Introduction:
The rear wiper motor is mounted in the tailgate of the car. There is a hole in the window or in the sheet metal of the tailgate through which the shaft of the wiper motor protrudes. The wiper arm with the wiper blade is mounted on this shaft. The shaft obviously cannot make full rotations, because then not only the rear window but also the rest of the tailgate or rear bumper would be wiped clean by the wiper blade. That is why there is a mechanism in the motor that ensures that the shaft can move a maximum of 180 degrees.
The rear wiper motor always has one speed. It can be switched on and off with the wiper switch and usually has an interval; after switching on, the motor will be actuated every few seconds.
The wiper always returns to its starting position after it is switched off. If that did not happen, the wiper arm would come to a stop halfway across the window when the switch is set to the “off” position. Instead of the supply voltage to the motor being cut off, it remains present until the zero position is reached.
Components of the wiper motor:
To allow the wiper arm to move back to the starting position, it contains a contact plate with sliding contacts on the inside. Using the following images, the operation of the wiper motor is explained.
The rear plate of the wiper motor has been removed here. The red arrow indicates where the round cam of the mechanism moves back and forth in the rear plate. The mechanism ensures that the rotating motion of the yellow plastic gear is converted into a reciprocating motion of the output shaft. In the image, the output shaft is upright. The wiper arm is mounted on this shaft.
The image on the right shows the cutaway wiper motor with the worm gear and the plastic gear. The mechanism has been removed here.
The following text relates to the image below. The yellow plastic gear is now turned over. The notches and recesses of the conductive contact disc are clearly visible here. Red, blue and green indicate at which position the sliding contacts touch the contact disc.
To make it clear at which positions the sliding contacts make contact with the conductive contact plate, they are indicated in red, blue and green. Below is what the sliding contacts are for:
Red: This always has 12 volts present with the ignition switched on.
Blue: This sliding contact is responsible for the zero position.
Green: This is the ground. With this, the motor is connected in the zero position.
The three sliding contacts “slide” over the gold-colored contact plate when the motor is running. A notch and a recess have been made in the contact plate. The sliding contacts therefore never touch the contact disc all three at the same time. The middle one (indicated in blue) is responsible for the movement to the zero position. The contact plate is conductive; if the motor is not yet in the starting position, then the inner (red) and the middle (blue) sliding contacts are connected to each other. The voltage is transferred via the contact plate from the red to the blue contact. As a result, the motor can continue to run until the red sliding contact reaches the notch. At that moment, it can no longer transmit voltage to the blue one. The control of the motor is then stopped.
At that same moment, the outer sliding contact makes contact via the recess (indicated in green) and the contact plate with the blue sliding contact. The green sliding contact is connected to the vehicle ground. This sliding contact acts as a kind of brake. This brings the wiper motor to a stop. The ground is passed on from green to blue. The motor is short-circuited to ground on both sides and therefore remains in the zero position.
Connecting the wiper motor:
To connect the wiring of the wiper motor, the operation of the contact plate and the sliding contacts, among other things, must first be studied. Only when one understands at which points voltages will be present, can one proceed to measuring and connecting the wiring.
The wiring harness in the tailgate for the rear wiper motor often consists of three or four wires. A constant voltage, switched voltage and a ground must be measured on these wires. On the remaining wire on which nothing is measured, a supply voltage (with a ground-switched motor) or a positive voltage (with a positive-switched motor) is usually present when the motor is in the rest position. A measurement can therefore only be taken on this wire when all wires are connected and the wiper motor is in the starting position. In all other cases, nothing will be measured.
There are positive-switched and ground-switched wiper motors. That means that the switch is located on the positive or on the ground side of the electric motor. It is very important to know this before starting measurements. In the sections below, each step is described in detail. Pay close attention to the differences between the positive-switched and ground-switched versions!
Positive-switched wiper motor:
Based on the diagrams, it can also be seen how this should be connected, for example for the notorious component of the practical exam. Below is a diagram with a legend of the positive-switched rear wiper motor. The wiper motor is stationary, and switch “0” is closed.
The electric motor (7) only receives direct battery voltage at a constant speed. In that case, switch 1 is closed and switch 0 is open. The electric motor (7) drives the worm gear (6), which in turn causes the gear (4) to rotate. The gray conductive contact disc is attached to the plastic gear and will therefore also rotate. When the wiper switch is set to the off position, the contact disc (5) and the sliding contacts A, B and C (2) ensure that the motor stops in the correct position. This is clarified below.
