The "Y-Factor"

Y-Factor is the term used to describe how much heating effort or cooling effort (expressed as a percentage) is required by the microprocessor-controlled climate control system (IHKR/IHKA) for it to achieve the desired passenger compartment temperature. The terms "adjusting factor," "master controller," and "guide control" are sometimes used in place of Y-Factor.

Fig 1: Identifying Y-Factor

Courtesy of BMW OF NORTH AMERICA, INC.

The climate control system control module computes the Y-Factor (actually two Y-Factors, one for the driver's side of the vehicle, Y_L , and one for the passenger's side, Y_R ), using the information it receives from some of the inputs.

Left Desired Temperature
Interior Temperature
Left Heater Core Temp.
Right Desired Temperature
Ambient Temperature
Right Heater Core Temp.

The Y-Factors (left & right) computed from these inputs have numerical values ranging from -27.5% to +100% and can be displayed on the DIS for diagnostic purposes . (Except E46)

Values between -27.5% and 20% indicate that the system is working to cool the interior; values from about 20% to 100% indicate that it is warming the interior.

Fig 2: Identifying BMW Diagnosis Display

Courtesy of BMW OF NORTH AMERICA, INC.

The lower the Y-factor number, the harder the system is working to cool down the interior; the higher the number, the harder the system is working to warm up the interior. In the middle region, the system is working to maintain the existing interior temperature.

Fig 3: Identifying Y-Factor Working System Graph

Courtesy of BMW OF NORTH AMERICA, INC.

The desired temperatures (left and right) and interior temperature are the primary inputs used to compute the Y-factor . (The control module monitors the temperatures that the passengers want, monitors the existing interior temperature, and controls outputs to make the two match.) The left desired temperature input has priority over the right, when the left input is set to the minimum or maximum values.

For a given Y-Factor, the control module can perform the following actions to achieve the desired interior temperatures:

pulse a coolant flow control valve to regulate left heater core temperature
pulse a coolant flow control valve to regulate right heater core temperature
operate a relay to turn an electric coolant pump on and off (except E36 & E46)
operate stepper motors to recirculate interior air, instead of using outside air
operate stepper motors to control air discharge location (IHKA only)
boost blower speed to decrease interior cool down or warm up time (IHKA only)

Some sample temperature readings with approximate Y-factor values appear below and on the following page.

With the large difference between the desired and interior temperatures, Y-Factors of 100% should be expected since the system will work as hard as possible to increase interior temperature.

As the interior warms up, the Y-Factors drop steadily to avoid grossly "overshooting" the desired temperatures. Coolant valves, initially held "wide open," are pulsed more frequently to reduce coolant flow (and heater core temperatures). Due to the extremely low ambient temperature, the control module will "boost" (or in the case shown above, "create") a difference between interior and desired temperatures to offset rapid heat loss to the atmosphere.

Despite the "match" between interior and desired temperatures, the Y-Factors will still be 100% due to the low heater core temperature. The control module initially keeps the coolant valves wide open to utilize all heat available from the engine. As soon as the heater cores begin to warm up, though, the Y-Factors will drop rapidly since sufficient heat can be obtained with very low flow through the cores.

Fig 4: Identifying BMW Diagnosis Display (1 Of 3)

Courtesy of BMW OF NORTH AMERICA, INC.

With the engine warmed up, the climate control system expends very little effort to maintain interior temperature. The coolant valves are seldom opened, maintaining heater core temperatures just above the desired temperatures.

Fig 5: Identifying BMW Diagnosis Display (2 Of 3)

Courtesy of BMW OF NORTH AMERICA, INC.

With such a large difference between the interior and desired temperatures, maximum cooling power is required. Heater core temperatures will drop rapidly since the coolant valves will be kept closed. The module will activate recirculation.

Fig 6: Identifying BMW Diagnosis Display (3 Of 3)

Courtesy of BMW OF NORTH AMERICA, INC.

Although the interior temperature is approaching stabilization, the system must continue to work hard due to the high ambient temperature.

The heater cores are now nearly at evaporator temperature since they are placed directly in the flow of cold air and the coolant valves are kept closed. Once interior temperature drops a few more degrees, recirc. mode will be discontinued and the system will draw in only fresh air.