Retractable rear head restraints (RHR (KAF)) function - GF91.16-P-0001FL

MODEL 212.0/1 

Fig 1: Retractable Rear Head Restraints Function Block Diagram - Model 212.0/1

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• No overvoltage or undervoltage

Retractable rear head restraints, general 

By lowering the rear head restraints, the driver is able to create an unimpeded view through the rear window as needed.

The rear head restraints can only be raised by hand.

The retractable rear head restraints system comprises the following subfunctions:

• Function sequence for head restraint lowering 
• Function sequence for temperature protection 
• Function sequence for fault storage 

Function sequence for head restraint lowering 

To lower, the lock of the rear head restraints is released by means of the following motors:

• Left rear head restraint seat adjustment motor
• Right rear head restraint seat adjustment motor
• Center rear head restraint seat adjustment motor

The rear head restraints are lowered by their own weight. After activation, there is a break of t = 3 s before the head restraint seat adjustment motors can be actuated again.

The head restraint lowering comprises the following subfunctions:

• Function sequence for head restraint lowering by means of releasing the rear seat backrest (with code (275) Memory package for electrically adjustable front seats and code (287) Through-loading feature) 
• Function sequence for head restraint lowering via rear head restraint button 

Function sequence for head restraint lowering by means of releasing the rear seat backrest (with code (275) Memory package for electrically adjustable front seats and code (287) Through-loading feature) 

The release of the rear seat backrest is recorded by the left rear backrest lock switch or the right rear backrest lock switch. The status of the respective rear backrest lock switch is read in directly by the rear SAM control unit. This then actuates the rear head restraint seat adjustment motors directly.

If the status of the left rear backrest lock switch and the right rear backrest lock switch is no longer recognized during this actuation, the actuation is completed.

Upon folding over the left or right rear seat backrest, the left rear backrest inclination switch (S54/5) or right rear backrest inclination switch (S54/6) is switched from locked to unlocked. The respective backrest inclination switch in the rear uses the inclination to recognize whether the backrest is lifted or folded over. The head restraints in the rear are lowered accordingly and the driver seat or front passenger seat moved forward. If the rear seat backrest is not folded over and not unlocked, a corresponding message is shown in the multifunction display (A1p13) of the instrument cluster (A1).

Additional function requirements for head restraint lowering via rear head restraint button 

• Terminal 15 ON
• Rear backrest not folded over (with code (275) Memory package for electrically adjustable front seats and code (287) Through-loading feature)

The electronic ignition lock control unit sends the status of circuit 15 via the interior CAN to the rear SAM control unit.

Function sequence for head restraint lowering via rear head restraint button 

The status of the rear head restraint button is read in by the front SAM control unit via instrument panel LIN and sent via interior CAN to the rear SAM control unit. This then actuates the rear head restraint seat adjustment motors directly for t = 600 ms.

Function sequence for temperature protection 

To protect the rear head restraint seat adjustment motors against overheating, after actuation a counter is increased by one and after t = 7 s decreased by one, irrespective of further requests. As long as the counter value is in range of 0 to 20, requests are permissible. If the counter reaches the maximum value, the next actuation is not possible until after t = 7 s at the earliest.

Function sequence for fault storage 

The following faults are recognized and stored in the fault memory:

• Malfunction due to short circuit to ground
• Malfunction due to short circuit to positive or due to an open circuit

These faults are recognized after the rear head restraint seat adjustment motor is actuated 10 times. As a result, the head restraints in the rear are not lowered.