Wiper motor switched on:
In this situation, the wiper motor is switched on. The supply voltage is delivered via the red positive wire. Switch 1 is closed, so the motor receives a constant supply voltage. The other side of the motor is connected to ground, so the motor will run at a constant speed. The worm gear is driven by the wiper motor and will therefore rotate as well. In this case, that has no influence on the power supply to the motor.
Switch in off position, wiper still moving:
In the image below, the wiper switch is set to the “off” position. This opens switch 1 and closes switch 0 (of the zero position). At this moment, a current flows via sliding contact A, via the gray contact plate to sliding contact B. The current then flows from sliding contact B via switch 0 to the wiper motor. Because the gear is driven by the wiper motor via the worm gear, the contact plate will also rotate. The motor will continue to run until the notches in the contact plate reach the top again.
Switch in off position; wiper stops:
The gear continues to turn until the notches of the contact plate are at the top. This interrupts the contact between sliding contacts A and B. Sliding contact A is insulated by the (yellow) plastic gear, so no current can flow to sliding contact B. No current will then flow to the wiper motor either. When the contact plate is rotated far enough, sliding contact C also makes contact with the small conductive part of the contact plate. At this moment, sliding contacts B and C are connected to each other. Because C is always connected to ground, B now also makes contact with ground via the contact plate. The wiper motor is at this moment connected to ground on both sides, so that it comes to a stop immediately. This therefore actually works as a kind of brake. In this way, the wiper motor always stops in the same place.
Animation:
In this animation, the different positions of the switch and the contact plate can clearly be seen. Below is a brief summary of the explanation given above.
- switched off: the switch is in the zero position and the electric motor is short-circuited with positive and ground.
- switched on, constant speed: the switch is in position 1 and the contact plate makes two revolutions clockwise. In this position, the contact plate is not used.
- Switch position 0, rotating to zero position: the contact disc provides the power supply to the motor until the notches reach the sliding contacts.
- A-B (positive interrupted), B-C make contact. This has a braking effect on the motor, which then comes to a stop almost immediately.
When, for example, during a practical exam, wiring has to be connected, the correct positions of the switch must be determined. With the wiper motor diagram, it can be seen which pin in the connector is responsible for power supply, ground or the zero position. By measuring on which wire in the vehicle’s wiring harness 12 Volts is present, this one can already be connected. Using a resistance measurement, it can be determined which connection is the ground. The ohmmeter will indicate a resistance value of less than 1 Ohm at this connection. The negative lead must of course be held on a good ground point on the bodywork. By then moving the switch through several positions, it is possible to find out which wire belongs to which position of the switch. The diagram can then be used to determine which wires must be connected to each other.
From theory to the manufacturer’s diagram:
The theory of the rear wiper motor was covered in the previous section. In the diagrams it can be clearly seen how the contact plate in the cutaway wiper motor ensures that the motor receives power to rotate back to the starting position. This section explains how this diagram can be translated into a manufacturer’s diagram.
The electrical diagram below is based on the rear wiper motor of a Hyundai Getz. The wire colors (blue, brown, white and black) correspond to the colors in the car.
Numbers 1 to 4 in the diagram on the right and below show the pins of the connector with which the wiper motor is connected to the vehicle’s wiring harness. The numbers and wire colors in both diagrams correspond. The diagram below was retrieved from HGS-data.com. The rear wiper motor has the component code: M51.
In both diagrams it can be seen that the blue wire (pin 1 in the connector) is the constant positive wire from the fuse. The brown wire (pin 2) is responsible for returning to the zero position. In the diagram below, the contact plate is shown as a mechanical switch. The switched positive wire from the switch is connected to the white wire (pin 3). The black wire is the ground wire (pin 4) and is connected to a ground point on the body (G55).
In the rest position, the electric motor is short-circuited to ground; the white and brown wires are connected to each other via the contact disc.
Two-speed wiper motor:
So far, only the single-speed wiper motor has been discussed. This version is mainly used for the rear window. The wiper motor for the front window has two rotational speeds: a low speed for normal wiping and the interval, and a high speed for intensive wiping.
A single-speed wiper motor has one positive and one negative carbon brush. In a two-speed wiper motor, a second positive carbon brush has been added. Depending on the position of the wiper switch, one of the two positive carbon brushes is energized.
