AOH :: THROTPS.TXT|
About TPS - Throttle Position Sensor
A throttle position sensor (TPS) is a sensor used to monitor the
position of the throttle in an internal combustion engine. The sensor is
usually located on the butterfly spindle so that it can directly monitor
the position of the throttle valve butterfly.
The sensor is usually a potentiometer, and therefore provides a variable
resistance dependent upon the position of the butterfly valve (and hence
The sensor signal is used by the engine control unit (ECU) as an input
to its control system. The ignition timing and fuel injection timing
(and potentially other parameters) are altered depending upon the
position of the throttle, and also depending on the rate of change of
that position. For example, in fuel injected engines, in order to avoid
stalling, extra fuel may be injected if the throttle is opened rapidly
(mimicking the accelerator pump of carburetor systems).
More advanced forms of the sensor are also used, for example an extra
closed throttle position sensor (CTPS) may be employed to indicate that
the throttle is completely closed. Some ECUs also control the throttle
position and if that is done the position sensor is utilised in a
feedback loop to enable that control.
Related to the TPS are accelerator pedal sensors, which often include a
wide open throttle (WOT) sensor. The accelerator pedal sensors are used
in "drive by wire" systems, and the most common use of a wide open
throttle sensor is for the kickdown function on automatic transmissions.
TPS (throttle position sensor) Auto Repair Technician Observation
The TPS is a three wire sensor that measures the throttle plate opening
as well as its rate of change. This sensor is a variable resistor, also
called a potentiometer, that is directly linked to the throttle plate
shaft. The TPS outputs a voltage directly proportional to the throttle
opening. As the accelerator is depressed the throttle plate opens and
the TPS voltage increases. This sensor is also one of the main inputs to
the transmission computer (TCM). The TCM uses the TPS input signal to
control the transmission shift points and the torque converter (TCC)
solenoid lock-up. The TPS together with MAP or MAF sensor are the main
ECM indicators of acceleration and load. In other words, the ECM looks
at these sensors to calculate engine operation upon acceleration.
Some manufacturers use the TPS signal as sole indicator of engine load
when there is a faulty MAP/MAF sensor. In such cases, the MAP/MAF sensor
values are calculated from the TPS signal. This means that the ECM
substitutes the faulty MAP/MAF value from a look-up-table stored in its
ROM memory, so the vehicle can continue to operate until the driver
reaches a repair facility.
The TPS sensors usually tends to fail at the lower range of its
movement. This is where the driver is usually at, most of the time (as
in cruising speed). This sensor usually works with a 5.00 volt reference
voltage and ECM provided sensor ground. The signal is output through the
signal wire, where all measurements should be made. Problems to any of
the ground or power (5 volt) feed lines will cause an incorrect reading
from the TPS.
CONDITION THAT AFFECT OPERATION
Most TPS sensors reset when the ignition key is cycled. This means that
whenever the vehicle is shut off and turned back on again, whatever
voltage signal the ECM sees (TPS base voltage) it will take as 0 degrees
of throttle opening. This ignition key resetting feature also means that
as the throttle bore gets dirtier the idle speed/IAC operation will also
be affected. Another curious drawback of a re-zeroing or resetting TPS
is in the event of a momentary signal drop-out glitch. In this case, if
the TPS signal momentarily drops to very low levels, the ECM will take
the low reading as the 0.00 % or closed throttle point. Then, as soon as
the output signal snaps back to normal, the ECM will perceive the new
signal as a wider than normal throttle opening. The result is an
increase in injector pulse-width and wrong transmission shift points.
TPS sensors are directly linked to the throttle plate shaft. Any binding
of the throttle plates can greatly influence the TPS signal. A
carbonized th rottle bore may also influence this signal. Some TPS
sensors are combined with a throttle switch. This is a simple on-off
switch that closes whenever the throttle is fully closed. This way the
ECM knows that the throttle plates are closed. This combo TPS/Idle
switch is mostly found on Euro and Asian imports. It is important to
remember that a shorted sensor or actuator that shares the same power
feed and ground with the TPS will also have an adverse effect on its
signal. If a shared sensor shorts out it may also short the TPS
reference voltage or ground line..
TPS sensors are of the three wire type. The sensor ground, reference
(usually 5 volt) voltage, and the signal wire. The sensor ground is
provided by the ECM, as well as for all its other sensors. A voltage
drop test should be performed across it and battery post ground to
verify no more that 100 mV voltage drop during KOER. The reference
voltage is also provided by the ECM and it is a 5.00 volts regulated
feed line. It provides the TPS sensor with its working voltage. A
shorted 5.00 volt reference line, either at the wire or another sensor
that is shorted, will directly affect the TPS sensor reading and the
entire engine. The signal line is the signal return to the ECM. It is
the one providing the ECM with the actual TPS sensor reading. This is
the line to tap when performing actual tests. A shorted TPS signal line
will also skew its reading.
The first analysis of the TPS signal voltage is done with the KOEO. This
base voltage signal is taken by the ECM to be 0.00 % or degrees of
throttle opening. It is very important that this signal be within exact
Second, using a DSO sweep the TPS while measuring the voltage output.
There should be no glitches or sudden drops in its voltage reading.
Refer to Fig 3. Beware of the flat spots. Most TPS failures occur at the
lower travel of the throttle plate. The TPS usually develops flat spots
in this region and are not easy to detect. A faster frequency setting
should be selected so as to be able to detect these flat spots.
Third, make sure that the signal reaches close to the WOT voltage level.
There is always the possibility that the throttle is binding or the TPS
is defective at the high range of its travel. This would cause problems
with vehicle hesitating at higher cruise speeds or upon heavy
Always make sure that proper idle speed is attained with the TPS fully
closed. If idle speed stays high with TPS at base voltage the ECM will
assume that it can't control idle speed and set a code.
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