| Free A.S.E. Study Guide An ASE Study Guide for the Automotive Technician |
 |  |  |  |
| Throttle Position Sensor |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
The Throttle Position (TP) sensor is a potentiometer located on the throttle body of a
fuel injected engine. A simple potentiometer is a three wire resistor with a sliding arm
that swipes across a resistance strip in turn changing the resistance which increases
or decreases the signal voltage back to the PCM. In a nutshell a TP sensor is a device
that changes voltage by varying internal resistance. Some TP sensors have extra pins
for extra internal functionality but for this discussion we will stick to the three wire type.
To help explain this we will look at the three leads and each of these leads/wire/PCM
or TCM function as it relates to either engine or transmission performance.
One of the leads is connected directly to the computers 5 volt voltage supply. This
voltage pin is in turn connected to one end of the resistance strip inside of the TP
sensor. Another wire is connected to the other end of this internal resistance strip and
in turn connected to ground.
The third wire is connected to the movable arm that will swipe across this internal
resistance strip sending a varying voltage signal back to the PCM or TCM indicating
the drivers’ intentions. This is often called the signal circuit of a sensor because it is the
lead the sends the varying or changing voltage signal to the various computers
indicating the changes that are occurring during vehicle operation.
This device is attached to the throttle plate shaft and changes output voltage as the
throttle plate opens and closes. The output voltage will be low or around 0.45 volts if the
throttle is closed and will be high or 4.5 volts at WOT “Wide Open Throttle”.
The fuel injection and ignition timing are adjusted by the PCM as the throttle plate / TP
sensor is moved. If the driver suddenly accelerates their vehicle the PCM would read
this 4.5 volt signal and increase the fuel injection timing to compensate.
The TP sensor is also used by the TCM to indicate load and drivers intentions. For
instance this helps the TCM determine the shifting schedule. If the throttle plate were to
suddenly approach WOT (4.5 volts) this would indicate that its time for a down shift.
It’s best to diagnose a TP sensor with a scanner or scope and would be displayed as a
percentage. The PCM coverts the voltage to a percentage on the display screen of a
scanner. Just plug the scanner in, locate the TP sensor on the screen, and with key on
engine off steadilly depress the accelerator to th efloor board. The percentage should
increase steadily from 0 to 100% or whatever the specifications dictate. Check with the
manual for the appropriate specifications.
You may also use a DMM to diagnose this sensor using the ohm and voltage settings.
First select the Ohm setting and connect the leads to the signal and ground pins of the
sensor. As you manually open the throttle to fully open and fully closed you should see a
smooth movement in the resistance reading. Any infinity readings or skips in the
sweep indicate a bad sensor.
Set the DMM to voltage and install test leads between the pins in the connector and
sensor. With the KOEO “Key on engine off” move the throttle between fully open and
fully closed and insure the reading is smooth with a low reading of 0.45 volts closed to
4.5 volts open.
fuel injected engine. A simple potentiometer is a three wire resistor with a sliding arm
that swipes across a resistance strip in turn changing the resistance which increases
or decreases the signal voltage back to the PCM. In a nutshell a TP sensor is a device
that changes voltage by varying internal resistance. Some TP sensors have extra pins
for extra internal functionality but for this discussion we will stick to the three wire type.
To help explain this we will look at the three leads and each of these leads/wire/PCM
or TCM function as it relates to either engine or transmission performance.
One of the leads is connected directly to the computers 5 volt voltage supply. This
voltage pin is in turn connected to one end of the resistance strip inside of the TP
sensor. Another wire is connected to the other end of this internal resistance strip and
in turn connected to ground.
The third wire is connected to the movable arm that will swipe across this internal
resistance strip sending a varying voltage signal back to the PCM or TCM indicating
the drivers’ intentions. This is often called the signal circuit of a sensor because it is the
lead the sends the varying or changing voltage signal to the various computers
indicating the changes that are occurring during vehicle operation.
This device is attached to the throttle plate shaft and changes output voltage as the
throttle plate opens and closes. The output voltage will be low or around 0.45 volts if the
throttle is closed and will be high or 4.5 volts at WOT “Wide Open Throttle”.
The fuel injection and ignition timing are adjusted by the PCM as the throttle plate / TP
sensor is moved. If the driver suddenly accelerates their vehicle the PCM would read
this 4.5 volt signal and increase the fuel injection timing to compensate.
The TP sensor is also used by the TCM to indicate load and drivers intentions. For
instance this helps the TCM determine the shifting schedule. If the throttle plate were to
suddenly approach WOT (4.5 volts) this would indicate that its time for a down shift.
It’s best to diagnose a TP sensor with a scanner or scope and would be displayed as a
percentage. The PCM coverts the voltage to a percentage on the display screen of a
scanner. Just plug the scanner in, locate the TP sensor on the screen, and with key on
engine off steadilly depress the accelerator to th efloor board. The percentage should
increase steadily from 0 to 100% or whatever the specifications dictate. Check with the
manual for the appropriate specifications.
You may also use a DMM to diagnose this sensor using the ohm and voltage settings.
First select the Ohm setting and connect the leads to the signal and ground pins of the
sensor. As you manually open the throttle to fully open and fully closed you should see a
smooth movement in the resistance reading. Any infinity readings or skips in the
sweep indicate a bad sensor.
Set the DMM to voltage and install test leads between the pins in the connector and
sensor. With the KOEO “Key on engine off” move the throttle between fully open and
fully closed and insure the reading is smooth with a low reading of 0.45 volts closed to
4.5 volts open.
| For more information about ASE, visit the official ASE website at www.ase.com |
© Copyright 2008-09
® All Rights Reserved