June 24, 2020
WHAT YOU NEED TO KNOW BEFORE YOU REPAIR OR DIAGNOSE A VEHICLE
How many technicians have received a repair order which states the vehicle has a clunking noise? Or the check engine light is on? Or the air conditioning doesn’t blow cold air?
The important details—for example, the clunking noise only occurs when backing out of the driveway—are typically not included. Maybe the noise only occurs at low speeds over speed bumps, or the noise is coming from the right front. If the person who dropped off the vehicle isn’t the primary driver, a lot of this information may not be collected. The service writer might not have thought to ask those questions at the time, or it could have been an overnight drop off when no one was available.
In an ideal scenario, technicians would receive a ticket that states:
- The check engine light is on, and it came on several weeks ago
- The car has low power under high throttle conditions
- It feels like it won’t go above 70 miles an hour
Instead, technicians are in shops surrounded by problem cars, deadlines, tools that are inadequate or outdated, insufficient training and customers who don’t have the budget to fix issues. Sounds familiar?
WORKING FASTER AND ELIMINATING MISTAKES DURING THE REPAIR
Let’s go over a typical day with the typical repair order, and how you could move through the scenario with more speed and efficiency, and fewer mistakes.
The key is having a repeatable, step-based plan which includes repair assistance—because no one can be expected to know everything, and repair assistance is not a substitute for skills. Being too prideful to ask for help or consult a database in today’s environment doesn’t help fix cars.
As a technician, at times you are a detective collecting facts, a troubleshooter, reverse-engineering someone else’s mistakes, the parts guy, and a trainer for new techs. In short, technicians have a full plate, and help is simply another tool in the toolbox. Besides, sometimes a simple check engine light is not as simple as checking the engine.
EXAMPLE: 2004 VOLKSWAGEN JETTA
Let’s say we get a repair order for a 2004 VW Jetta, but we don’t have a lot of experience on European vehicles since our shop mostly works on domestic vehicles. But the clock is working against us, and the only thing we learn from the ticket is that the check engine light is on.
Scanning the Vehicle
First we grab our scan tool and pull out a code P0101 Mass or Volume Air Flow Circuit Range/Performance. It’s a pretty generic code because the range of the sensor can be affected by either a mechanical or electrical issue. Since we have the scan tool on the car, we can go ahead and start the car up to pull data out. We go into vehicle PIDS and pull up the mass airflow sensor data, which reads an unusual value MGH. This is actually milligrams per stroke, which is a term Germans use when providing mass airflow data on this diesel engine.
This creates another snag. Diesel engines work a lot differently than gasoline engines. There is no traditional throttle plate on this diesel engine, the intake manifold is wide open, and airflow is a byproduct of how much fuel is added and RPM and eventually boost. Simply put, the airflow increases as the engine gains RPM.
The other variable here is airflow, which is affected by how much EGR is added. In other words, adding more EGR will reduce airflow coming into the engine by the MAF sensor. At idle we read roughly 350 milligrams per stroke, and the car has a requested value that matches that. This helps inform us that the mass airflow sensor appears to be reading normally at idle and we can rule issues like the mass airflow sensor being dead, unplugged, or having damaged wiring. At this point we will need to perform a test drive.
Testing the Car
One thing worth mentioning is that you always want to respect the owner’s vehicle and drive it in a respectful manner. However, sometimes this may not be the best approach and you need to put the vehicle through a bit of stress to see how it behaves.
This vehicle seems to be operating normally, the values on the scanner line up, and at this point it doesn’t look like anything’s wrong. When we take it on the freeway to see what happens, we notice the vehicle doesn’t really want to rev past 3500 RPMs, and it seems lazy and sluggish. Up to that point it felt pretty decent.
Being unfamiliar with diesel does not help, but it’s indicating on the scanner that it wants a lot more airflow than we’re getting, so we cruise back to the shop and do more testing on the mass airflow sensor.
We open up the air cleaner and inspect the air filter to make sure it’s not completely plugged up. When we restart the car it doesn’t really change any of the values, meaning the engine seems to be breathing OK at idle and the airbox is clean.
At this point we’re at a crossroads. Do we go down the road of testing to see if the signal is breaking down and that’s triggering the corresponding loss of power? Or is there a mechanical problem that’s causing this? Considering we have little information about the car, and not a lot of experience with this vehicle, wouldn’t it make the most sense to go to Identifix?
By doing so, we can plug in the car to see what the common trends are. It’s likely we’re not the only person working on a 2004 Jetta right now, and this probably isn’t the only 2004 Jetta that has experienced a problem like this.
And remember—the clock’s working against us (and we’ve already been interrupted with different questions, wait repairs, and emergencies in the last 30 minutes).
The first line mentions checking for carbon build up in the intake manifold, which would certainly affect its ability to breathe.
With the hose off and a little light on the situation, we can see what it looks like in there.
Well… it’s going to be hard to get air through that wad of carbon. It looks like we will need to pull the manifold and do some serious cleaning.
Now we have a solid direction, as well as labor and parts for the writer. Nothing better than having a picture of the problem that a customer can clearly see and understand. And it is certainly better than breaking out a lab scope and troubleshooting a sensor circuit.
The Diagnostics and Repair Checklist
Remember, when diagnosing and repairing cars you should always have a process or checklist that you follow for every vehicle that comes to your shop. These four steps below should serve as your starting point as you build out a process that works best for you:
- Know what problem you are trying to fix and gather accurate information
- Have the necessary tools and information to diagnose/repair
- Consult a fix database to pull trends, tips, and diagnostic expertise
- Phone a friend before you get in too deep (Virtual Tech is in this with you!)
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