Subscribe to the Free
Newsletter

Shop Class: The Basics of Vehicle Repair Troubleshooting

Troublesome Troubleshooting: Breaking down some common breakdowns

Alex Steele
Aug 2, 2016
Photographers: ASE Master Technician
The ability to accurately diagnose car and truck problems separates the men from the boys. Whether it’s a suspension noise, misfire, late transmission shift-point, or a complex electrical failure—dealership technicians typically have the upper hand. Their manufacturer training, extensive experience with the specific brand, special tools, and unlimited sources of information make the difference.
That’s the reason owners and general repair shops often use the brand’s service department as a last resort following extensive time and expensive “parts swapping” trying to troubleshoot an atypical failure.
Now don’t get us wrong—top dealership technicians can get stumped too, often due to intermittent, hard-to-duplicate conditions. And new car buybacks do occur on rare occasions, where the vehicles get shipped to engineers in a final attempt to correct the problem.
Faults occur bumper to bumper. Here’s a breakdown of common diagnostic categories.
Photo 2/6   |   Noise Diagnostic
Noises
Some noises technicians love ($), others they hate. As with diagnosing anything, experience plays a major role. Say a truck comes in with a ticking noise from the engine when hot. For a technician who’s heard it many times before, and familiar with the common camshaft failure on this particular engine, it’s a no brainer. Supply a quick estimate, complete the repair, and out-the-door it goes.
Same truck, same noise, same failure, being handled by a tech with lesser experience and knowledge, it can become a big deal. There may be extensive time involved, potential misdiagnosis, or an unnecessary replacement of the entire engine.
Engine/Transmission
Let’s say that the technician does not immediately recognize that same ticking noise. First step, with help from the manufacturer’s service information program, would be performing a quick search for “noise” under the specific model and year. There may be a technical service bulletin or service news article dealing with the problem.
Being an uncommon engine noise—and with no specific service information to work with—it’s time to play doctor. The best method is the use of an automotive stethoscope attempting to narrow down the area of the noise. This can eliminate external sources causing the sound, such as a rattling engine mount, loose component bracket, etc. Noise gone following removal of the drive belt tells us it’s a belt driven component causing the problem (AC compressor, alternator, belt tensioner, etc.).
When there is no applicable service information, the sound is confirmed to be internal, and the general area of the tick is confirmed, engine teardown will be the next step. This is when a thorough visible inspection and measurements of suspected engine parts are completed to determine the exact cause of the failure.
Photo 3/6   |   Suspension Issues
Suspension/Chassis
Technicians deal with squeaks, rattles, creaks, pops, and so on. These noises can occur during various conditionsL over bumps, making turns, specific speeds, etc.
Of course, if you’ve heard the complaint before, it’s a gimmie—or at least a quick diagnosis following a visual inspection of the suspect component.
Once service information has been checked, it’s an unfamiliar complaint and a visual inspecting comes up blank, we want to find the most vulnerable situation to duplicate the condition. The best scenario is duplication “in shop.”
If we’re working on a squeak from the right rear over bumps, and note that we can duplicate the noise by jouncing the suspension up and down with the vehicle at a standstill, that’s a homerun. Have one tech jounce and the other tech pinpoint the squeak with a stethoscope while the sound occurs. The squeak may be a loose bushing, bad strut, worn/dry ball joint, etc.
The toughest cases cannot be duplicated in the shop and often require the use of electronic microphones placed in strategic positions throughout the chassis/suspension. This allows the tech to hear the noise while driving, and pinpoint the area by noting at which microphones the sound is most pronounced. It sounds easy, but it is not.
Body
Body noise issues usually involve body creaks and wind noises.
Body creaks, similar to suspension noises, can occur while turning, braking, accelerating, or traveling over dips in the road. But the causes are often points where body sheet metal panels meet, and are welded together. The body of every vehicle will flex small amounts while driving. This flexing can cause improperly welded or positioned panels to rub against each other and creak. Once the area is confirmed, through methods similar to diagnosing a suspension noise, the fix is typically repositioning and/or lubricating the specific body joint. Yep, sometimes a good shot with a hammer or a pry bar is all it takes.
Wind noise can be caused by an air leak or turbulence around external body parts. Diagnosis involves narrowing down the point of origin. This can be done by taping off small body sections/seams one at a time while road testing. Noise gone? That’s the spot. Projected powder or smoke can be helpful locating air leaks, including confirmation of tight contact at weather strips. A leaf blower can also be a valuable tool simulating on-road wind conditions in-shop.
Repairs involve parts replacement (mirror, external trim, weather strips) and body alignment and sealing (doors, body panels, glass).
Photo 4/6   |   Cooltech 34788
Heating, Ventilation, and Air Conditioning (HVAC)
We can breakdown HVAC diagnosis into three basic categories: electronic testing, pressure analysis, and finding refrigerant leaks.
Electronics can vary from blower operation, compressor clutch and cooling fan function, and temperature, mode, and recirculation door actuator performance. Some conditions can be diagnosed with simple testing of circuits, relays, fuses, and components, while others require a scan tool to confirm correct control module incoming data and outgoing commands.
Pressure analysis requires refrigerant recovery, recycling, and recharging equipment. Each system has a high-pressure and low-pressure service valve. While viewing abnormal pressures on both sides we can determine a weak A/C compressor or a blockage elsewhere in the system. Sometimes the complete contamination of a system (Black Death) will require the replacement of, well, everything.
