| Scanner Diagnostics: Throttle-Position Sensor |
From "Weekend Mechanic"
episode WKM-204 |
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Check-Engine Light Diagnostics: Throttle-Position Sensor Since the scanner diagnostics identified two problem areas -- the throttle-position sensor and coolant-temperature sensor -- the next steps are to inspect and check those sensors and their related engine components. We begin by checking the throttle-position sensor. Important: Included below are the steps in this automotive procedure as shown in the episode. These steps are general guidelines that are applicable to most vehicles. With any particular vehicle, there may be procedures, specifications, settings, tolerances, components, etc. that are specific to that vehicle. There are also variations according to the type and brand of repair kit you select. Always consult your vehicle's service manual when undertaking significant automotive repairs, and read and follow the manufacturer's directions and precautions that come with your kit or replacement parts.
- The first step in checking the throttle-position sensor is to unplug the sensor itself (figure A).
- The sensor-terminal has three separate wires connected to it, and three corresponding electrical contacts in the plug (figure B). The first wire is a low-voltage (5-volt) connector from the sensor to the onboard computer. The center wire is a signal-return from the computer. The last wire is a ground-wire, also supplied by the computer.
- First, a digital volt/ohm meter (DVOM) is used to check the voltage going to the sensor. One jumper wire is connected to the 5-volt side of the terminal, and one connects to the ground circuit. The leads from the DVOM are then connected to the jumpers. With the meter connected in this way, a reading can be taken on the meter. This tests for the amount of voltage being supplied to the sensor. If everything is functioning properly, a reading of about 5 volts (positive or negative) should be indicated on the digital readout (figure C).
Important: Be careful that the leads from your DVOM don't touch the engine. This could cause a short, resulting in inaccurate readings on the meter, or could cause a problem with your car's computer. Bend the jumpers so that they don't touch each other. - OHM is a measure of electrical resistance within a circuit. By switching the meter from the "voltage" to the "ohm" setting, the resistance to the ground side of the connector can be checked. One lead from the meter is connected to the ground side of the plug, and the other is connected to the negative terminal of the vehicle's battery (figure D).
- A reading of a small amount of resistance (figure E) is normal since the length of wire in the circuit itself can create a small amount of resistance. A high resistance-reading would indicate a problem in the wiring between the sensor and the battery. The reading on our meter indicated that the ground wiring was OK.
- The next step is to check the sensor itself. To do that, the sensor is plugged back in. Using the scan program on the laptop, the function of the throttle-position sensor can be monitored. While Bruce alters the position of the throttle, simulating use of the accelerator, his assistant Paul is able to check the graphic readout on the laptop screen. Spikes in the graphic readout (figure F) indicate a problem with the sensor.
- In this way, the problem has been narrowed to the sensor itself, or the throttle-shaft to which it's connected (figure G).
- The next step is to remove the sensor. The sensor-wire harness is disconnected, and the screws holding the sensor in place are removed using a small ratchet and driver-bit (figure H).
- The throttle is then manipulated manually (figure I) to ensure that it is working freely.
- The sensor is then examined closely. Two tabs inside the sensor (figure J) are what come into contact with the end of the throttle-shaft. As the throttle-shaft moves, it rotates the sensor. If the potentiometer inside the sensor breaks, the sensor becomes nonfunctional. This was the case with our sensor.
- The old sensor is replaced with a new one (figure K). This part costs around $60. Simply screw the new sensor in place, plug the terminal back in, and reconnect any hoses or connections that were removed to get to the sensor.
In the segment that follows, the coolant-temperature sensor is checked.
RESOURCES :
The Complete Idiot's Guide to Trouble-Free Car Care
Author: Dan Ramsey
ISBN: 0028635833
Alpha Books
How Cars Work
Author: Tom Newton
ISBN: 0966862309
Publisher: Black Apple Press
Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896
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