Mazda6 and Mazda3 Driveability Diagnostics, Electronic Throttle Control

Tech Feature: Mazda6 and Mazda3 Driveability Diagnostics and Electronic Throttle Control

Drive-by-wire throttle control systems are pretty much trouble-free but have to be taken into consideration as you perform routine service and diagnose problems, and it changes the way you service these cars when it comes to cleaning the throttle plates, advises Bob Dowie, import specialist contributor, who also covers electronic throttle control's tie in with ABS and VSC systems.

PESKY MISFIRES
As I mentioned earlier, misfires that are evident shouldn’t be a real challenge to diagnose. To protect the cat converter, misfires are monitored closely and it’s rare not to have the check engine lamp even blinking along with a solid misfire. Your first step should be to get the codes and other info the system is willing to share.

You would expect a misfire code directing you to the offending cylinder, but don’t overlook the other available information. An accompanying fuel-trim code, for example, is an important clue as to what could be causing the misfire and, if nothing else, it should have you looking for additional issues as the misfire is diagnosed. Don’t forget to check the freeze-frame information for conditions when the code was set. This is especially true if the car is currently running well and the misfire is intermittent.

Before the initial road test, look at the live data to see if the temps are accurate, whether the throttle switch is changing, if the air mass sensor is reacting as expected (and look at the long-term trim to see if the computer is adding or removing fuel to maintain the correct mixture), and see if short-term trim is close to zero. You should keep these things in mind as the diagnosis moves forward.

In recent years, Mazda has changed over to the coil-on-plug system to better control those catalytic converter-killing misfires. While I’m sure this system does a great job of dealing with misfires better than the old ignition wires, it is not without its problems. In many cases, the easiest way to confirm a coil problem is to switch it with an adjacent cylinder and confirm the misfire moves with the coil. Don’t forget to look at the condition of the spark plugs and it takes only a couple of minutes to test compression.

With extended service intervals, drivers tend to ­forget about routine maintenance. With that said, if the plugs are shot, chances are the car is in need of a service. Fix the misfire first, but don’t overlook the ­opportunity to recommend the rest of the service.

Our diagnostic and repair strategy changes a bit when you’re dealing with a six cylinder where it’s more labor intensive to access the coils. In the case of a solid misfire, before you dig into removing the manifold, listen to the injectors with a stethoscope to confirm they are working and sound similar. Also double-check for any severe vacuum leaks. Of course, either of these problems would result in a high positive fuel trim number. If we suspect a coil failure, we always recommend that all coils in that bank be ­replaced along with the spark plugs.

I know we’re all familiar with diagnosing and ­repairing misfires and it’s just our nature to get into the job and fix it with an “I know how to do this” attitude. But, in this day and age, it’s more important than ever that you get into the service information and online forums to check for service bulletins and recalls. You can spend a lot of time looking for that random miss at 1,500 to 2,000 rpm, only to find out that there was a reflash offered that took care of it. Make it one of the first steps, even if there are no bulletins, as I’m sure you’ll get some useful tips on the repair.

ELECTRONIC THROTTLE CONTROL
The latest technology from Mazda that’s finding its way to us is drive-by-wire throttle control. These systems are pretty much trouble free but have to be taken into consideration as we perform routine service and diagnose problems. It’s really a fairly simple system; a 12-volt motor opens the throttle as controlled by the PCM based on various inputs.

Along with the electronically controlled throttle body, an accelerator pedal assembly was added that mimics the feel of a mechanical throttle, yet houses the necessary accelerator pedal position sensors that tell the PCM what the driver is asking for. It’s easy to think that we replaced a simple cable-operated throttle with a complex electronic unit, but this has really simplified things, while providing engineers with even more options for vehicle performance, convenience and vehicle stability.

It changes the way we service these cars when it comes to cleaning the throttle plates. To prevent damage to the electronics, it’s important that we use very little solvent in the process. The best strategy is to apply the solvent to the shop rag or brush that’s being used in the cleaning process, with very little or no solvent being sprayed into the housing. If you have to use some solvent in the housing, avoid the shaft area where it could find its way into throttle actuator and position sensors.

One of the advantages of this system is its ability to open the throttle and maintain the target idle speed, even if the plate is dirty, eliminating the need for and the limitations of an idle speed control motor. That certainly doesn’t mean the plates should be ignored; it’s still important that they are cleaned to ensure smooth operation.

These systems have been very reliable. While there were some problems when the system was first introduced in 2003, most of those troublesome units were replaced under warranty, although we have recently seen one. The service bulletin is available in your service information, but the short description is if there is AA, AB or BA identifying designations on the throttle body, suspect a bad unit and replace it with a unit carrying a BB.

When it comes to diagnosing this system, a scanner with OEM capabilities will be a big help. As you would expect, there are codes related to every part of the system that has limp modes connected to ensure the safety of the vehicle. RPM may be limited to a predetermined point or the throttle body may be held to an idle speed until the codes are cleared.

Don’t be intimidated by all those wires on the throttle body. You have two wires that control the throttle motor, with the others dedicated to letting the PCM know what’s going on with redundant throttle position switches that are part of the throttle body assembly.

When looking at the position switches on the throttle body, be aware they work in opposite bias, using a 5-volt reference or input; switch one will read high voltage at idle (approx. 4.3V), while switch two will be low voltage (0.8V). Look for the opposite voltages as the plate moves toward wide-open throttle.

The PCM will make its decisions by looking at what the throttle is doing, along with the accelerator pedal position switches located in the pedal assembly; again these are redundant switches for reliability. These switches are different in construction being a non-contact Hall effect-type sensor rather than the mechanical sweep-type switch. They are tested in the same way by looking for smooth voltage output throughout the range of movement with no dropouts or spikes.

Both pedal switches read low voltage at idle and increase as the pedal is depressed. Be sure to check the specs as its normal for switch one to read half a volt more than switch two. There have been no reports of problems with the pedal assembly with most of the failures leading to throttle body replacement.

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