You May Now Be Seated

March 15, 2012 | By | Reply More

Power seats have been around for decades. In early applications, a simple set of switches provided power and ground to DC electric motors. Today’s memory seating, however, requires some sophisticated electronics.

Who’s In Charge?

power supply

You may have checked fuses, but that does not mean adequate voltage has made it to the control unit and seat motor. On this E38 chassis, the power distribution point for the motors (the R/Br wire) has been immersed in water and corrosion has created a voltage supply problem.

In the early ‘90s, BMWs were equipped with manual or electric power seats. Higher-end models such as the 7 Series had memory seats as an option. This required a control unit to make the necessary logic decisions, and BMW opted for a combination Seat/Steering Column Memory Control Module (SM/LSM) to perform this function on early- to mid-’90s vehicles. However, it is not the only module controlling memory functions.

There is a matrix of modules working in concert to control the movement of the side-view mirrors, steering column and, of course, power seats. The SM/LSM controls the memory functions for the seats and the steering wheel. The power mirror memory functions are stored in their respective door modules.

The memory switch input is wired to the driver’s door module. The modules communicate the memory selection via a P-Bus to the SM/LSM. The door modules and SM/LSM operate the various motors to move them into their set positions. The motors are solid-state controlled. They have a power supply, a ground and a single control wire that carries a square-wave signal from the SM/LSM. This signal positions the motor. You can use a scope or graphing multimeter to monitor the signal.

Evolution Of The Breed

Door Control Corrosion

Sometimes the first step in the diagnosis is the most critical. If the power seats are working fine, but power mirrors and other memory functions are not, remember that the switch signals may pass through a door control module. This one shows signs of water damage.

Fast forward to the E46 chassis and you find the more things change the more they stay the same. Control units still control most memory features, but there are now two modules. One is the Driver’s Seat Memory Control Module, and the other is for the passenger side, and they are essentially identical. If replacement is required, you do not have to code for the driver’s or passenger’s side application. Harness pin #13 will inform the Seat Memory Control Module if it is mounted on the driver’s or passenger’s side. On the passenger’s side, pin 13 is grounded; on the driver’s side it’s left open. If the wire in pin 13 on the passenger’s side were broken, the circuit would be open and the K-Bus line would not be able to differentiate between the driver’s and passenger’s seat. As a result, neither seat may operate as designed.

Some BMW Cabriolet models are equipped with the Easy Entry feature. You can determine if a vehicle has this option by looking


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Category: Electrical, the bimmer pub

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