Customer’s description
When applying the footbrake, the brake pedal appears hard to press intermittently whereby the customer has to apply more effort in order to bring the vehicle to rest. Upon the next brake pedal use the feel of the brake pedal and the effort required to stop the vehicle returns to normal. The symptom had been very intermittent regardless of temperature/road condition and had been evident for over two weeks.
Technical description
Although the symptom was described as “intermittent” by the customer, during my road-test I was alarmed at just how pronounced this symptom became!
When creeping (using the footbrake to hold the vehicle against the auto transmission) in slow moving traffic I noticed the feel of the brake pedal became hard, with additional effort required to hold the vehicle stationary via the footbrake.
Whilst holding the vehicle stationary, I then noticed the vehicle begin to creep forward (even with the footbrake applied) where I then had to literally stand on the footbrake to stop the vehicle and switch off the engine! Here we suffered from a complete failure of engine assisted braking.
Ironically after restarting the engine the operation of the brake pedal returned to normal and I managed to limp back to the workshop. I think it goes without saying that the vehicle could not be driven until this fault has been rectified (much to the dismay of the customer!).
Diagnosis
With the customer complaint verified, the vehicle’s ID and specification were confirmed. The customer interview confirmed the vehicle had full service history with no accessories installed and no previously related repairs.
The basic inspection confirmed that visible brake servo pipework was secure and routed correctly, and that the brake fluid level was normal with no leaks. The engine oil level was also inspected and found to be normal. This may seem an odd inspection for brake assistance failure, however when we consider the vacuum required for the brake servo is generated by an engine driven vacuum pump, it becomes quite relevant.
It is at this point we check for Technical Bulletins (software updates, initialisation, recalls & campaigns etc.). This is often a step in diagnosis that is missed, as it is very easy to get ahead of yourself during an investigation. On this occasion, we struck gold.
A little research soon revealed Land Rover had recalled a number of Range Rover Sport vehicles for the possibility of engine oil entering the Brake Servo or, sticking of the non-return valve in the closed position at the brake vacuum pump after a cold start.
Bulletins and known issues must be checked before diagnosis begins as it is mighty embarrassing to discover the fault you have been chasing for days is a known issue rectified by a software update or modified component (speaking from experience)!
Armed with this knowledge a quick check at our local Land Rover centre confirmed the recall mentioned above had been actioned and no further attention was required. We therefore must approach this concern as an unrelated fault following the necessary test procedures.
The description and operation data was referenced for in-depth knowledge of component functions. This knowledge is imperative when diagnosing any system and highlights the need for continual research and training.
Servo operation check
Following the typical operational check for a brake servo revealed an interesting result:
The usual test is to switch off the engine and drain the servo of any vacuum by repeatedly pressing the brake pedal until the pedal’s resistance increases and its travel is reduced. With the brake pedal pressed firmly down, re-start the engine and note how the pedal begins to creep or yield and pedal travel increases. This indicates normal servo operation, and for us this test was passed!
An additional test is to allow the servo to become fully primed with negative pressure, with the foot brake pressed switch off the engine. Maintain pressure on the brake pedal (applying a fixed pressure) and note any change in pedal travel. The pedal should remain fixed in this position if there are no vacuum leaks.
This test failed as the brake pedal began to return to the rest position, opposing the pressure I was applying requiring additional pressure from the driver (brake assistance was lost due to vacuum leakage).
The nature of such a safety related fault commands a thorough diagnostic approach where we can categorically confirm the component responsible for the failure whilst confirming other associated components remain serviceable.
Before diving in with this one I wondered how we could qualify what appeared to be a Servo failure using PicoScope. How about measuring Pressure against Time?
Luckily the Range Rover Sport utilises a quick-coupler within the relevant pipework between the servo and vacuum pump allowing for the insertion of the WPS500X pressure transducer.
