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Using the Scan Tool for Maintenance, Part 1

One of the buzzwords in the automotive industry is “prognostics.” It comes from the Greek words pro, meaning “before,” and gnosis, meaning “to know.” The idea is to recognize and anticipate a problem before a component failure leads to a system or vehicle failure. It is another step in our industry’s efforts to take full advantage of the computer capabilities we are gaining each year.

The advantages of this kind of system are at once obvious and compelling. If we could be warned that the alternator bearing or the fuel pump winding was about to fail, we could schedule the repair at our convenience, rather than at the convenience of the tow truck operator.

Rather than waiting for prognostics to come down the pike from the manufacturer, is there anything we can do to accomplish a similar result? The answer is “yes.” We all have in our shops a great little tool called the scan tool which we use to communicate with the vehicle computers. Scan tools can read data and codes, freeze frame records, and much more.

First, let’s look at data. There are many computers in the average vehicle: the PCM (Powertrain Control Module), the BCM (Body Control Module), the ICM (Instrument Cluster Module) and many others. On most modern vehicles, they are linked in a CAN (Controller Area Network) bus system. This provides for a series of networked computers with (usually) a primary, or a server and several client modules multiplexed together.

By adding a scan tool to the network, technicians can monitor these modules by means of a data stream and, in certain cases, command various output devices for diagnostic purposes. Let’s take a look at an example of what a scan tool can tell you.

Most of us know that when the “Check Engine” light or MIL (Malfunction Indicator Lamp) comes on we need to have our car checked out by a competent technician. What most people do not realize is that the MIL is unrelated to performance or fuel economy. It is a federally mandated item that is simply and only tied to emissions. If the MIL is on, the PCM believes that federal tailpipe emission standards are being exceeded by one and a half times. If the MIL is flashing, the emission standards are being exceeded by enough to damage the catalytic converter.

This principle of operation, along with the fact that computers are “adaptive” (i.e., they modify their commands based on what they learn from feedback during engine operation), means that a failure can be well along the road to occurring without any symptom or indication that anything is wrong.

One of the primary adaptable items (or parameters) in the PCM is fuel control. The PCM monitors the amount of air entering the engine, usually by means of an MAF, or Mass Air Flow sensor. It then turns the injectors on for a calculated amount of time. This “on time” is referred to as PWM (Pulse Width Modulation). Using the oxygen sensor(s), the exhaust is monitored so that the PCM can respond if the fuel amount is too much or too little.

The PCM then adapts the PWM to the injectors to provide a proper air to fuel ratio for low emissions and proper performance. If the injectors start to clog, the PCM will recognize that it is taking more “on time,” or “pulse width,” to have the proper effect on the oxygen sensor. It can adapt and increase without the driver even being aware that anything is happening. However, it can only adapt so far.
Two sets of fuel control adaptives exist. One is called STFT (Short Term Fuel Trim) and the other is LTFT (Long Term Fuel Trim). STFT is a rapid response to minor and momentary variations. LTFT is a slower response to more chronic changes. If the fuel injectors start to clog, the STFT will add pulse width to make up the difference. As they get worse, the STFT will not be able to make enough change to keep the engine in fuel control and within emission standards. When this happens, the LTFT begins to add fuel.

The LTFT is usually retained even when the vehicle is shut down to provide a starting point for the STFT when the engine is next started (Both are erased whenever trouble codes and computer memory are cleared). There is, however, a limit to how far the fuel trims can go (both rich and lean). Once it exceeds a certain amount, the MIL will illuminate to alert the driver that the PCM may not be preventing harmful emissions from exiting the tailpipe.

Let’s say the injectors have a little leakage. This sends more fuel into the intake stream than the PCM intends. The oxygen sensor will then report that to the PCM, and adjustments will be made to shorten the PWM signal to the injectors. Or, if the fuel pump is losing efficiency, the fuel line pressure may drop, and the PCM will again have to increase PWM to maintain fuel control. On a Ford product with a duty cycle-controlled fuel pump, the duty cycle can be maxed out and still not supply enough fuel to keep the STFT and LTFT within design parameters.

One warning on using the scan tool for data: If the PCM detects a problem that leads to a code being set, it can substitute a default value in place of the problem detected. The government allowed this in the interest of minimizing tailpipe emissions. The key is this: You can access engine data from a manufacturer specific or a generic side of the scan tool.

If you use the manufacturer specific side, you may be lied to by the PCM. If you access data from the generic OBD2 side, you will get the straight story. The government mandated this in the interest of allowing the tech to get accurate information to repair the vehicle. The importance of understanding this point cannot be overstated.

By using a scan tool at each service, the technician can monitor the data stream and look at, among other things, the fuel trims. If the LTFT is at more than 15% and the STFT is at more than 13%, he knows that there is something going on, either with the injectors, the fuel filter or possibly the fuel pump. Each vehicle’s idiosyncrasies will have to be learned by the tech to recognize the kinds of problems that can be foreseen by this method. In this way, further diagnosis can be accomplished while the vehicle is still in the shop, thus saving everyone time and money.

Next time, we will look at Diagnostic Troubleshooting Codes (DTCs).

Kevin Roberts is the president of Roberts Repair in Rhinelander, WI. The company has specialized in emergency vehicle maintenance since 1989. Roberts is an ASE Certified Master Automobile and Master Truck Technician. He can be reached at

Published in Police Fleet Manager, Nov/Dec 2009

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