For tire pressure, we shared a Maserati case in the post: How To Fix Maserati Quattroporte Tire Pressure Light On? And in this post we will share a repair case of BMW tire pressure loss. Following are details:
“Type pressure loss, stop carefully” message pop up while driving.
1. Use Autel MaxiTPMS to test TMPS sensor, for tires are 2.2bar, the sensor is normal.
2. Connect Autel MK908P with car via OBD port to scan the car, no error code read.
3. Reset tire pressure, drive test, appear “tire pressure loss” message again, check the tire pressure again, still normal with 2.2bar.
4. Here review the TPMS knowledge: tire pressure monitor system RPA: whether the tire loses pressure by comparing coaxial wheel speeds.
5. RPA analysis: Wheel speed sensor signal is inaccurate? Incorrect tire pressure? Uneven braking force on four wheels? Differential gear causes inconsistent left and right wheel speeds? DSC analysis error?
6. We have ruled out tire pressure at above test. Connect the diagnose computer with car for a road test, and check the value of the wheel speed sensor when driving straight:
Left front wheel: 58 right front wheel: 54
Left Rear wheel: 55 right rear wheel: 50
7. The speed of 4 wheels is different, RPA can't recognize the pressure loss of a certain tire, so it prompts 4 wheels of pressure loss.
8. Check the wheel speed sensor and bearing target wheel and find no abnormalities.
9. When inspecting the brake system, it was found that the right rear brake disc was abnormally worn. It may be that the right rear wheel brakes are slightly engaged, causing the vehicle to shift to the right, so the right side of the wheel speed data is lower than the left side.
10. Remove the right rear brake disc and find that when the brake is not stepped on and the handbrake is not pulled, the right rear wheel has high resistance to rotation, and the static resistance of the left and right rear tires is measured with an electronic torque wrench.
11. The wrench shows the right rear wheel brake is tight. Is it caused by the handbrake motor or the caliper? First analyze the working principle of the rear brake:
12. Swap the left and right handbrake motors and measure the tire rotation resistance again after installation. The result is that the right rear wheel has a large braking resistance, indicating that the fault point is on the piston of the brake caliper.
13. Replace the right rear brake caliper assembly, the test drive is normal without fault, read the wheel speed sensor value:
Front left: 50 Front right: 50
Rear left: 50 Rear right: 50
If we understand the working principle of the system, then we can solve the problem quickly.
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