Front axle braking without trailer attached

(Fig. 12.41(a-d)) When the semi-trailer is disconnected from its tractor the electronic control module responds by energizing the pressure-limiting valve solenoid. This results in the solenoid valve closing the direct by-pass passage leading to the proportional relay valve and opening the valve leading to the relay valve within the pressure-limiting valve unit (see Fig. 12.41(c)). This results in the solenoid valve shutting-off the front axle reservoir tank air supply from the proportional relay valve and at the same time re-routing the air supply via the pressure-limiting valve's relay valve which then reduces the maximum braking pressure reaching the proportional relay valve and hence the front axle brakes.

Limiting the air pressure reaching the front axle of the towing vehicle when the trailer is removed is essential in retaining the balance of front to rear axle braking power of the now much shorter overall vehicle base, thereby maintaining effective and stable vehicle retardation.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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