NOTE: This test requires a brake dynamometer. See TESTING ON BRAKE DYNAMOMETER .

Connect "T" plug with adapter "A" to ABS wiring harness connector. Connect "T" plug with adapter "B" to ECU. Connect service tester lead to adapter "A". Red light should come on.
Place rear axle on dynamometer. Ensure tester flashes REAR L/R. Turn ignition on. Check voltage between leads No. 1 and 10. Voltage should be greater than 11.5 volts. If necessary, idle engine.
Turn dynamometer on for both wheels. Apply brakes until dynamometer reading is about 450 lbs. (204 kg), and charge axle if necessary. Dynamometer display should be about 450 lbs. (204 kg) for front and 337 lbs. (153 kg) for rear.
Display should not drop more than 90 lbs. (41 kg) for front or rear brakes. Press ABS test button. After about 2 seconds, hydraulic pump will come on and control valve will be opened for about 4 seconds. Control valve will then move to retention phase.
Display will start to run. When 3 seconds are displayed, press test button. Display reading should drop to 112-214 lbs. (51-97 kg) for front brakes and to 90-191 lbs. (41-87 kg) for rear brakes.
With pedal force unchanged, display must not change by more than 67 lbs. (30 kg) within 3 seconds. If nominal values are not reached, check battery voltage and repeat TESTS 5, 6 and 7. Check hydraulic unit. Repair or replace components as necessary. See Fig 1 and Fig 2 .

Fig 1: Hydraulic Pressure Drop (Reduction) Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Test 9 - Hydraulic Unit Pressure Build-Up