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CHAPTER 6 THERMISTORS PAGES 1 6

A thermistor is variable resistor that reacts to temperature changes. There are two main types, positive and negative temperature coefficient resistors (PTC and NTC). In this chapter we will be focusing on the NTC type which is the design used for all of the temperature sensors on a car.

Once you learn the operation of these, testing them is universal and can be applied to every single thermistor out there. Even outside of the automotive industry.

Some examples of thermistors being used on the car: engine coolant, intake air, battery, transmission fluid, ambient air, in-car, EGR, cylinder head, hi and low sides of an AC system.

Topics covered

  • types of thermistors
  • description and operation of 1,2 and 3 wire temp. sensors
  • which wires can be shared with other sensors
  • introduction to the 5v regulator 10:25
  • thermistor circuit operation
  • sensor ground circuits
  • ROM look-up tables 22:23
  • identify opens and shorts in a thermistor circuit
  • dual circuit (stepped) thermistor operation
 
Chapter 6 Thermistors pages 6 7

 

Most people think when resistance rises, voltage drops. Is there ever a time where a rise in resistance can cause a rise in voltage? This is what is being discussed in this video and is key in understanding what is actually happening in a thermistor circuit.

Once you understand this concept, you will be able to apply this thought process on other electrical faults throughout the car.

Topics covered

  • how and why a stepped thermistor signal voltage changes
  • measuring voltage between two variable resistors
  • Ohm's Law review
  • Kirchhoff's Voltage Law
 
CHAPTER 6 THERMISTORS PAGES 8 9

Description and operation of an engine coolant temperature (ECT) sensor. What does the computer use this input for and what drivability problems can occur with a fault in this sensor input.

Topics covered

  • why the ECT is critical during start-up
  • what outputs are controlled based on the ECT input
  • symptoms of a faulty ECT
 
CHAPTER 6 THERMISTORS PAGES 10 11 16 17

We are now on to the testing portion of this chapter. Both scan tool and voltmeter testing will be covered. The material here can be applied to all thermistor circuits, not just the engine coolant temp. sensor.

Recommended tools

Raytek MT6 Non-contact MiniTemp Infrared Thermometer

Topics covered

  • ECT scan tool testing
  • quick tests to confirm accuracy of the ECT on a cold engine
  • cold engine vs warm engine testing
  • wiring quick tests on a thermistor circuit
  • substituted values
  • ECT voltmeter testing
  • signal fixed high/low
  • using scan data ECT readings to determine thermostat operation
  • the Snap-on Vantage voltage charts and why they are so helpful
 
CHAPTER 6 THERMISTORS PAGES 12 15

Topics covered

  • ECT no signal or signal out of range testing
  • Intake Air Temperature sensors (IAT)
  • why the ECT is more critical than the IAT
  • IAT functions and locations
  • IAT scan tool testing
  • how to use the IAT sensor to ID a stuck open EGR valve
  • Exhaust Gas Temperature sensor (EGT) function and testing
  • outside air temp. digital display on a rear view mirror case study
 
THERMISTOR VOLTAGE TESTING VARIABLES PG 11 EXPANDED CONTENT

If a thermistor circuit is fixed at 0 volts, measured at the computer, sensor unplugged, what other variables do we need to consider before replacing the computer?

Topics covered

  • what effect does low reference voltage have on the signal circuit of a thermistor
  • where to find voltage to temperature conversion charts
  • voltmeter testing of an ECT circuit
  • ohmmeter testing for opens and shorts to ground
  • a possible faulty PCM (not listed in the text) 15:50
  • signal fixed at 0 volts with sensor unplugged variables) 18:14
  • computer pin contact problems 24:35
  • symptoms of a shorted reference circuit
 
SENSOR WIRING AND THE 5V REFERENCE CIRCUIT

I am beginning my week 4 lectures with this current class, (Sections 6 through 11) and decided that this would be a good introduction into sensor wiring. For those following along, this would be a great place to start before beginning week 4.

Topics covered

  • the voltage sensing circuit in a computer
  • comparison to a pull-down switch input circuit
  • how much is current flow on a sensor input circuit
  • voltage levels between two resistors in series circuit
  • the 5v reference circuit
  • thermistor vs. potentiometer wiring
  • why the 5v ref. is the signal wire on a thermistor circuit
  • why we cannot share the 5v reference between certain sensors
 

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