Free ASE Prep: L1 Advance Engine Performance
BAR Specific Diagnostic Class: Throttle Position Sensor and Engine Coolant Temperature Sensor
Look at the Throttle Position Sensor and Engine Coolant Temperature Sensor Schematic, there is 0 volts on the signal wire of the TPS , name three things that will cause this?
ASE L1 Question
Throttle Position Sensor and Engine Coolant Temperature Sensor Schematic
Here is an ASE L1 question for you, if you are not there yet ignore this section because it will confuse you for the particular section you are studying for, you will be directed back here once you start L1.
IF a car came into your shop with a Malfunction Indicator Light “MIL” on and the code was for low Throttle position sensor voltage. The customer is complaining of hesitation and transmission shifting problems and an occasional stalling.
The code was for low Throttle position voltage, doing your diagnosis you discover there is a 5 volt feed to the sensor and .05 volts on the ground wire. Your next check is to check the voltage on the signal wire, your reading on the digital voltmeter is below specifications. The specifications call for 1.0 – 1.5 volts and you are reading .01 volts a short to ground it seems like.
But once you disconnect the sensor, the signal wire voltage shoots up to 5 volts, and stays there. Now initially the voltage was low plugged in, once disconnected it went up to 5 volts.
When I first seen this it was very confusing I initially thought I had a problem in the computer, with further diagnosing I discovered it was a defective TPS.
Now remember initially the voltage on the signal wire was low it only went up because the sensor was unplugged. This is normal, there is a special resistor inside the ECM/PCM and I forget the correct name at this time but when the sensor is disconnected the voltage goes to 5 volts default value on the signal wire.
The confusing part is this, think about it, why would you have any voltage on a signal wire when the sensor is out of the picture?
What happen; you remove the ECM/PCM from the shorted sensor so with a disconnected sensor the PCM will send out a default value on the signal wire. It’s a pull up circuit and very common on Chrysler’s, so the original problem was a shorted sensor.
Also remember if the signal wire was shorted to ground the voltage would had stayed low and not go to the 5 volt default number.
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