Refrigerant leaks are a common reason behind A/C performance issues or a complete lack of operation. Some can be visibly detected by signs of refrigerant oil at the point of the leak, while smaller leaks require the injection of ultraviolet refrigerant dye or the use of an electronic leak detector. Common sites are hoses, O-rings, or damage from road debris at the condenser behind the grill.
Photo 5/6   |   Engine Problems
Powertrain Performance
Men from the boys, remember? The difficulty diagnosing engine performance problems is due to the large number of systems working in conjunction and therefore having effects on each other during a failure. An engine misfire, even though a diagnostic trouble code was set specifying which cylinders misfired, can be caused by faults in a number of areas. Drivability concerns, without a DTC to help lead us in the right direction, may be even tougher to troubleshoot.
Again, experience with the condition and applicable service information are your best friends. Once the technician road tests and duplicates the complaint, the diagnostic scan tool comes into play.
Scan tools are becoming more and more complex, and therefore more helpful with the increasing amount of data and electronic testing available.
Example:
We have a customer complaint of a check engine light and a rough idle on a late-model V-6 engine. A scan tool shows diagnostic trouble codes P0300 (random misfire), P0301 (misfire cylinder #1), P0302, P0303, etc.
Right off the bat we know the fault is something that’s having an effect on all cylinders—fuel pressure, contamination, vacuum leak, and so on—and not a bad sparkplug, injector, or ignition coil, all of which would be isolated to one cylinder.
Now we observe all the data fields available. There may be more than 100 inputs, but training and experience helps us determine which fields are relevant to the condition and where to look first.
Data Fields (engine hot, running, at idle):
• Misfire counts: indicates moderate misfire at all cylinders
• Short-term Fuel Trim: .82
• Long-term fuel Trim: .83
• (Fuel trim below 1.00 indicates a rich condition, above 1.00 is lean)
• EGR (exhaust gas recirculation) valve command: 0 percent
• EGR position sensor: 1.67 volts (5 percent)
These specific powertrain control module data inputs mean a lot of nothing to most folks, including a few technicians. However, to others they clearly indicate a simple case of a partially stuck-open EGR valve.
• Current misfire counts confirm the problem is still present, in agreement with the misfire DTCs.
• Fuel trim is a PCM command for fuel injection and ignition timing, in correspondence to oxygen sensor data, attempting to maintain an optimum air/fuel ratio (14.7:1). Short term is an immediate response to changing conditions, and long term follows suit when the condition remains for a longer period of time. Bottom line is we have a rich condition (either too much fuel, or not enough oxygen) and the PCM is commanding less fuel to compensate.
• EGR valve command from the PCM at 0 percent (fully closed) at idle is correct. The valve opens and bleeds exhaust gas into the intake manifold only during open throttle conditions, in order to lower combustion temperatures and reduce harmful emissions (NOx).
• EGR valve position at 5 percent (or above 1.20 volts) tells us the valve is stuck open. Normally the position would correspond to the command of 0 percent (closed at idle).
The fuel trim is reacting to a rich condition indicated by the oxygen sensors. The EGR valve stuck open at idle sends exhaust gasses into the combustion chamber when it shouldn’t. The low-oxygen-content exhaust taking up space, instead of oxygen-rich outside air, reduces the needed amount of oxygen for combustion and leads to a rich condition, rough idle, misfire, etc.
All this may seem a bit confusing, but common sense for a lot of professionals. The fix: replace the EGR valve, clear the DTCs, recheck misfire, fuel trim and EGR data, and road test to confirm the repair.
Photo 6/6   |   Engine Wiring Problems
Electrical
Wow, where do we start? Bluetooth, crash avoidance, theft deterrent, electronic power steering, and the list goes on. Again, training, experience, and service information play vital roles. But knowing the theory behind how each system functions (along with a clear understanding of Ohms law) is just as important for accurate testing.
The mandatory tool is a digital multimeter, which measures voltage, resistance, and amperage. Additional special electronic tools are also necessary when diagnosing advanced components and systems.
Reprogrammable software is now becoming a big part of the automotive landscape. Updates distributed by the manufacture to correct bugs in many electronic control units are coming out on a daily basis.
Another advantage for dealership technicians in all types of diagnostics is access to the automaker’s techline. Here they can contact technicians and engineers for support. Along with their expertise, they have a database of atypical complaints and repairs, which are otherwise unavailable. Techlines can also provide the status of part modifications, recalls, and technical service bulletins currently in the works. It’s not uncommon on late-model vehicles where a complaint is verified, but the fix is under review or development, leaving customers in a holding pattern.
We’re just scratching the surface of what’s really involved in automotive troubleshooting. There are all kinds of sub-categories and off-the-wall complaints that drivers and technicians deal with every day, such as various leaks, vibrations, transmission performance issues, and all kinds of intermittent faults. Sometimes even normal characteristics of a vehicle are seen as a failure by customers.

POPULAR TRUCKS

Subscribe Today and Save up to 83%!

Subscribe Truck Trend Magazine

Subscribe to:

Truck Trend
Magazine

PRINT DIGITAL
Subscribe Diesel Power Magazine

Subscribe to:

Diesel Power
Magazine

PRINT DIGITAL
Subscribe Truckin Magazine

Subscribe to:

Truckin
Magazine

PRINT DIGITAL
SUBSCRIBE TO A MAGAZINE
CLOSE X
BUYER'S GUIDE
SEE THE ALL NEW
NEWS, REVIEWS & SPECS
TO TOP