A/C - Compressor Control Systems Description
90bmw14 Group 64 Heating and Air ConditioningBulletin Number
64 05 90 (3094)
Woodcliff Lake, NJ August 1990
Service Engineering
SUBJECT: Air Conditioning Compressor Control Systems
Models: 750iL (E32) and M5 (E34)
Situation: The above named models incorporate two different types of air conditioning compressor control systems, separate from IHKA/IHKR controls.
When troubleshooting one of these vehicles for A/C compressor clutch problems it is important to consider how these systems affect the compressor.
General
Information: Lock sensor control unit - 750iL (E32)
The M70 engine uses one Poly-Vee belt to drive both the water pump and the A/C compressor. Since a seized A/C compressor could damage this belt (and thereby impair engine cooling), a "lock sensor" control system is installed in the 750iL. Its operation, and troubleshooting procedures specific to it, are as follows:
Normal sequence of operation:
1. Ign. @ KL15, B + @ Pin 2, x77, B- @ Pin 3, x77.
2. Engine running, TD signal @ Pin 4, x77.
3. A/C compressor relay "on", high pressure & low pressure switches closed, B+ @ Pin 3, X80.
4. With the above conditions met, lock sensor applies B+ to Pin 1,
x77 and monitors Pin 1 & 2, x80 for frequency (in form of A.C. waveform).
5. If slippage exceeds programmed limits (comparing Pin 1 & 2, x80 with Pin 4, x77), power is withdrawn from Pin 1, x77. This power will not reappear until ignition switch is cycled (interrupting power @ Pin 2, x77).
6. Interruption of power or ground (Pins 2 & 3, x77) or B+ from compressor control relay (Pin 3, x80) will cause loss of B+ @ Pin 1, x77 only for the duration of that interruption.
7. Interruption of frequency signal (Pins 1 & 2, x80) causes loss of B+
@ Pin 1, x77. This will not reappear d signal is restored, unless key is cycled.
Troubleshooting Lock Sensor System
The lock sensor circuit interrupts power to the A/C compressor clutch if it monitors even a brief (< 1 sec.) slippage above a certain threshold. This interruption continues until the ignition switch is turned OFF and back ON again. Consequently, replacement of the control unit may appear to correct the problem when it actually has not.
If a complaint of "A/C Cuts Out" (or "Blows Hot Air") is received, verify that power is available to the lock sensor control unit (x80, pin 3). If no power is present at x77, pin 1 (to compressor clutch), the lock sensor control unit may have simply responded to slippage.
Before replacing the control unit, consider the following influences which can cause slippage:
- Defective belt tensioner.
- Oil or coolant on drive belt.
- Oil or coolant contamination of compressor clutch.
- Excessive "high side" pressures.
Please note the following points:
1. There is currently no approved method for cleaning either a drive belt or a compressor clutch which has been contaminated with oil and/or coolant. The component must be replaced.
2. "Excess high side pressures" can be momentary and therefore hard to diagnose. If the complaint of "A/C blows hot air" seems to be limited to startup after a hot soak, and/or under conditions of very high ambient temperatures, momentary excessive high side pressures should be investigated. Proceed as follows:
A. Verify proper operation of [1][2]auxiliary fan.
B. Verify proper R12 charge, recharging if necessary.
Note: Since approximately November, 1989, M70 equipped vehicles additionally use an engine coolant temperature input to the lock sensor control unit. At a monitored engine coolant temperature of 117~C, the lock sensor control unit cycles the compressor (20 sec. ON, 20 sec. OFF) in order to reduce coolant temperature. If the coolant temperature is reduced to 114~C, this cycling ceases and the compressor is again allowed to run continously (unless interrupted by the IHKA Control Unit, Pressure Switches, or Lock Sensor System). If the coolant temperature exceeds 120~C, compressor operation is discontinued until coolant temperature is reduced to 114~C.
Full Throttle Compressor Cutoff Control Unit - M5 (E34)
In accordance with design criteria for the M5 (smooth start-up and idle, and enhanced engine performance at full throttle operation), these vehicles are fitted with a full throttle compressor cutoff control unit.
This control unit is supplied with inputs for "A/C On", engine rpm, and A/C compressor clutch engagement is delayed by approximately 3-5 seconds after the engine is started (recognized by engine rpm signal).
Full throttle operation is controlled by the resonance flap control unit (located in E-Box, under DME main and fuel pump relays). A ground from this control unit causes A/C compressor relay K9502 to open and discontinue power to the compressor (K9502 is normally closed).
Further, power to the compressor is interrupted by the full throttle compressor cutoff control unit if the engine coolant temperature exceeds approximately 117~C.
Please consider these additional systems when troubleshooting air conditioning complaints on 750iL (E32) and M5 (E34) vehicles.
Warranty
Status: Information only.