Diagnosing air conditioning issues in a classic BMW, like the 1999 528i 2.8, can sometimes feel like chasing ghosts, especially when you encounter a situation where everything seems to be functioning, yet you suspect a deeper problem, perhaps even communication issues with your OBD2 port. Understanding the intricate workings of the BMW E39’s air conditioning system is crucial for effective troubleshooting. This guide delves into the compressor control mechanisms of your ’99 BMW 528i, providing insights drawn from BMW factory electrical information to help you pinpoint potential problems.
The air conditioning system in your BMW is designed with a focus on both performance and protection. When you press the A/C button, you’re initiating a complex sequence, not just a simple on/off switch. The system enters standby mode, indicated by the function light, but the compressor engagement depends on a series of “switch-on conditions” being met. Cold air production begins in the evaporator, and the system can even reheat this air to achieve your desired cabin temperature. A critical component is the evaporator sensor, which constantly monitors outlet temperature to prevent icing. The compressor itself is managed by an electromagnetic clutch, and its operation is governed by specific conditions to ensure efficiency and longevity.
To understand this better, let’s look at the detailed switching conditions for the electromagnetic clutch, as outlined in BMW’s documentation:
| ON (all conditions must be fulfilled) | OFF (one of the conditions must be fulfilled) | Timed mode (switching on and off in 20 s clock cycle) |
|---|---|---|
| Terminal 15 ON | Terminal 15 OFF | Coolant temperature greater than 117 o C and less than 120 o C |
| Air flow control wheeldriver out of zero position | Air flow control wheeldriver in zero position | |
| Air conditioning standby ON (function light ON) | Air conditioning standby OFF (function light OFF) | |
| Evaporator temperature > 3 o C | Evaporator temperature o C (controlled compressor o C) | |
| Coolant temperature o C | Coolant temperature > 120 o C – 3 o C hysteresis | |
| Air conditioning system signal sent from DME (compressor ON) | Full throttle signal from DME initiates shut-down with respect to time | |
| Delay time 5 s (on reaching engine speed > 600 rpm) elapsed | Speed dropped below minimum value (400 rpm) |
These conditions highlight the system’s intelligence. For instance, the compressor will only engage if the engine is running (Terminal 15 ON), there’s airflow demand, the system is in standby, and the evaporator isn’t too cold. Conversely, it will disengage if any of the “OFF” conditions are met, such as engine shutdown, no airflow, or an excessively cold evaporator. The timed mode at high coolant temperatures is a safety mechanism to prevent engine overheating.
Another vital safety component is the pressure switch, often a triple pressure switch, which protects the system from both low and high refrigerant pressure extremes. Here’s how it operates:
| Low pressure | Medium pressure | High pressure |
|---|---|---|
| ON | ON > 20.0 +/- 1 bar | ON > 33.0 +/- 1 bar |
| OFF > 1.9 +/- 0.2 bar | OFF | OFF |
This switch monitors refrigerant pressure and can interrupt power to the electromagnetic clutch if pressure falls too low (indicating a leak or low refrigerant) or rises too high (indicating overcharge or blockage), preventing component damage.
The evaporator temperature controller is crucial for preventing evaporator icing. It functions as a two-point controller, activating the refrigerant circuit when the evaporator temperature is above 3°C and deactivating it when it drops to 0°C (or lower in controlled compressor systems). This hysteresis of 1°K ensures stable operation and avoids rapid on-off cycling.
Coolant temperature also plays a role in A/C operation. The Instrument Cluster Electronics (IKE) monitors coolant temperature, and this information is relayed to the IHKA (Integrated Heating and Air Conditioning) control module. If coolant temperature exceeds 117°C, the electromagnetic clutch enters the 20-second timed cycle. If it surpasses 120°C, the compressor is fully switched off until the temperature drops below 117°C again, at which point normal operation resumes.
During periods of full throttle acceleration, the system implements a full load cutout. To maximize engine performance during acceleration, the electromagnetic clutch is deactivated for a short duration (4-10 seconds, engine dependent). This reduces the load on the engine, allowing for quicker rev-up.
To maintain smooth idling despite the compressor load, the BMW system incorporates an idle increase function, also known as anti-stall. When the A/C is switched on, the air conditioning control unit signals the DME (Digital Motor Electronics) to increase the idle speed. This compensates for the additional load imposed by the compressor, ensuring stable idle even with the A/C running. This idle increase can be further activated under specific conditions like rear window defogger use, high blower settings, or low voltage detection.
Compressor activation is a multi-step process. Pressing the A/C button initiates idle increase, and the control unit signals the DME its intention to engage the compressor. The DME, in turn, manages engine parameters to accommodate the compressor load and sends a feedback signal (DME_KOREL) to the control unit to activate the compressor. Even after switching off the A/C, the compressor may cycle on and off for up to 15 minutes to gradually dehumidify the evaporator and prevent excessive water discharge.
Finally, the air conditioning system also manages the auxiliary fan. Stage 1 of the auxiliary fan is activated when the compressor clutch is engaged and the outside temperature is above 10°C. It deactivates when the compressor clutch disengages or the outside temperature drops below 8°C. For safety, a thermostat switch also triggers stage 1 at 91°C coolant temperature, independent of the A/C control unit. Stage 2 of the auxiliary fan is activated by high coolant temperature (above 99°C) or high pressure in the refrigerant system, ensuring adequate cooling under extreme conditions.
When you’re facing a situation where your ’99 BMW 528i 2.8 A/C seems problematic, and you suspect OBD2 communication issues are hindering diagnosis, understanding these detailed operational aspects is paramount. While the OBD2 system should ideally communicate diagnostic codes related to the A/C system, communication problems can mask the true issue. Therefore, a systematic approach, starting with verifying basic electrical connections, checking fuses, and then methodically testing the switching conditions and sensor inputs as described above, becomes essential. Even if your OBD2 scanner isn’t providing clear answers, a solid grasp of the system’s logic, combined with careful component checks, will guide you towards effectively resolving your BMW’s A/C concerns.
