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Technically Speaking: Diagnostics

James Berry Feb 9, 2013

Q: I’ve been working at a repair shop for about six months. Recently, my boss let me start doing some diagnostics, using a diagnostic flow chart as a guide. Unfortunately, my results have been mixed. Are there any books dedicated to diagnosing fault codes? Do you have any suggestions for improving my diagnostic skills? One vehicle that has been in the shop for quite a while is a C4 with a “Throttle Position” code that no one can seem to fix. I would like to repair this car to gain some confidence, but I don’t know where to start.

Via email

A: I’m afraid there are very few praiseworthy publications for diagnosing fault codes. Most suggested diagnostic procedures fall into one of three categories, or a combination of the three. The first method involves a step-by-step diagnostic flow chart. Once you have acquired the fault code, this procedure takes you through a series of questions. Each answer directs you to another question, until you reach the most likely cause of the problem. Unfortunately, if you answer even one question incorrectly, you’ll end up with the wrong diagnosis, or multiple recommendations.

The second method involves simply replacing the part indicated by the fault code. For example, if the fault in question indicates a throttle-position-sensor problem, just replace the throttle-position sensor.

The third method involves checking the Internet to see what other people have done to repair a similar problem. This is actually a good idea if you use the information you find as a tool, and not your sole troubleshooting source. (Yes, it’s true: Not all of the information on the Internet is accurate.)

The problem with the first two methods is that they don’t help you learn how a system works. This is evident in other areas of the auto-repair field. For example, when it comes to failures in components such as alternators and starters, many of today’s automotive technicians do not know what, precisely, has failed. All they know is that something is wrong, so they replace the part. Older automotive technicians generally know how subcomponents such as alternators and starters work, because when they were younger, they actually rebuilt them. This experience often makes them better at diagnosing problems with such items. Knowledge, after all, is power.

There is a simple solution. Instead of learning how to follow a flow chart, try to learn the theory behind how a system works. Even if you still need to refer to a flow chart for help, knowing how the system you’re diagnosing functions will help you understand why you’re performing the specific tasks specified in the chart. This will ultimately make you a better technician.

The obvious benefit of this type of learning is that you only need to learn a few basic concepts in order to become proficient.

Research common failures: The first step in this technique is to check for any manufacturer Technical Service Bulletins (TSBs). This is also a good time to refer to the Internet to see what other people have done to repair a similar problem. There are also several pay sites that are excellent reference points, such as Alldata and Identifix.

Research how a system works: You’ll need to know how the system you are working with is supplied power, and how much. Also, what result is that system is trying to achieve?

Find your jumping off point: This is the point where you begin your diagnostics. The best jumping-off point is the most common failure item, as indicated by your research. By testing this device, you can determine whether it has failed, and which path you’ll need to take next.

Perform your diagnostics: Let’s begin by taking a look at a few of the sensors the engine controller uses to determine how it should perform in a given situation. A sensor is simply an input device that monitors driver inputs as well as engine temperatures, fuel mixture, and other parameters.

Sensors use an electrical signal supplied by the engine controller to help determine demands. Most sensors work in one of four ways:

1. Sensors can act as variable resistors (voltage diverters), signaling the engine controller of a change in position. Think of these sensors like a dimmer switch in your home: As you move the dimmer switch, it allows more or less voltage to travel through. In turn, the lights become brighter or dimmer. An example is the throttle-position sensor (TPS). The TPS monitors throttle inputs using an internal variable resistor and voltage readings, then sends them to the engine controller.

2. Sensors can act as on or off switches. Think of these sensors like a light switch in your home: They’re either on or off. An example of this sensor is the brake switch. Either the brakes are applied or they aren’t.

3. Sensors can act as low voltage producers, similar to a microphone, which uses a piezoelectric generation to produce an electrical voltage signal from mechanical vibration or pressure. An example of this is the oxygen sensor, which acts like a miniature generator and produces its own voltage.

4. Sensors may act as thermistors, which are devices that change resistance as temperature changes, much like a thermometer. An example of this is the coolant-temperature sensor (CTS).

Let’s perform a diagnostic procedure on your C4 TPS problem without using a scanner. Your research will likely indicate that the TPS is the most common failure. Now we’ll use the TPS as our jumping-off point and try to understand how it works.

Your first step, when working with anything that may be controlled by a fuse, should be to test all of the fuses with a test light (Image A). First, turn the key to the On position. Probe the back side of each fuse; there are two places to check on each one. If the test light does not illuminate on both sides of the fuse, that fuse is most likely blown. If the test light doesn’t illuminate on one side of the fuse or the other, you’ll need to look in the owner’s manual to see what that circuit feeds. Certain circuits will require that you activate something to illuminate the test light, such as turning on the headlamp switch to energize that particular fuse.

The engine controller sends a constant 5-volt reference voltage to the TPS variable resistor (Image B). You’ll need to test for proper input voltage with a Digital Volt Ohm Meter (DVOM). All the tests at the sensor must be performed with the TPS connected to the wiring harness and the key turned to the Run position.

If the reference voltage is 4.5 to 5 volts, the engine controller is capable of supplying voltage to the TPS. This tells us that the engine controller and reference-voltage wiring are not faulty.

If there’s no reference voltage, you’ll need to refer to a wiring diagram so you can back-probe the reference-voltage wire at the engine controller. If there are 5 volts coming from the engine controller for a reference signal, you’ll need to ohm-check the reference wire from the engine controller to the TPS. You’re looking for a cut or burned wire.

If there is no voltage coming from the engine controller, the controller is most likely faulty.

Check for signal voltage (return voltage). This is the voltage returning to the engine controller from the TPS (Image C).

You’ll need to test for proper return voltage with your DVOM. All the tests at the sensor must be performed with the TPS connected to the wiring harness and the key turned to the Run position.

The voltage with the throttle closed should be approximately 0.4 to 0.85 of a volt. As you slowly open the throttle, the voltage should steadily rise, until it reads 4.5 to 5 volts at WOT. If it does, the TPS is working correctly.

If the voltage drops to 0 and then returns to normal during the test, or jumps around erratically, this could signify a faulty TPS. Some sensors will read properly when the engine is cool, then fail at operating temperatures. As a result, this test may need to be rerun while the engine is hot. Tapping on the TPS to simulate vibration could produce a failure.

You’ll then need to check for a ground at the TPS (Image D). This is best done with the DVOM in ohms mode.

For testing purposes, let’s say there is no ground. Thinking through the process, you should realize that the next logical step is to check the wiring and whatever source is supplying the ground.

I hope the foregoing information will convince you to use common sense and the process of elimination as alternatives to simply following a flow chart. Once you understand how a system works, the diagnostic process becomes much less confusing.

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at Be sure to put “Technically Speaking” in the subject line.



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