Technically Speaking: Diagnostics

James Berry Feb 9, 2013 0 Comment(s)

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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