The brake booster vacuum switch (F190) informs the DIGISWING control unit whether the pressure in the brake system is satisfactory.
Switch closed: below 0.7 bar (vacuum pressure too low). Measuring block status: "1"
Switch open: above 0.7 bar (vacuum pressure OK). Measuring block status "0"
If the switch closes (vacuum pressure too low), the engine is off, and the vehicle is moving, the engine is immediately restarted to restore vacuum to the brake circuit via the mechanical exhauster.
If this switch is closed, the priority shift valve is closed and the clutch and brake circuits are separated, the engine will not switch off.
Incorrect information exists in the SSP regarding this switch. If this switch fails, engine control running lamp K111 does not illuminate, however, the engine is still prevented from switching off.
Observe safety precautions!!
Test procedure without diagnostic equipment:
-Start engine and leave ecomatic mode on
- Find a slight downhill gradient or open stretch of road which is safe to carry out the procedure below. Important! If the vacuum switch is faulty you will need to push the brake pedal harder to stop the car!
-Allow vehicle to start rolling down the gradient with engine off
-Keep pressing brake pedal on and off.
-As soon as the brake pedal starts to feel ever so slightly more harder to press, the engine should start up and restore brake vacuum.
-If the brake pedal starts to feel much harder to press before the engine restarts, or the engine does not restart at all after servo assistance has gone, it is possible the switch is worn and is not operating at specification, or otherwise not working.
Part number: 1H0 919 825
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