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A variable three-wire resistor-type electrical sensor which sends a
signal to the electronic control unit relative to the throttle position.
Different kinds of problems can happen with a throttle position sensor (TPS).
- The most common symptom of a bad or misadjusted TPS is hesitation or stumble during acceleration.
- The fuel mixture leans out because the computer does not receive the right signal telling it to add fuel as the throttle opens.
- Eventually, the oxygen sensor senses the problem and adjusts the mixture, but not before the engine stumbles.
- On carburetor and TBI systems, fuel can migrate into the sensor.
- In cold weather a TPS gets cold and the feather (wiper arm) sometimes does not wipe against another metal strip.
- If the strip wears away, momentary interruptions of the electrical signal can occur. These interruptions are called glitches.
A throttle position sensor has a wiper arm that rubs against a resistor strip.
- When the engine is under a heavy load, the air conditioning
compressor is shut off by the computer. A bad TPS can shut off an air
- Also, on computer controlled cars, the key should be turned on
before depressing the throttle or the computer can receive a faulty TPS
reading and the car might not start.
The TPS can be checked with either a voltmeter or an ohmmeter.
A throttle position sensor can be checked with an ohmmeter or a
voltmeter. The elongated slots in this TPS allow for adjustment.
Courtesy of Ford Motor Company.
To perform ohmmeter tests, the switch must be disconnected. That
makes the voltmeter (or scope, which is actually a voltmeter, too) the
tester of choice.
Three tests are made for TPS operation.
- Reference voltage must be present at the switch with the key on.
- The base voltage is also compared to specifications.
- Finally, voltage should change gradually and evenly as the throttle
is opened and closed. If the voltage does not rise, or there are skips
(glitches) in the voltage measurement, the sensor is bad.
A glitch in a DSO pattern on a TPS test. Courtesy of Edge Diagnostic Systems.
When checking voltage output of a TPS, typically there would be less
than 0.1 V at idle. Typical voltage at half throttle would be 2.5 V.
Remember, there is a 5-volt reference signal. That
is why there is a 2.5 V signal when the throttle is half open. At WOT
the signal would be 4.5 V.
A sensor can be tested with a voltmeter while its electrical wiring
is still connected (this is called backprobing a connector). Move the
throttle slowly from closed to open. At different throttle openings,
varying resistances are specified. Watch for any glitches.
- With the ignition switch on, connect a voltmeter between the reference wire to ground.
- Normally, the voltage reading should be approximately 5 volts.
- If the reference wire is not supplying the specified voltage, check the voltage on this wire at the computer terminal.
- If the voltage is within specifications at the computer, but low at the sensor, repair the reference wire.
- When this voltage is low at the computer, check the voltage supply wires and ground wires on the computer.
- If these wires are satisfactory, replace the computer.
- With the ignition switch on, connect the voltmeter from the sensor ground wire to the battery ground.
- If the voltage drop across this circuit exceeds specifications, repair the ground wire from the sensor to the computer.
When the throttle is opened gradually to check the
throttle position sensor voltage signal, tap the sensor lightly and
watch for fluctuations on the voltmeter pointer, indicating a defective
- With the ignition switch on, connect a voltmeter from the sensor signal wire to ground.
- Slowly open the throttle and observe the voltmeter.
- The voltmeter reading should increase smoothly and gradually.
- Typical TP sensor voltage readings would be 0.5 volt to 1 volt with
the throttle in the idle position, and 4 to 5 volts at wide-open
- If the TP sensor does not have the specified voltage or if the voltage signal is erratic, replace the sensor.
With the wiring disconnected, the TPS can be tested with an ohmmeter.
- Connect the scope to the sensor's output and a good ground and watch the trace as the throttle is opened and closed.
- The resulting trace should look smooth and clean, without any sharp breaks or spikes in the signal.
A normal TP sensor waveform while it opens and closes.
- A bad sensor will typically have a glitch (a downward spike)
somewhere in the trace or will not have a smooth transition from high
The waveform of a defective TP sensor. Notice the glitch while the throttle opens.
These glitches are an indication of an open or short in the sensor.
On some carbureted engines, an open in the TP sensor will show up as an
upward spike. This is because the computer supplies a 5-volt signal to
the sensor's output wire if the TP sensor has an open.
Some throttle position sensors are adjustable. Follow manufacturer's instructions.
- On older engines, correct adjustment is crucial to proper system operation.
- On newer engines, this is not as important because the computer
uses whatever reading it takes at idle as base voltage (adaptive learn
Incorrect TP sensor adjustment may cause:
- Inaccurate idle speed,
- Engine stalling, and
- Acceleration stumbles.
Follow these steps to adjust a typical TP sensor:
- Backprobe the TP sensor signal wire and connect a voltmeter from this wire to ground.
- Turn on the ignition switch and observe the voltmeter reading with the throttle in the idle position.
- If the TP sensor does not provide the specified voltage, loosen the
TP sensor mounting bolts and rotate the sensor housing until the
specified voltage is indicated on the voltmeter.
A TP Sensor with elongated slots for sensor adjustments. Courtesy of General Motors Corporation - Chevrolet Motor Division.
- Hold the sensor in this position and tighten the mounting bolts to the specified torque.
Most throttle position sensors are made so that their adjustments
are tamperproof. They have screws that are either soldered or staked.
To remove the switch, these might need to be drilled or filed off.
After the new switch is installed and adjusted, the new mounting screws
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