Theory of Operation
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.
NOTE: TPS signal
problems can greatly affect transmission operation. On a vehicle
with an incorrectly shifting transmission, a careful analysis of the
TPS signal should be made. A high TPS voltage reading during KOEO
could also signal the ECM to go into FUEL CUT-OFF MODE. In which
case, the ECM reacts as if the engine is at WOT and cuts pulsation
to the injectors and preventing the engine from starting. Again,
this is what is called fuel cut-off mode.
Conditions 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 throttle 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.
Component Testing
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 tobe 0.00 % or degrees of throttle opening. It is
very important that this signal be within exact specifications.
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 acceleration.
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 cant
control idle speed and set a code.
NOTE: A minimum
air-rate out of adjustment with TPS voltage too high or too low will, in
many cases, cause an IAC (Idle Air Control) valve code. In such cases the
scanner DESIRED idle speed can not be maintained because the throttle plates
are over adjusted. When this happens, a reading of 0 steps on the IAC is
seen on the scan tool and the idle speed will remain high. Properly
adjusting the minimum air rate will correct this problem.
