Electronic mirror adjustment:
Two electric motors are mounted in the mirror housing of a car with electrically operated exterior mirrors. One electric motor moves the mirror glass up or down. This is the vertical movement. The other electric motor is for the horizontal movement; left and right.
To operate the exterior mirrors, a button is integrated into the interior of the car. This can be, for example, in the dashboard, the center console, or the door.
The signals from the switch reach the control unit (the ECU). Usually this is the comfort control unit or the door control unit. These control units are often connected to the CAN bus. When the control unit receives a signal from the switch, this switch (in older vehicles) or the ECU (in newer vehicles) will use an H-bridge to control the electric motors to move the mirror glass.
The waterfall diagram shows that the power supply and ground are connected to pin 7 and pin 11. The CAN-high and CAN-low are connected to the ECU on pin 8 and 9.
The switches for the vertical movements are connected to pin 1 and 2. Pin 3 is the power supply for these two switches. The same applies to pin 6, which is the power supply for the switches for the horizontal movements on pin 4 and 5 of the ECU.
The electric motors are connected to pins 8 and 10. The left electric motor (pin 8) is for the vertical movements. The right one (pin 10) is for the horizontal movements. Pin 9 has a wire that is connected to both electric motors. Depending on the situation, the ECU can switch either a positive voltage or a ground to this wire. How this works is described in the following paragraphs.
Vertical adjustment:
This explanation refers to the two images below.
When adjusting the mirror glass upwards, the left electric motor is controlled by switch B on pin 1. The control unit then switches a supply voltage to pin 8, which is connected to the electric motor by a wire. Pin 9 is switched to ground. The left electric motor will run until switch B is opened again.
When adjusting downwards, switch O will close. The supply voltage is now applied to pin 9 of the control unit and pin 8 is ground. Because the positive and ground on the electric motor are reversed, the electric motor will rotate in the opposite direction (downwards instead of upwards). This can be seen in the images below.
When the electric motor for vertical adjustment is running, the electric motor for horizontal adjustment must remain stationary. Both terminals of the motor that must remain stationary are shorted by pin 9 either to ground or to positive. On pins 8, 9 and 10, either a supply voltage or ground can be switched independently of one another.
Reversing positive and negative to change the direction of rotation of the electric motor is possible with a so-called H-bridge. This circuit of four transistors or FETs is built into the ECU. The ECU controls the correct transistors in pairs (one for positive and one for ground) to determine the current direction to the electric motor.
Horizontal adjustment:
This explanation refers to the two images below.
When adjusting to the left, switch L will close. The right electric motor receives a supply voltage via pin 10 on the control unit. The ground is via pin 9.
Because the left electric motor (for vertical adjustment) must not run now, it is also connected to ground at pin 8. The left electric motor is therefore shorted to ground. The right electric motor runs at a constant speed until switch L is no longer operated.
When adjusting to the right, the supply voltage and ground on the right electric motor are reversed again. The electric motor will now turn in the opposite direction.
Now pin 8 of the left electric motor no longer receives ground, but the supply voltage. Otherwise it would also start to rotate. With both sides now switched to positive, the electric motor remains stationary.
Mirror adjustment with memory function:
Modern vehicles have mirror adjustment with a memory function. The ECU controls actuators until they have reached the desired position. Position recognition takes place by means of potentiometers. More information about this can be found on the page about the potentiometer.
In the diagram we see potentiometers G791 and G792. The potentiometers receive a 5-volt supply and ground from the ECU. The signals are sent to pin 1 and 9 of connector T16a.
The ECU recognizes the position of the actuators V17 and V149 by reading the signal voltages from the potentiometers. For a value stored in memory, the ECU continues to control actuators V17 and V149 until the signal voltage is equal to the stored value in memory. Example:
- The car is unlocked with key A;
- The stored value of V17 is 3.6 volts;
- The measured value of V17 is 2.9 volts;
- The ECU controls electric motor V17 until the potentiometer provides a signal voltage of 3.6 volts;
- When the car is unlocked with key B, with a stored value of 2.9 volts, the same process takes place to reach the stored value.
The operation, versions and possible faults are covered on the Potentiometer page.
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