By connecting the supply voltage to a different positive carbon brush, the point at which the current through the armature windings is commutated changes. As a result, the Lorentz force acts at a different rotor position. Because the electromagnetic motor torque is equal to the Lorentz force multiplied by the effective moment arm (T=F*d), the electromagnetic motor torque that the electric motor can generate also changes.
At low speed, the positive carbon brush is used that produces the greatest electromagnetic motor torque. This positive carbon brush is located directly opposite the negative carbon brush. As a result, the balance between the electromagnetic motor torque and the load torque is reached at a relatively low speed.
At high speed, the diagonally placed positive carbon brush is energized. This results in a smaller electromagnetic motor torque. The motor therefore continues to accelerate until a new balance is reached between the electromagnetic motor torque and the load torque. The result is a higher steady-state speed.
The full physical explanation of the two speeds with three carbon brushes, including the Lorentz force, the electromagnetic motor torque, the counter EMF and the steady-state speed, is elaborated on the page Electric motor.
The diagram of the two-speed wiper motor is very similar to the diagram already discussed above. The wiper motor here is again positive-switched. Three switch positions can now be seen.
– Position 1: low speed, running continuously (or interval).
– Position 2: high speed, running continuously.
– Position 0: switch off, return to starting position (zero position).
In the diagram on the right, the first position is switched on. This is the low speed.
Here, position 2 is switched on. The motor now receives the positive voltage via a different carbon brush. There is now a lower counter voltage in the electric motor, causing the speed to be higher than when connected to the other carbon brush.
In this diagram, position 0 is selected. The motor is switched off, but first returns to the starting position. The contact plate connects sliding contacts A and B to each other so that the wiper motor still has a supply voltage. When the contact plate has rotated a further 180 degrees, the contact between sliding contacts A and B is interrupted, causing the supply voltage to be cut off.
The operation with the contact plate and the sliding contacts is the same as with the single-speed wiper motor.
In this situation, the contact plate has rotated again, so that sliding contacts B and C now make contact with each other. The motor is now connected to ground on both sides. In this position, the wiper motor will remain until it is switched on again.
LIN-bus controlled wiper motor:
The previously mentioned systems use control voltages that come from the wiper switch. In modern cars, control via LIN-bus is increasingly being used. The control unit operates the wiper motor. Multiple inputs, from both the switch (S) and the rain / light sensor (RLS), provide a signal to the ECU to switch the wiper motor (RWM) on, to let it wipe at a different speed, or to switch it off.
The diagram shows the components that control the wiper motor.
The switch (S) is connected to the ECU with the three green wires. The position of the switch is transmitted via these wires.
The switch therefore has no direct connection to the RWM, as was the case with the conventional control. The RLS receives its power supply from the ECU (12 volts), receives its ground via a ground point, and transmits its signal via the LIN-bus wire to the other connected components. The RWM is controlled by means of a signal on the LIN-bus. The control unit in the RWM (recognisable by the transistor symbol) provides the actual control of the electric motor.
With the conventional wiper motor, the position of the conductive contact plate ensured the movement to the zero position. In a LIN-bus controlled wiper motor, this contact plate has been replaced by a position disc and Hall sensors. The position of the position disc depends on the position of the plastic gear wheel, and therefore on the position of the wiper arm. The position disc is divided into a number of north and south poles (the N of North and the S of South). Because each north and south pole on the position disc has a different size, the control unit in the RWM can determine the exact position of the gear wheel by means of the Hall sensors. When the RLS or the switch ends the control of the wiper motor, the control unit in the RWM continues to operate the electric motor until the position disc has reached the “zero position”.
Advantages of this type of control are:
- PWM control makes it possible to run at different speeds.
- The direction of rotation of the electric motor can be reversed; when rotating clockwise, the wiper arms move upwards, and when rotating counterclockwise, the wiper arms move downwards. This allows for a smaller installation space for the wiper mechanism.
- The zero position can vary; by sometimes moving the wiper blades up just a tiny bit, the rubber of the wiper blade tilts the other way. The wiper blade now does not always rest on the same position on the windscreen. This has a positive effect on the service life of the wiper blade.
The LIN-bus signal can be measured with an oscilloscope. The displayed scope image shows the communication between the ECU (the master) and the rain / light sensor and the wiper motor (the slaves).
On the page LIN-bus the structure of a LIN-bus message is described. The communication of the wiper system is also described in detail and it is explained how faults in the LIN-bus signal can be identified.
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