THERMISTOR OHMS FINAL APP

Test purpose:
This test is designed to monitor the resistance value of the thermistor and determine if it is within range of the operating specifications. Thermistors are either NTC or PTC type
and their resistance values are variable depending on the temperature that they are
sensing. A thermistor can either be open, shorted or operating correctly or incorrectly. In
this test we are looking to determine if the resistance change in the thermistor is smooth
and consistent over the increase and decrease in temperature. Although this test is not
a definitive “pass / fail” or “green light / red light” the tool is automatically set up for a
fast resistance capture with stable readings and ranged correctly. Once connected to a
thermistor, the graphing screen will be the way to monitor output.
Tool Setup:
• Top Chanel: N/A
• Bottom Channel: Resistance - Ranged to 10MΩ
• Set Min/Max to Max
• Speed: 64MS (256) or (4000 Hz 256 SMPL)
Test Procedures:
• Place Leads on tool to Ohms and Common
• Use Probe Tips on leads or clamps
• Remove the thermistor from a power source.
• Connect the positive probe tip or clamp to the positive output of the
thermistor
• Connector the ground probe tip or clamp to the ground of the thermistor
• Look for “out of range” , a “stuck” Ω value or a slightly fluctuating μΩ
reading.
• Set the Meter Mode to Graph
• Begin to heat up the thermistor.
• Watch the characteristics of the reading for a gradual change in the
resistance or Ω reading.
Recommendations
A thermistor is not actively powering itself, but rather creating a voltage drop across
itself based on temperature. A faulty thermistor reading on the vehicle may be the
cause of the thermistor, the thermistor circuit, or the computer monitoring the
thermistor value.
Diagnosis of a thermistor should be broken into the 3 categories of component,
circuit and computer.
Diagnosing the thermistor:
• A thermistor can either be open, shorted, operating correctly or incorrectly.
• If the thermistor is open, the reading will be “out of range” or a the tool will see
fluctuating resistance values due to ghost voltage (a fluctuation of voltage
randomly from the air and other electronics around us).
• If the thermistor is shorted it will either see an Ohms value on the screen that
fluctuates but does not intentionally increase or decrease or at least an ohms
value reading that does not change due to the nature of how ohmmeters work.
• If the thermistor is operating incorrectly, the changes in its internal resistance
values will change sharply as consistent temperature is applied to the element
and the readings will appear to “jump” or “drop”significantly as element is being
heated and monitored.
Diagnosing the circuit:
If the thermistor is found to be operational or operating as expected but the issue
still exists the next step is to diagnose the thermistor circuit.
• Begin with the connectors, then the feeds or cables and end with the grounds.
• Visually inspect and wiggle test the circuits.
• Use the “loaded cable” or “cable tests” from the voltage drop menu to verify there
is no undue voltage drop on the system. (If possible and safe a loaded test of the
cables disconnected from any computer circuits would be preferable.)
• If the problem still exists, attempt to diagnose the monitoring system of the
thermistor using a wiring diagram to identify what is monitoring the thermistor
value.