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Throttle Position Sensor (TPS) Diagnostics and Troubleshooting
PURPOSE
The function of the Throttle Position Sensor (TPS) is to sense the current
throttle valve position and relay that information to the ECM . Throttle
position information allows the ECM to generate the required injector control
signals (base pulse).
OPERATION
If the TPS senses a opening throttle, a voltage signal indicating this condition
is sent to the ECM. The ECM then increases the injector pulse width, permitting
increased fuel flow.
As the throttle valve rotates in response to movement of the accelerator pedal ,
the throttle shaft transfers this rotation to the TPS . A potentiometer
(variable resistor) within the TPS assembly changes its resistance (and voltage
drop) in proportion to throttle movement. By applying a reference voltage (5.0
volts) to the TPS input, a varying voltage (reflecting throttle position) is
available at the TPS output. For example, approximately 2.5 volts results from a
50% throttle valve opening (depending on TPS calibration). The voltage output
from the TPS assembly is routed to the ECM for use in determining throttle
position.
If the TPS is held in a wide open throttle (WOT) position during engine start,
the ECM will interpret this as a clear flood mode. The ECM will then pulse the
injectors at a ratio of 20 to 1 in order to clear a flooded condition. The ECM
will hold this injector rate as long as the throttle stays wide open, and the
engine speed is below 600 rpm.
LOCATION
The Throttle Position Sensor (TPS), is mounted on the side of the throttle body
opposite the throttle lever assembly.
Throttle Position Sensor (TPS) Diagnostics
Verify that minimum idle is at specified value.
VOLTAGE SPECIFICATIONS: THROTTLE POSITION
SENSOR :
With Throttle Closed
...................................................................... 0.55 +/-
0.05 Volts
Wide Open Throttle
....................................................................... 4.5 -
5.0 Volts
Make all checks/adjustments with engine at operating temperature.
Carbureted Models: Remove aluminum plug covering the adjustment screw.
Remove the screw and connect a digital voltmeter from the TPS black wire (-) and
either of the other two colored wires (+). If voltage is approximately 5 volts,
this is the reference voltage source. Connect DVOM to the other wire in this
case. Apply thread locking compound to the screw and with ignition on and engine
not running (as applies), quickly adjust screw to obtain specified voltage at
indicated condition.
Fuel Injected Models: Disconnect harness connector from TPS . Using three
six-inch jumper wires, reconnect harness to TPS. With ignition on and engine not
running (as applies), connect a digital voltmeter to black wire (-) and either
of the other two colored wires (+). If voltage is approximately 5 volts, this is
the reference voltage lead. Connect DVOM to the other wire in this case. Check
reading against specified value. If TPS is adjustable, loosen the unit retaining
screws and rotate the unit to reach specified value but most TPS are not
adjustable.
TPS Adjustment Check (Only for Adjustable TPS)
NOTE: Not all TPS
sensors are adjustable.
1. Install three jumper wires between TPS and harness connector or use a
"Scan" tool.
2. With ignition "ON," use a digital voltmeter connected to terminals
"B" and "C," or dark blue and black wire terminals. Adjust
TPS to specifications. See SPECIFICATIONS for voltage reading.
3. When voltage reading is satisfactory, tighten the retaining screws and and
remove the voltmeter and jumper leads. Reconnect the TPS connector.
4. If proper voltage reading cannot be obtained, replace TPS sensor.
Throttle Position Sensor (TPS) Removal and Installation:
REMOVAL:
1. Disconnect the throttle position sensor electrical connector.
2. Remove the TPS attaching screws.
3. Remove the throttle position sensor .
CAUTION: The TPS is an electrical component and
must not be soaked in any liquid cleaner or solvent, as damage may result.
INSTALLATION:
1. With the throttle valve in the closed position, Position the TPS on the
throttle shaft, then align with the screw holes.
2. Install the TPS attaching screws, and tighten to 2.0 Nm (18 lb.in).
3. Connect the throttle position sensor (TPS) electrical connector.
GM Good Throttle Position Sensor
Testing and Inspection using Digital Storage Oscilloscope (DSO)
Background and Application Information:
A TPS is a variable resistor (potentiometer) that tells the PCM the position of
the throttle. This is an important sensor because its signal input to the PCM is
used to calculate engine load, ignition timing, EGR control, idle control and
other PCM controlled parameters such as transmission shift points. A bad TPS can
cause hesitation and idle problems, among other driveability problems, and even
I/M emission test failures. The TPS is a three wire sensor. One wire supplies
the sensor with 5 volts via the PCM's VRef circuit, another wire provides the
sensor ground, and the third wire is the sensor's signal output to the PCM.
