Checking Compressor Cut-Out

COMPRESSOR - CHECKING CUT-OUT
Engines 103, 116, 117, as of 09/85

Test Conditions:
a) Compressor switching off when the a/c compressor's electromagnetic clutch fails to engage
- check, Test Steps 1-6
b) Switching off of electromagnetic clutch of a/c compressor via compressor cut-out control unit (N6)
- check

Commercially Available Tester:
Multimeter e.g., Sun DMM 5

Operation Of Compressor Cut-out
General:
Should the a/c compressor lock up, a control device switches off the electromagnetic clutch to protect the single-belt drive system.

Components of the compressor cut-out system:
1. Speed sensors
2. Control unit

1. Speed sensors
The speed sensors monitor the rpm's of the a/c compressor and of the engine. They consist of a magnetic core and a coil. When the ring gear and the a/c compressor shaft rotate, alternating voltage is generated in the coils of the speed sensors, with this being relayed to the control unit (N6).

1 A/C compressor speed sensor

The alternating current and the frequency rise and fall proportionately to speed.

NOTE: The engine speed is monitored at Terminal TD on the diagnostic terminal block.

L3 Flywheel ring gear speed sensor

2. Compressor cut-out control unit (N6)
The control unit compares the rpm's of the engine and the a/c compressor, and switches the compressor off when the difference reaches approx. 30 %.

N6 Compressor cut-out control unit

Compressor Cut-Out, Description Of Operation:
On vehicles up to 01/88, when an engine speed of approx. 600 rpm is reached. there is a delay of about 10 seconds before the a/c compressor is engaged (to allow engine speed to stabilize). On vehicles as of 02/88 the a/c compressor is engaged after approx. 4 seconds.

There is a delay of approx. 2 seconds after a/c compressor activation (due to initial clutch slippage) before the control unit (N6) begins to compare the speeds of the ring gear and the a/c compressor. If a variation in excess of 30% is present when the a/c compressor is switched on, this condition is then examined for 200 milliseconds. If the speed differential has disappeared after 200 milliseconds, the a/c compressor remains engaged. If the speed differential remains due to stiffness in the compressor, the control unit (N6) immediately shuts down the compressor.

The procedure is repeated only after the ignition has been switched off and the engine then started again.

A/C Compressor Emergency Cut-out, 09/85 Through 08/87:
To prevent excessive thermal stress on the 8-cylinder engines, the a/c compressor is switched off as follows:

On vehicles from 09/85 through 08/87 a temperature switch (S25/5) initiates the shut-down via the compressor cut-out control unit. At a coolant temperature in excess of 115 °C the temperature switch is grounded, causing the compressor cut-out control unit to shut down the a/c compressor.

When the temperature drops to 108 °C, the temperature switch opens and the compressor cut-out control unit immediately switches the a/c compressor back on.

Position of temperature switch (S25/3) on 6-cylinder engines

Position of temperature switch (S25/5) on 8-cylinder engines

On all vehicles as of 09/87 the emergency cut-out is controlled by a temperature sensor (S11/7 or B10/8) with NTC resistor element. This temperature sensor relays a resistance value corresponding to the coolant temperature to the control unit. This processes the information and then shuts down the compressor in two stages, as follows:

1st stage:
At a coolant temperature of 117 °C the operating time is reduced by 50%. The compressor is switched according to a set frequency that is approx. 20 seconds on, 20 seconds off. When the coolant temperature gets down to 114 °C, the compressor is again switched to full operation.

2nd stage:
At a coolant temperature of 120 °C the a/c compressor is switched off completely. When the coolant temperature gets down to 117 °C, the compressor is switched on again for frequency controlled operation (1st stage).

Position of coolant temperature sensor (B 11/7) as of 09/87 (B 10/8):

Engine 116

Engine 103 as of 09/87

Test Conditions:
The compressor activation must function correctly for the test to be performed.

Checking Actuation:
Switch on ignition. Switch on blower and air conditioning by selecting (image).

Connect voltmeter to positive, and then check both pins of the pressure switch (S31/1) to see whether battery voltage is present. If there is no voltage on the pins, check the a/c compressor control.

If voltage is present at only one pin, check air conditioning system high pressure. If the pressure is >3 bar replace pressure switch (S 31/1). If the pressure is <2 bar, recharge the air conditioning system with approx. 200 g refrigerant (R12), find and repair leak.

a) Compressor Cutout - Checking When A/C Compressor Electromagnetic Clutch Fails To Engage