Challenging current techniques

By Frank Massey | Published:  18 July, 2017

Frank Massey looks at how you need to always keep an open mind on diagnostic methods

Accepting my reservations, I always believe in challenging any current techniques with an open mind with the possibility that a better method may exist. My reservations are based on the following reasons.

Reservations

Electronic measurements are based on sensor signatures compared with known confirmed values. Many engines now employ variable valve lift and timing. Added to this complexity, often there is no mechanical locking of sprockets to shafts. Confirming actual mechanical camshaft position regarding the crankshaft would require sensors monitoring all camshafts and all sprockets.

With exception of highly advanced engines this is not the case. So how can we predict actual valve functionality before opening the spanner drawer?

Functionality

Recently we filmed a detailed diagnostic process with Pico exploring a reliable method. Determining electronic plausibility is relativity easy. The answer to actual valve operation is reliant on accurate pressure evaluation in the cylinder. This must be overlaid in real time with the timing sensor signatures. There is still an element of error as you will need to calculate camshaft rotational angle against the crankshaft profile. The Pico software helps by converting the oscilloscope base line into the 4-stroke rotational position. So far so good, but can poor mechanical condition be predicted more easily? Well yes it can, remember the vacuum gauge. Volumetric efficiency is directly proportional to correct valve position.

Hang on a moment; Where should the valve position be? It’s not simply a function of a chain or timing belt. The computer now has an opinion on optimum timing angle. It’s not that easy anymore. However, I do support the premise of predictive evidence.

Boxter

Let’s visit an actual event in our workshop from very recently; a Porsche Boxter 3.4 supported with Bosch Motronic med control. With no obvious cause or warning it developed a drop in power and throttle response. No definitive DTC. You didn’t think I was going to make it that easy for did you?

Serial data confirmed two key symptoms. Misfire count and rich fuel trim more than 20% on one bank only. It is a low mileage car with no obvious signs of poor care. Removing the spark plugs confirmed excessive soot as the symptoms of incomplete combustion. Note my words carefully. Further examination excluded the unlikely cause as poor ignition energy. Back to basics, the vacuum gauge confirmed too high pressure in the intake system specific to the offending bank. The next action was to attach the pressure transducer to each bank in turn comparing the pressure profiles.

Direct measurement

To achieve this, direct measurement from each bank is required. Running the engine with the pressure sensor directly in one cylinder from each bank confirmed a differential in pressure between banks. It must be a mechanical problem then. The next decision is more intuitive than evidence driven as this engine has variable valve timing adjustment via electro-hydraulic control.

Serial data could not provide actual camshaft position data though, as the sensor is only motoring the sprocket position. This is the very point I made earlier. We could get clever and monitor control duty cycle and current through each actuator. Unfortunately, the computer does not monitor a problem, so there will not be any correction made.

Complete restoration

Examination of the service history confirmed long life servicing. This prompted us to flush the engine and replace both hydraulic camshaft control actuators. This resulted in a complete restoration of performance; obviously, we reset the adaption values to fuel trim, throttle angle and misfire count.

As is often the case, a combination of hard evidence and common sense prevailed. I often say to entry level diagnostic students, the most complex problem in diagnostics is you.

Further information
Please contact Annette on:
01772 201 597 or email enquiries@ads-global.co.uk for further information on upcoming training courses and events.

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    Just imagine how many functions require wheel speed and rotational differential data.

    ABS, dynamic stability, hill start, audio volume, navigation, self park, all wheel drive, active steering assist, electronic handbrake etc. Sharing this data on a high speed can network ensures very accurate vehicle motion dynamics.

    Older variable reluctance sensors (VRS) rely on a coil generating an alternating voltage when rotation occurs. The problem is they are not directional sensitive and cannot report motion at very low speed. Air gaps were critical as they affect signal amplitude. They are often referred to as passive sensors. So, the introduction of digital or active sensors was inevitable.


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    The ground or return signal value will vary between 0v or 1.4/1.8v. You could have a sensor or circuit fault; let me try and explain the subtle differences, and how to prove which is which. Remember the golden rule if in doubt compare a wheel circuit that works normally.

    First unplug the sensor and measure both circuits in the loom. With no load applied the supply voltage should jump up to NBV

    Next check the ground circuit if its true ground then it’s a pull-down type and the signal will be on the power line, and may only be around 200mv

    If a small voltage exists then it’s a pull up type and the signal will be on this wire not the supply. The digital signal will be very small when the wheel rotates. It could be small around 200/400mv, or as high as 0.5/1.8v, depending on the manufacturer variant

    Common sense would dictate the serial route is easiest, however how would you determine an intermittent fault? It could be a faulty sensor, faulty encoder, or a circuit error. The only way is using a scope. Should we measure voltage or current though? Both change in the circuit. Unless you have a very special current clamp, go for voltage and select a AC coupling.

    The specific question I am often asked is current measurement, well I can tell you in a pull-down circuit its around 7-15 ma with a 400mv voltage change. The pull up type will produce around 6/13ma with 0.2/0.35mv.     However, these voltage values can vary due to the value of the two parallel internal sensor resistors these are normally 1.4k ohms, with a much higher resistor in the meg ohm range, within the ABS pcm.

    I hope this helps. The pico image was taken from a VW Golf 1.4 TSI. The easy bit is replacing the wheel sensors. Ever since metal housings were replaced with plastic they never corrode in the housings
    do they…?

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