Almost all TPS 's from all vehicle manufacturers operate in the same way.
Therefore the following DSO setup and test procedure will work for most makes
and models with three wire Throttle Position Sensors. Generally, TPS's will
produce just under one volt with the throttle closed and produce just under 5
volts with the throttle wide open.
Connect and Setup the Digital Storage Oscilloscope (DSO):
1. Connect the "COM" probe to the TPS ground circuit, engine block, or
negative battery post.
2. Connect the "CH1" probe to the TPS signal wire to the PCM. Use a
wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 1 V/div.
4. Set the time base to 500 ms/div (1/2 second per division).
5. Select "GND" coupling and position the trace on the second division
line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" or "MIN/MAX" acquire mode.
8. "TRIGGER" mode is usually set automatically (Roll Mode).
Exercise the Sensor:
With the key on, engine off, slowly open the throttle from closed position to
wide open throttle and back to closed throttle again. Repeat this process
several times. It is best to do this slowly so the waveform is spread out on the
display like the example.
Image Good Throttle Position Sensor Pattern (Typical)
Expect This Waveform Result:
Check the manufacturer's specifications for exact voltage range specifications,
but generally the sensor's voltage should range from just under one volt at idle
to just under 5 volts at wide open throttle. There should be no breaks, spikes
to ground or dropouts in the waveform. Pay particular attention to the waveform
as it begins to rise (in the first 1/4 of throttle movement). This is the
portion of the sensor's carbon track that is most often used while driving
(during cruise, light throttle, etc.). The first 1/8 to 1/3 of the sensor's
carbon track usually wears out first.
IMPORTANT NOTE: Beware of TPS 's on 4.0L Jeep
Cherokees. 4.0L Cherokees have two throttle position sensors: One is for the PCM
and one is for the transmission controller. The transmission TPS operates
opposite from the engine TPS. The transmission TPS produces just under 5 volts
at idle and goes to just under one volt at wide open throttle. There are a few
other instances where this might be encountered.
GM Bad Throttle Position Sensor Testing and Inspection using Digital Storage Oscilloscope (DSO)
Background and Application Information:
Symptom: The vehicle would buck and jerk at just less than half throttle and
then run perfectly for a period of time. The TPS waveform above was captured
from a sensor that would intermittently malfunction. The sensor did not exhibit
the same magnitude of failure each time the throttle was opened and closed.
Sometimes it would even get in a good mood and work perfect for half an hour.
Connect and Setup the Digital Storage Oscilloscope (DSO):
1. Connect the "COM" probe to the TPS ground circuit, engine block, or
negative battery post.
2. Connect the "CH1" probe to the TPS signal wire to the PCM. Use a
wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 1 V/div.
4. Set the time base to 500 ms/div (1/2 second per division).
5. Select "GND" coupling and position the trace on the second division
line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" or "MIN/MAX" acquire mode.
8. "TRIGGER" mode is usually set automatically (Roll Mode).
Exercise the Sensor
With the key on, engine off, slowly open the throttle from closed to wide open
throttle and back to closed again. Repeat this process several times. It is best
to do this slowly so the waveform is spread out on the display like the example
above.
Bad Throttle Position Sensor Pattern (Typical Failure)
Expect This Waveform Result if the Sensor Is Bad:
Check the manufacturer's specifications for exact voltage range specifications,
but generally the sensor's voltage should range from just under one volt at idle
to just under 5 volts at wide open throttle. There should be no breaks, spikes
to ground or dropouts in the waveform. Pay particular attention to the waveform
above as it begins to rise (as it reaches Approx. 2.8 volts). This is the
portion of the sensor's carbon track that is usually damaged or broken. The worn
out or broken carbon track in this sensor did not communicate with the PCM using
the correct critical dimension for that throttle angle. Therefore, the PCM could
not calculate a correct fuel mixture command for the given engine conditions,
causing a driveability problem.
