Compressor Control
IHKA E38 COMPRESSOR CONTROL
IHKA E38 compressor control is significantly different from that on previous IHKA systems:
- The engine control module (ECM) and the IHKA control panel/module communicate via three direct circuits; there are no relays.
- The IHKA control panel/module powers the A/C compressor clutch coil directly; again, there is no relay.
When the "snowflake" button is pressed, the control panel/module signals the ECM that engine load is about to be increased (signal AC). The ECM immediately boosts engine idle speed by 100 to 200 RPM, regardless of A/C compressor clutch status.
- Signal AC is continuously active (high) while the "snowflake" button LED is illuminated.
If evaporator temperature is above 37°F (3°C), the control panel/module then signals the ECM that it is about to engage the A/C compressor (signal KO). So long as the ECM does not signal back (signal KO-REL) that A/C compressor operation is not allowed, the control panel/module engages the compressor.
- Signal KO cycles high and low as the IHKA control panel/module monitors evaporator temperature and cycles the compressor on and off.
If the ECM detects "full-load" conditions, it sends signal KO-REL to the IHKA control panel/module to disengage the A/C compressor. The conditions for full load are:
- Vehicle speed is less than 10 mph, and
- Throttle is wide open or
- Engine coolant temperature is too high
The compressor is turned off for a maximum of 4 minutes.
As on previous IHKA systems, refrigerant pressure switches can interrupt compressor operation, but cutout pressures are slightly higher on the E38 system:
- The high-pressure switch opens it refrigerant pressure exceeds 479 psi (33 bar), and closes if pressure then drops to 334 psi (23 bar).
- The low-pressure switch (still on the high side of the system) opens if pressure drops below 28 psi (1.9 bar), and closes if pressure rises to 41 psi (2.8 bar).
IHKA E38 includes several additional compressor control strategies:
- If the "snowflake" button is on at engine start-up, the control panel/module delays compressor clutch activation until engine speed exceeds 600 RPM for at least 5 seconds.
- As on E31 vehicles, if the driver switches "Off" the A/C compressor while the vehicle is moving, the control panel/ module continues to cycle the compressor on and off (for increasingly longer "off" times) for up to 15 minutes. This allows the evaporator to warm up gradually, helping to prevent windshield fogging. If the evaporator reaches ambient temperature in less than 15 minutes, the compressor is shut off sooner.
- A unique E38 feature is ambient temperature compensation. When ambient temperature exceeds 68°F (20°C), the control panel/module activates the compressor at an evaporator temperature above 33°F (1°C), instead of 37°F (3°C). This provides additional cooling.
A/C Compressor Control (5/97 and later 740, 9/97 and later 750)
Activation of the compressor clutch is an output control function of the IHKA module. Activation is carried out directly through a final stage in the module (no relay required).
The signals KO and AC have been combined into one signal providing multiple data through a stepped increase in signal duty cycle.
The individual switching stages are transferred by way of pulse width modulation with a frequency of 5 Hz.
Diagnosis of the line is carried out by the ECM.
SIGNAL KOREL:
The engine control module signals the IHKA (Signal KOREL) when the idle increase has taken place and the IHKA will then activate the compressor clutch, as long as all other compressor requirements are satisfied. Compressor cut off conditions include:
- Evaporator temperature below 2°C
- Coolant temperature > 117°C - causes the compressor to run in a pulsed operation.
- Coolant temperature > 120°C - the compressor clutch is deactivated until the temperature drops below 117°C
- Full acceleration at low speeds - below 10 mph with a full throttle input will cause the compressor to shut off for 10 seconds
A/C COMPRESSOR CONTROL '99 MODEL YEAR
For '99 Model Year, both 74 Oil and 75 Oil IHKA now receive a variable input from the refrigerant pressure "sensor". The sensor is mounted in the receiver/dryer.
Based on the refrigerant pressure, the pressure sensor input allows the IHKA module to determine the "start up" torque of the A/C compressor. If the pressure is too low or high the compressor will be deactivated. The IHKA also anticipates the necessary [1][2]auxiliary fan speed and passes this information on to the ECM via the K-bus.
This change now removes the pressure sensor from the compressor activation circuit.