Comfy Customers How BMW HVAC Systems Work, & How to Work on Them

June 15, 2012 | By | Reply More

Being some of the most sophisticated vehicles in the world, just about every aspect of BMWs is a technological marvel. Think about how these vehicles have evolved over the last couple of decades and you might put the engineering that went into them right up there with that of the space shuttle. Take their HVAC systems, for example. To say they’re extremely advanced is an understatement, which requires considerable discipline on your part if you want to understand how they work and interact with the rest of the vehicle. Never forget, however, that the basics still apply. You still need to check such things as the refrigerant charge, blower operation, blend door movement, coolant level and condition, etc.

Once you go beyond those essentials, you must learn a considerable amount of additional information about the various climate control systems BMW offers so you will be prepared when problems arise. Knowing the subtleties of how these systems function can make the difference between being profitable and struggling to break even. You need to know how the A/C compressor is controlled, how heat is provided, and what controls blower motor operation. This may sound simple enough, but there are three available options and all are electronically controlled to some extent, which takes this area of service onto another level.

In The Old Days

BMW has used several systems to control the interior temperature of its vehicles over the years, each with its own level of sophistication. For example, IHKS is a manual system typically found on 3-Series cars such as the 318ti, Z3 and Z8. The driver is in control of temperature settings, mode door position, and blower speed. Despite the fact that it’s basically manual, many of the components are electronically-controlled. For instance, the A/C compressor is ruled by the DME, which also handles fuel injection. It directly controls the compressor through a relay, and it can increase idle speed to handle the additional load. An input from the IHKS system tells the DME when to energize the clutch. Blower speed is handled by multiple resistors through a switch. Mode door control is by means of a manual cable except for the motor that handles fresh air recirculation. This motor receives a pulse width-modulated signal from the IHKS control unit and opens or closes the door accordingly.
The next level of sophistication is IHKR. This was an option on the 3-Series and standard on most 5-Series, and is considered semi-automatic. Blower speed is still controlled with multiple resistors, and mode door control is still by means of cable, but cabin temperature is regulated by the IHKR control unit. Sensors send interior and exterior temperature readings to the electronics, which use that data to make decisions about how best to control the compressor and blend door.

Both IHKS and IHKR have pressure switches that will only pass along the A/C request if there is enough refrigerant in the system. They also shut off the compressor if the pressure exceeds approximately 450 psi


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Category: HVAC, the bimmer pub

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