Building the better garage

In the second part of his look at how to put together your diagnostic service offering, Frank looks at how you should always work to a plan

Published:  14 November, 2022

The most important first step begins with vehicle and owner triage. Listen carefully, ask searching and relevant questions regarding the complaint, do not accept anything until you have confirmed the condition, and never accept previous work or opinion as correct. The customer must accept this cost or walk away. The triage may include, visual inspection, road test, or a preliminary global vehicle scan, i.e., all systems. None of this is free. It is part of a progressive methodical process. Agree a separate contract for this allowing either party to walk away.
    
One especially crucial point to understand before you begin any repair or diagnostic investigation, you must fully understand how the system functions and the specific responsibility of each component in that system, how it operates and how to test it.
    
Check DTCs that are relevant to the symptoms, not forgetting pending and confirmed errors in EOBD. Also check for incomplete default flags. These cannot be cleared unless all flag parameters have been satisfied during drive cycles.
    
Next, you need to cross-reference specified, actual, and corrected data. A fault code will not register unless the component parameters have been exceeded, in some cases for a considerable time, so fast intermittent drive concerns may not be registered in the fault memory. Previous experience over the past 50 years has convinced me of the value in using gauges when confirming, fluid, pressure, and flow.
    
For example, when testing fuel pump performance, flow is just as important as pressure. Also check the pump current. It is linear with pressure, therefore faults may be predicted by checking current across the relay or fuse without accessing the pump or supply hoses.

Intermittent variable vane turbo faults are easily monitored with a gauge. We could not source suitable gauges, so I designed our own. In fact, many of our tools have been modified to suit challenging tests.
    
Data log selected serial data so that focused analysis can be carried out. The selected items will depend on the nature of the fault under investigation. This can then be downloaded into graphing software like datazap.  If you are interested refer to my VW Amarok SCR repair article from the June issue.
    
There may be a technical bulletin or software update dealing with the complaint so access to the manufacturers repair information system is mandatory.

Component testing
It is at this point where component testing may commence, like each stage of an investigation there are rules that govern and guide your response. Before the output of a sensor is suspected, you must check the ground reference and power supply at the sensor. Output deviation can be caused by wiring errors, sensor error, or a genuine environment value error. It may be necessary to cross-reference the value by alternative means, where possible.
    
For example, with a cold vehicle, all temperature sensors will have a similar value, as will pressure sensors on a static engine. Exhaust gas temperature sensors will reduce by approximately 50°C as they pass further down the exhaust stream.
    
Sensors fall into set groups; Position, range or movement, temperature, pressure, angle etc.
They also fall into three output categories; Linear/analogue, digital, and sent. Because of the complexity in vehicle systems control, it is inevitable that an oscilloscope needs to be used to confirm correct functionality.
    
An oscilloscope, like all tools, fall into one of three groups; The good, the bad and the ugly. They demand two special skill sets; Set up and image interpretation. They provide a unique insight to mechanical and electronic functionality.
    
This brings me to current and ongoing problems: Accessibility, and the cost risk ratio in the diagnostic process. Many of the tools that can be used with a scope find their roots in other hi-tech industries.
    
Cylinder pressure analysis, WPS, is the best example. The use of an absolute pressure sensor directly in the cylinder reports real time pressure differential above and below atmosphere. With minimal component removal and the engine running, the precise valve open/close position can be established.
    
The catch here is fully understanding the image as correct. It may require confirmation from a good known vehicle. The other problem is variable valve lift and timing control. This will affect pressure readings and must be confirmed via serial data evaluation.

Complex
Vehicles manufactured today are a complex mixture of mechanical systems, all of which share one unique property; Mass, acceleration, and frequency. The latest technique in systems diagnosis is NVH, or vibration analysis. With the aid of a three-dimensional accelerometer and analytical software, each individual component can be identified by its frequency signature. Everything from a cylinder misfire to a defective bearing can be isolated.
    
Some of our more individual specialist tools include an injector test bench. This helps identify combustion imbalance from our vibration analysis. With the onset of direct drive turbo actuator control, we invested in an actuator drive simulator. Driving the wastegate through precise angles whilst monitoring the current draw confirms correct movement, range, and mechanical resistance.
    
It is occurred to me writing this two-part piece that several subjects identified would make good subjects for future articles, so watch this space.  It has also reminded me of the remarkable skills that automotive technicians need to repair and service vehicles. Have pride in your achievements and don’t work cheap!


Related Articles


Facebook


©DFA Media Group
1999-2022
Terms and Conditions