Would you like to diagnose more vehicles first time?

John Batten explores the one area of diagnosis that could stop misdiagnosis forever

Published:  03 April, 2019

As we reach March, 2019 is well and truly underway. In fact by the time you read this one third of the year will have whizzed by never to be seen again. Now, I’m not one for New Year’s Resolutions (they’re so last year), but I am the type of chap that likes constant progress when it comes to developing a technician’s career.
    
There’s so much to be said for small steps taken everyday that on first look appear don’t appear to make a difference, but when gazed back upon over a 12 month period have a staggering affect on your capability to diagnose a vehicle first time, in a timely manner.

Pitter-patter of tiny feet
Small steps are all well and good but where do you start? After all, you don’t know what you don’t know, and you’d like to start your journey to diagnostic success off on the right foot. In this instance I’d start with the end in mind and reverse engineer the outcome you desire. It’s a logical process that works, and can be replicated time and time again in your diagnostic routine.
    
Your ‘end in mind’ in this instance is a vehicle where the fault no longer exists, that won’t appear back across the threshold of your workshop anytime soon. But how do you guarantee that?

One test to rule them all
I love nothing more than when the delegates working through our training programs have a technical epiphany. This happens at many points on their path of learning, but none more than with bypass testing.
    
Bypass testing is step nine in Johnny’s diagnostic circle of love (our 15 step routine), and often the key element in the first time fix. The good news for you is that it doesn’t require mythical creatures to forge their magical powers into an object that only one technician can possess. It’s something that every tech can learn, and become a diagnostic wizard.

What is bypass testing?
Quite simply it’s fixing the vehicle before you fix the vehicle. Let me explain.
    
Wouldn’t it be great if you suspected that a Mass Air Flow sensor was at fault and you could prove that you were right before you fitted a new part, or spoke to the owner of the vehicle. If you could do that then the positive effect it would have on you and the business you work for would blow you away.
    
Picture this: Your customer has reported that the vehicle is low on power. You’ve diligently questioned them, experienced the problem with them on a road test, and the bought the vehicle into the workshop.
    
You’ve pulled codes and found none present, followed by taking a look through serial data to hunt for diagnostic clues. It doesn’t take you long to identify that the MAF sensor frequency looks a little low at 1.5 Khz and your fuel trim data is incorrect and making a positive corrections. You’ve seen a bunch of these before and know that 1.85 Khz is a suitable value for this vehicle.
    
You’re keen to prove that the serial data is leading you in the right direction so confirm the sensor output with your oscilloscope. The oscilloscope frequency mirrors that of the serial tool and your starting to get that warm fuzzy feeling that an you’re onto something.

Steady the buffs
You’ve been close to success before though, only to be thwarted in the final moments so you’re keen not to be caught out twice. You know that documenting the reasons that the MAF output could be incorrect is the way to go, and duly make a list of tests required to confirm your theories.

  •  Faulty MAF Sensor
  •  Restricted Inlet
  •  Restricted Exhaust
  •  Leaky Inlet System
  •  EGR open
  •  Etc, etc…


With your extensive list of tests in hand, and your pants on the outside of your trousers (is there a diagnostic superhero?) you systematically work through the test routines and come to the conclusion that it must be the air mass meter.
    
So, what next? Time to talk to the customer and order the new part?

Not quite.

Where the magic happens!
You have another option and that is the Bypass test. Quite simply you manufacturer a circuit, or apply a sensor simulator to emulate the signal of a Mass Air Flow (MAF)  sensor. Setting the frequency of your bypass device to 1.85Khz will simulate a good input (on this vehicle) to the PCM at idle.
    
With the correct input in place you’re now in a good position to re-inspect serial data and make sure that the fuel trim and any other erroneous values and now back in line with normal parameters. You find that they are and give yourself a well earned pat on the back.
    
Not only that but you’re now in a great place to call the customer, confident in the knowledge that when you fit the new MAF you’ll have fixed the vehicle and not to have to make any of those “well, it’s like this...It needed the MAF but...” phone calls.
    
Here’s the really cool part. Bypass testing isn’t just for MAF sensors. So long as you understand fundamental electrical theory, can use an oscilloscope, know how a component and system functions electrically and mechanically, then you’ll be able to build a circuit or choose the right bypass test too safely confirm you diagnostic theories.

Just one word of warning!
It’s addictive! The feeling you get knowing that when you bolt a component onto a car that you’ll be right in your diagnosis is hard to give up once you’ve experienced it. You’ll have created a monster and be spending your downtime devising new techniques to guarantee the fix.
    
This has an incredible upside though. As your knowledge builds your time to diagnose drops. In fact Batten’s Law states that time spent bypass testing is inversely proportional to your average diagnostic time. What’s not to like about that. Not a lot!
    
If you’d like to learn how to hone your diagnostic skills and see if Batten’s Law stacks up then give John a call at Auto iQ on (01604) 328500. We’re here to help you be the best technician you can be. You’ll have to bring your own pants though…
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