The "Y-Factor"

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.

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

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

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







The desired temperatures (left and right) and interior temperature are the primary inputs used to compute the Y-factor. The control module looks at the temperatures that the passengers want, looks at the existing cabin temperature, and does what it can 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.

The control module uses the ambient temperature input "correct" or exaggerate the difference between desired and interior temperatures when the ambient temperature is low. This feature compensates for the fact that people will "feel" cold, even though they are in a 70 °F cabin, because their bodies (and the cabin) are rapidly radiating heat to the cold atmosphere.

The lower the ambient temperature, the more the control module exaggerates the difference; the lowest ambient temperature allowed for is -40 °F at which temperature the control module "creates" an 8 degree difference between desired and interior temperatures.

For a given Y-Factor, the control module can perform the following actions to achieve the desired cabin 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)
- operate stepper motors to recirculate cabin air, instead of using outside air
- operate stepper motors to control air discharge location (IHKA only)
- boost blower speed to decrease cabin 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 cabin temperature.







As the cabin warm 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.







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







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 recirc. mode, but the occupants should partially open windows and the sunroof for the first few minutes to expel hot air trapped in the cabin.







Although the cabin 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 cabin temperature drops a few more degrees, recirc. mode will be discontinued and the system will draw in only fresh air.