Class D bus with an air-brake pressure loss and an out-of-date tachograph calibration
A 2016 Class D bus (two-axle, 18,000 kg GVW, full air brakes, 53 seats) is presented for its CVRT. The driver mentions that the low-pressure warning buzzer 'sometimes comes on at idle'. On test you find: with the system charged and the engine stopped, pressure falls from 8.0 bar to 6.4 bar in 60 seconds with the service brake released; the low-pressure warning operates at 4.2 bar; the compressor cuts back in at 6.8 bar and cuts out at 8.1 bar; the tachograph calibration plaque shows the last calibration was 26 months ago; a rear marker lamp is not working; and one emergency exit door opens but the audible warning does not sound when the vehicle is moving. The presenter says the bus is needed for a school run in two hours and asks which items can be 'left for a retest later'.
1. Assess the air-brake pressure-loss finding against the standard and state the consequence for the test result.
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Pressure loss. With the system charged and the engine stopped, a loss from 8.0 bar to 6.4 bar in 60 seconds is 1.6 bar per minute with the service brake released. That is far in excess of the permitted leakage - the standard allows only a small drop over a much longer period (roughly a fraction of a bar per minute for a released service brake system). A drop of this magnitude indicates a leak in the system and is a major defect.
Consequence. The vehicle fails. Because the defect affects the braking system, it is a dangerous/major defect and the vehicle must not be used on the road until repaired. It cannot be 'left for later'.
Related checks. The cut-in at 6.8 bar and cut-out at 8.1 bar are within the normal governor range, and a warning at 4.2 bar is plausible, but the leakage is the controlling finding. Note that a low-pressure buzzer 'sometimes at idle' is consistent with the leak - the system cannot hold pressure.
Result: Refuse the certificate; record the air leak as the major defect.
2. Deal with the tachograph, marker lamp and emergency exit items, and answer the presenter's question about deferring items.
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Tachograph calibration. The plaque shows 26 months since calibration. Calibration intervals are set by regulation (typically two years for this type of vehicle), so this is overdue and is a defect. It does not affect the mechanical roadworthiness of the bus, but it is a testable item and must be recorded. It cannot be deferred.
Rear marker lamp. A lamp not working is a defect. Depending on its position and function it may be a minor or major item, but it must be recorded and rectified. It cannot be deferred.
Emergency exit audible warning. An emergency exit that opens without the audible warning sounding while the vehicle is moving is a safety defect - passengers could open an exit in motion. This is a major defect and must be rectified before the vehicle is used.
Answer to the presenter. There is no mechanism for 'leaving items for a retest later' in the sense he means. The test produces a result; defects are recorded; the vehicle is either passed or refused. A refused vehicle may not be used on the road (certainly not for a school run) until the major/dangerous defects are fixed and the vehicle is re-presented. The school run commitment is a commercial matter for the operator, not a reason to release an unsafe bus. Advise him that all recorded defects must be rectified before re-presentation, and that the air leak alone makes the vehicle unfit for service.