GF40.15-P-0001GM Tire Pressure Monitor Function
GF40.15-P-0001GM Tire pressure monitor function
- with CODE (470) Tire pressure monitor (base)
- with CODE (475) Tire pressure monitor (premium)
Function requirements, general
^ Circuit 15 On
^ Tire pressure monitor (TPM [RDK]) functional
TPM [RDK], general information
The TPM [RDK] system assists the driver in checking the tire inflation pressure thus contributing to active safety.
The TPM [RDK] system monitors the air pressure in the individual tires. A previously stored specified pressure is used as a comparative value. In the event of a relevant pressure loss a warning message is issued to the driver.
The tire pressure measuring range extends from p = 0 to 5.5 bar.
The TPM [RDK] comprises the following component functions:
^ Tire pressure sensing function sequence
^ Warning message function sequence
^ Manual restart function sequence
^ Automatic fill detection function sequence (except code (494) USA version)
^ Learn mode function sequence
Tire pressure sensing function sequence
During a journey the TPM [RDK] system continuously monitors the tire pressure of all rotating wheels (with circuit 15 OFF run-on time of t = 20 min).
During a journey the left front TPM [RDK] wheel sensor (A69/1), right front TPM [RDK] wheel sensor (A69/2), left rear TPM [RDK] wheel sensor (A69/3) and right rear TPM [RDK] wheel sensor (A69/4) transmit data telegrams every t = 60 s.
These data telegrams contain the pressure, tire pressure temperature, individual serial number and status information (wheel acceleration, operating mode etc.).
The data are received by the central tire pressure monitoring antenna (A2/97) and forwarded straight to the TPM [RDK] control unit (N88).
For vehicles with code (475) Tire pressure monitoring (Premium), trigger pulses are generated via the left rear TPM [RDK] wheel antenna (A2/46) and the right rear TPM wheel antenna (A2/47) for rear wheel position finding.
This allows wheel-specific pressures to be displayed. The position of the front wheels is first detected when cornering using the steering angle and the different wheel speed values at the front. In addition, the TPM [RDK] control unit incorporates the outside temperature and the vehicle speed in the warning logic. The data are sent over interior CAN (CAN B).
The steering angle is recorded by the steering angle sensor (N49) and transmitted by the steering column tube module (N80) to the TPM [RDK] control unit.
The vehicle speed is transmitted by the instrument cluster (A1). The outside temperature is recorded by the outside temperature indicator temperature sensor (B14) and forwarded by the AAC [KLA] control and operating unit (N22) (without code (581) Comfort automatic air conditioning) or from the comfort AAC control and operating unit (N22/7) (with code (581) Comfort automatic air conditioning) to the TPM [RDK] control unit.
Any warning or information messages are shown in the multifunction display (A1p13) in the instrument cluster (A1).
Warning message function sequence
A warning is issued when a relevant loss of pressure has been detected in at least one of the tires.
The warning does not specify which tire is losing pressure on vehicles with code (470) Tire pressure monitor (standard).
The warning does specify which tire is losing pressure on vehicles with code (475) Tire pressure monitor (premium).
The driver receives the feedback via the instrument cluster's multifunction display or via the tire pressure monitor warning lamp (A1e66).
The warning messages are made up of the following subfunctions:
^ Soft warning function sequence
^ Hard warning function sequence
^ Leak detection function sequence (rapid pressure loss)
Soft warning function sequence
The warning is issued immediately after a trip is ended (circuit 15 OFF).
Tires lose air over the course of time through diffusion.
This is why convenient feature provides the driver with a message in good time about the need to correct tire pressures.
Warning threshold:
- Pressure loss of p greater than 0.25 bar
If the tire pressure of at least one wheel is continuously under the warning threshold for t = 12 min during a trip, a warning signal is set, stored in the memory of the TPM [RDK] control unit and displayed in the instrument cluster at the end of the trip (circuit 15 OFF).
If the tire pressure increases again to at least p = 0.1 bar under the stored specified value, the warning message is suppressed.
Warning cancellation:
- Value is no longer below warning threshold
- Successful automatic fill detection (except code (494) USA version)
- Manual restart
Hard warning function sequence
While driving the hard warning informs the driver of a significant pressure loss which is relevant to safety.
Warning threshold:
- Pressure loss of 20 % or p ≥ 0.5 bar
In the event of a pressure drop below the warning threshold, a warning message is displayed in the multifunction display after t = 5 min at the latest and the tire pressure monitor warning lamp is actuated. If the pressure loss is very high (p a0.2 bar under warning threshold), the warning message is issued after max. t = 1 min.
if a leak is detected (air extraction p greater than 0.2 bar/min) the driver is notified with a message "Warning! Tire defect".
The tire pressure monitor warning lamp is actuated together with a hard warning, which is repeated during the next ignition sequence. Warning cancellation:
- Successful automatic fill detection (except code (494) USA version)
- Manual restart
Leak detection function sequence (rapid pressure loss)
If progressive pressure loss is detected during a certain period of time while driving, it is assumed that a tire is defective. In this case a warning is triggered, even before the triggering criterion for the hard warning is reached. The warning is then issued at the latest t = 120 s after the start of a pressure loss of p = 0.2 bar/min and is shown as a warning message (red) "Warning! Tire defect" on the multifunction display.
