This was the first time I have ever seen 12v on a potentiometer style throttle position sensor. The spec for this sensor is around .5v at idle at around 4v at wide open throttle and this Jeep was fixed at 12v all the time. Scan data was misleading as the TPS data parameter showed a constant 5v regardless of throttle angle. Also when performing signal circuit integrity type testing, you will now understand why we use a resistor instead of a jumper wire.

Engine Performance Diagnostics chapter 7

Topics discussed


  • check engine light (P0123)
  • new throttle position sensor but keeps setting a code for the TPS
  • incorrect idle speed
  • hesitation on acceleration
  • stalling
  • no start with aftermarket computer chip removed

fix 50

  • repair bent computer pin

test shown

  • how to test a throttle position sensor using scan data
  • how to test a TPS with a digital voltmeter
  • why scan data can be wrong and cause misdiagnosis
  • why you should use a resistor when performing a signal circuit integrity test
  • how to identify a short to power

Tools used

  • scan tool
  • lab scope (a digital multimeter may be used instead)

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(Chapter 7) Potentiometer-position sensor tests TPS, EVP, APP, VAF etc

Related videos:

How to test throttle position sensor wiring with a resistor (any car):


How to test TPS wiring with a scan tool (any car):


Scan tool data graph vs Lab Scope waveform (should I buy a scope?):


1996 Jeep No Start Case Study Part I:


1996 Jeep No Start Case Study Part II:


1996 Jeep No Start Case Study Part III:


What can cause an O2 sensor to read near 5 volts? (Chrysler bias voltage):


System Too Rich P0172, P0175 from a blown O2 heater fuse (Jeep):


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For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available as an eBook or paper book.

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