| Scanner Diagnostics: Coolant-Temperature Sensor |
From "Weekend Mechanic"
episode WKM-204 |
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Check-Engine Light Diagnostics: Coolant-Temperature Sensor With the engine-position sensor checked and replaced, the next step is to check the second sensor detected as a potential problem by the scanner diagnostics -- the coolant-temperature sensor (figure A). The sensor has a sensing tip that extends into the coolant flow. The tip feeds information to the car's onboard computer about engine temperature, supply of fuel to the engine, when to turn on the cooling fan and (on some vehicles) transmission shifting. 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 scanner diagnostics indicate that some condition is preventing the sensor from properly supplying temperature information to the onboard computer. With the laptop hooked up to the car's computer, the scanner interface provides several methods for determining if the sensor is working. With the engine running, the graphic temperature readout on the laptop is monitored to see if the sensor is detecting change in engine temperature (figure B). The straight line on the graphic readout indicates that the sensor is not operating.
- An infrared digital thermometer is used to check the actual engine temperature (figure C). These sophisticated thermometers cost around $150, and are useful in that they allow you to target individual areas of the engine for checking temperature at specific locations. The rising temperature readout confirms that the coolant-temp sensor is not working.
- The next step is to disconnect the terminal from sensor (figure D) so that the sensor can be checked, using the DVOM, while the engine is running.
- With the DVOM set to the "ohm" setting, the leads on the DVOM are connected to the terminals of the sensor -- using caution that the leads don't touch each other.
- With the meter connected in this way, the readout --indicating resistance -- should change over time as the engine is running. The readout (figure E) shows that there is resistance, indicating that this sensor itself is good. The problem must lie elsewhere.
- The terminal that connects to the sensor is checked (figure F) to make sure that the contacts are not damaged.
- The wiring harness and wires that connect the sensor to the computer are also checked (figure G).
- Close inspection reveals that there is a break in the wiring harness and a frayed wire (figure H). Thus, the source of this problem has been identified. Judging from the melted plastic on the nearby thermostat-housing, it appears that the wires have come into contact with the hot metal surface and the insulation has melted away, causing the connection to fail.
In the segment that follows, the broken wire is repaired, and follow-up diagnostics are used to confirm the effectiveness of the repairs.
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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