Warning cancellation:
During an ignition sequence, the warning message can be canceled by successful filling detection or by a manual restart. When the engine is switched back on after a leak warning message, the tire pressure monitor warning lamp in the instrument cluster is initially activated.
If after the start of a journey new pressure values exist and no warning criterion is met, the tire pressure monitoring warning lamp is switched off and the warning message is cleared. The warning message is canceled through a successful fill detection (except code (494) USA version) or a manual restart.
Manual restart function sequence
When restarted, the tire pressures set by the driver are adopted by the TPM [RDK] control unit as the new specified values.
The left multifunction steering wheel button group (S110) and the right multifunction steering wheel button group (S111) can be called up by the TPM [RDK] menu in the instrument cluster. The submenu for restarting the TPM [RDK] system can be accessed via the trip odometer reset button (A1s3).
The signals are read in by the steering column tube module and transmitted to the instrument cluster via the engine compartment CAN (CAN C). The pressure thresholds for the warnings are determined on the basis of the pressure values at the time of the restart.
Operation feedback is issued immediately over the instrument cluster.
The TPM [RDK] control unit filters the new pressure values for t = 3 min as of the start of the journey and checks them for plausibility.
The smallest pressure value on each axle is then stored as the new specified value for pressure monitoring.
With new wheels or a longer parking time of t greater than 20 min first the separate wheel detection and axle assignment must be completed via the TPM wheel sensors.
A new TPM wheel sensor is monitored after driving for approx t = 10 min.
Automatic fill detection function sequence (except code (494) USA version)
Automatic fill detection is performed if a change in pressure of more than p = 0.2 bar was detected on at least two of the wheels. Temperature-compensated and filtered pressure values can be used here for comparison.
The new reference values are stored if the plausibility check is passed.
Learn mode function sequence
The learn mode is comprised of the following subfunctions:
^ Function sequence for determination of new specified pressures
^ Function sequence for wheel position identification (with code (476) Tire pressure monitor (premium))
^ Function sequence for learn mode in workshops
Function sequence for determination of new specified pressures
Manual setting of new specified pressures is performed by manually restarting the TPM [RDK] system in the instrument cluster.
On vehicles with code (475) Tire pressure monitor (premium), the specified pressures are determined by axle.
On vehicles with code (470) Tire pressure monitor (standard) the two higher pressures and two lower pressures are each assigned to an "imaginary axle".
Function sequence for wheel position identification (with code (476) Tire pressure monitor (premium))
Following a parking period of at least t = 20 min the TPM [RDK] control unit activates the left rear TPM [RDK] wheel antenna and the right rear TPM [RDK] wheel antenna once a journey starts. First the left trigger signal is transmitted to the left rear TPM wheel antenna until a corresponding radio telegram is received as an answer from the left rear TPM wheel sensor, then the right trigger signal from the right rear TPM wheel antenna. The procedure is repeated two more times. This allows the TPM [RDK] control unit to identify the position of the rear wheels by identification number. There are no wheel antennas mounted on the front axle. The right left assignment is determined with the aid of the radial acceleration transferred by the left front TPM wheel sensor and right front TPM wheel sensor.
When cornering, the wheel at the outside of the curve rotates slightly faster than the wheel on the inside. Position identification of the wheels on the front axle can thus be completed when the vehicle drives through a number of curves.
If a spare tire is present, it is not monitored.
On vehicles with code (475) Tire pressure monitor (premium), after new wheels are mounted the pressure picture is displayed as soon as position identification of the rear wheels is complete. The position of the front wheels is not yet reliably identified at this point and is corrected later.
Function sequence for learn mode in workshops
The learning process in workshops or follow-up operations by entering the identification number takes place with the Diagnosis Assistance System (DAS).
When working with DAS the "initial start-up" routine should be followed. There are two variants: "Initial start-up with unknown wheel IDs" and "Initial start-up with known wheel IDs".
Working with DAS or CANDI: New wheel sensors can be taught in with the routine "Check for WU Remeasure Transponder". With this routine, it is possible to choose between teach-in with Trigger tool MB2000E or teach-in via pressure change. This routine is started via diagnosis and is identical for all wheel positions. The respective TPM wheel sensor must be triggered when teaching in with the trigger tool. As a result of this the TPM wheel sensor transmits a telegram from which the identification number is read out.
For teach-in via pressure change, the pressure at the corresponding wheel must be changed by p = 0.3 bar. The wheel sensor then transmits telegrams with the changed pressure value (delta_p Frames) to the TPM [RDK] control unit. The identification number is read out from these telegrams.
The result is queried with the service "Check for WU Remeasure Transponder: Request results WU D". The identification numbers determined must then be written to the respective position by the operator in diagnosis service download "Transmitter IDs" (Left Front, Right Front, Right Rear and Left Rear).
OK must also be written to the service download "Axle/Side Location Status".
The TPM [RDK] control unit checks the plausibility of the identification numbers and only permits one identification number with the same number.
