from 09/85 to 08/87:
To prevent any thermal overload of the 8-cylinder engines, the refrigerant compressor is switched off as follows
a) In vehicles from 09/85 to 08/87, the compressor is switched off by a temperature switch (S 25/5) via the compressor cutout control unit. At coolant temperatures >115 °C, the temperature switch switches to ground causing the compressor cutout control unit to switch off the refrigerant compressor.
If the engine temperature drops (see table) the temperature switch opens and the refrigerant compressor is immediately switched back on again by the compressor cutout control unit.
Arrangement of temperature switch (S25/5):
Engine 103 up to 08/87
Engine 116/117
Engine 103, B 11/7 or B 10/8
Arrangement of temperature switch (S25/11)
Engine 603 TURBO up to 08/87
b) In all vehicles as of 09/87, the emergency cutout is controlled by a temperature sensor (B 11/7 or B 10/6) with NTC resistor. This temperature sensor sends its resistance, which varies according to the coolant temperature. to the control unit; this then processes the value and switches off the refrigerant compressor in 2 stages as follows:
1st stage:
At a coolant temperature of 117 °C in gasoline engines and 122 °C in diesel engines, the switch-on duration is reduced by 50 %. The compressor is switched on and off cyclically, i.e. approx. 20 sec. off, 20 sec. on. If the coolant temperature drops to 114 °C in gasoline engines and 117 °C in diesel engines, the compressor cuts back in again fully.
2nd stage:
At a coolant temperature of 120 °C in gasoline engines and 128 °C in diesel engines, the refrigerant compressor is switched off entirely. If the coolant temperature drops to 117 °C in gasoline engines and 122 °C in diesel engines, the compressor is switched back on again cyclically (1st stage).
Arrangement of coolant temperature sensor (B 11/7 as of 09/87 B 10/8)
Engine 116/117
Engine 103 as of 09/87
Engine 603 as of 09/87
Test Condition:
A prerequisite for the test is that the actuation of the refrigerant compressor functions correctly.
Checking Actuation:
Switch on ignition, blower and air conditioner with function key "Defrost".
Connect voltmeter to positive and test each connection of pressure switch (S31/1) in turn to ascertain whether the battery voltage is present. If there is no voltage at both connections, test actuation of refrigerant compressor (refer to "Function Test, Checking With Test Cable test step 24"). Testing and Inspection
If voltage is present at one connection only, test high pressure of air conditioner. If pressure is >3 bar, replace pressure switch (S31/1). If pressure is <2 bar, top up air conditioner with approx. 200 g refrigerant (R 12), and localize and eliminate leak.