System Description
General
The power seat is available as a factory-fitted option. It can be installed instead of the standard seat on both driver and passenger positions.
The power seat has three memory buttons for storing three different seat positions.
1992-1995 cars have an earlier type of on-board diagnostic (OBD) system.
From 1996 models onward, an improved on-board diagnostic (OBD) system has been introduced.
NOTE: When replacing the control module with a new module always check that the new control module has P/N 9174904 (left seat) or 9174905 (right seat). The earlier type of control module has a different type of diagnostics. The older versions operate correctly with the different seat functions, but their diagnostic system cannot communicate with the Volvo Scan Tool (ST).
The control module located under the seat cushion operates four different motors in all. Three of the motors are located under the seat, the fourth is located in the backrest.
The control module with memory receives information on the seat position from signals from four position sensors. These are potentiometers driven by a worm gear on the output shaft of each motor.
Seat controls
General
There are six control buttons - the seat squab control shaped like the seat squab, the seat backrest control shaped like the seat backrest and the tour memory buttons.
Pressing more than one control or memory button at the same time will result in the seat coming to an emergency stop.
Functions
- The control shaped like a seat squab controls the seat height at the front and rear and moves the seat backward and forward.
- The control shaped like a backrest controls the backrest angle. The lumbar support is adjusted manually.
- The three memory buttons, which are marked 1, 2 and 3 are used to store and recall seat adjustment settings. To move the seat into a pre-set and stored position, hold a memory button down until the seat has reached that position. If the button is released while the seat is moving to the stored position it will stop moving immediately.
To store a seat position:
- Operate the power seat controls until the seat is in the position required.
- Then press the programming button, marked MEM and keep it pressed in.
- At the same time press one of the memory buttons and the seat position is now stored in the control module memory.
Motors and potentiometers
Motors
The four motors control the following functions:
- Motor 1 moves the seat backward and forward.
- Motor 2 controls the backrest angle.
- Motor 3 controls the height of the rear of the seat cushion.
- Motor 4 controls the height of the front of the seat cushion.
Potentiometers
The potentiometers (5) act as position senders and pro. vide the control module with feedback information on the position of the seat. They are driven by a worm gear located on the output shaft of each motor.
The resistance in the potentiometer is proportional to the output shaft position.
Control module
General
The control module is located underneath seat. The control module has a microprocessor and a number of relay functions, a dual relay for each motor and a main relay for the main power supply.
Seat positions are stored in an EEPROM type memory. This type of memory stores information indefinitely, even if there is no power to the control module.
Safety functions
Power to the relays and electronics is controlled directly by the control buttons. When the seat is not being operated all power to the motors is cut to ensure that the seat stops moving.
If more than one button is operated at the same time, the seat will stop. The only motors which can be operated simultaneously are those controlling the seat cushion height.
If an open-circuit, short-circuit to ground or supply occurs in the leads to the motors or sensors, this will be detected when trying to operate the motor because the sensor signal will not change. The motor concerned is then disabled.
If a button is held down for longer than 60 seconds, the power is cut to the main relay.
This function is controlled by a timer circuit in the control module.