TPS sensor
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Engine Throttle Position Sensor

Copyright AA1Car

Late model engines with feedback carburetion or electronic fuel injection use a "Throttle Position Sensor" (TPS) to inform the computer about the rate of throttle opening and relative throttle position. A separate idle switch (sometimes called a "Nose" switch) and/or wide open throttle (WOT) switch may also be used to signal the computer when these throttle positions exist.

The throttle position sensor may be mounted externally on the throttle shaft as is the case on most fuel injection throttle bodies, or internally in the carburetor as it is in the Rochester Varajet, Dualjet and Quadrajet.

The TPS is essentially a variable resistor that changes resistance as the throttle opens. Think of it as the electronic equivalent of a mechanical accelerator pump. By signaling the computer when the throttle opens, the computer can richen up the fuel mixture to maintain the proper air/fuel ratio.

The initial setting of the TPS is critical because the voltage signal the computer receives back from the TPS tells the computer the exact position of the throttle. The initial adjustment, therefore, must be set as closely as possible to the factory specs. Most specs are given to the nearest hundredth of a volt! And since there is no range of "acceptable" specs given for a specific application, the TPS should be adjusted as closely as possible to those in the manual.

This is accomplished by reading the TPS voltage at a specific throttle position using a 10k ohm impedance digital voltmeter, or on GM vehicles, using a hand-held scan tool that plugs into the vehicle diagnostic connector.

Engine Throttle Position Sensor . .
The TPS sensor monitors throttle position.

TPS ON DRIVE-BY-WIRE CARS

Most late model cars and trucks have no throttle cable. A small electric motor is used to operate the throttle using inputs from position sensors on the gas pedal. When the gas pedal is depressed, the electrical resistance of the potentiometers inside the pedal sensors change. The control module notes the change in position and commands the throttle to open. A pair of throttle position sensors on the throttle shaft note the change in throttle position and provide feedback signals to the control module so the module knows the exact position of the throttle and that everything is working correctly.

DRIVABILITY SYMPTOMS

The classic symptom of a defective or misadjusted TPS is hesitation or stumble during acceleration (in other words, the same symptoms a bad accelerator pump would produce). The fuel mixture leans out because the computer doesn't receive the right signal telling it to add fuel as the throttle opens. The oxygen sensor feedback circuit will eventually provide the necessary information, but not quickly enough to prevent the engine from stumbling.

Throttle position sensors typically experience the most wear in the position just above idle, since this is the throttle's position for most driving. A worn sensor may cause a skip or drop in the reading when the throttle opens, causing a momentary loss of input to the PCM. The result is usually a hesitation or stumble because the PCM fails to provide the necessary fuel enrichment.

If the TPS mounting is loose, it will produce an erratic signal leading the ECM to believe the throttle is opening and closing. The result can be an unstable idle and intermittent hesitation.

If the TPS is shorted, the computer will receive the equivalent of a wide open throttle signal all the time. This will make the fuel mixture run rich and set a fault code that corresponds to a voltage signal that's too high.

If the TPS is open, the computer will think the throttle is closed all the time. The resulting fuel mixture will be too lean and a fault code that corresponds to a voltage signal that's too low will be set.

SENSOR CHECKS

First, check for the presence of any fault codes. Codes that may indicate TPS problems include:

P0120....Throttle/Pedal Position Sensor/Switch A Circuit

P0121....Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem

P0122....Throttle/Pedal Position Sensor/Switch A Circuit Low Input

P0123....Throttle/Pedal Position Sensor/Switch A Circuit High Input

P0124....Throttle/Pedal Position Sensor/Switch A Circuit Intermittent

P0220....Throttle/Pedal Position Sensor/Switch 'B' Circuit

P0221....Throttle/Pedal Position Sensor/Switch 'B' Circuit Range/Performance Problem

P0222....Throttle/Pedal Position Sensor/Switch 'B' Circuit Low Input

P0223....Throttle/Pedal Position Sensor/Switch 'B' Circuit High Input

P0224....Throttle/Pedal Position Sensor/Switch 'B' Circuit Intermittent

P0225....Throttle/Pedal Position Sensor/Switch 'C' Circuit

P0226....Throttle/Pedal Position Sensor/Switch 'C' Circuit Range/Performance Problem

P0227....Throttle/Pedal Position Sensor/Switch 'C' Circuit Low Input

P0228....Throttle/Pedal Position Sensor/Switch 'C' Circuit High Input

P0229....Throttle/Pedal Position Sensor/Switch 'C' Circuit Intermittent

On older pre-OBD II vehicles, the codes for the throttle position sensor include:

* General Motors Pre-OBD II: 21, 22

* Ford (EEC-IV) Pre-OBD II: 23, 53, 63, 73

* Chrysler Pre-OBD II: 24

If you find a code, then refer to the appropriate diagnostic chart and follow the step-by-step checks to isolate the cause. If you don't find any codes, you can still do the following scan tool and voltage checks.

use a scan tool to check TPS sensor output

SCAN TOOL CHECKS

A scan tool that can display sensor data will usually show the throttle position as a percentage of opening. Professional grade scan tools may also be able to display the TPS sensor's actual voltage, depending on the software. Plug the scan tool into the vehicle's diagnostic connector, turn the key ON, and note the throttle opening reading. At idle is should be zero or a couple of degrees. Press down on the gas pedal very S-L-O-W-L-Y until the throttle is all the way open. You should see the percentage of throttle opening gradually increase to 100 percent at wide open throttle.

No change in the scan tool reading would indicate no input from the throttle position sensor. Or, if you see more than 5 percent open at idle, or less than 90% open at WOT, it could indicate a problem with the sensor.

Note: Most scan tools do not update their readings quickly enough to detect a momentary glitch in the TPS reading during a TPS sweep from idle to WOT. If the TPS has a worn spot, most likely it will be between 0 and 20 percent throttle opening. Try holding the throttle between 0 and 20 percent to see if you get a steady reading. If the reading suddenly drops while holding the gas pedal or throttle linkage steady, it may indicate a fault with the sensor.

tps voltage checks

TPS VOLTAGE CHECKS

If your scan tool cannot display a voltage value for the TPS, you can measure the sensor's output voltage by packprobing the sensor conenctor with a voltmeter. First, check for the presence of voltage at the TPS with the key on. The TPS cannot deliver the proper signal if it does not receive reference voltage from the computer. Refer to a wiring diagram for the reference connection and look for 5 volts.

The second check is the base voltage reading. Compare the voltage reading to the manual specifications. TPS voltage values are often specified to the nearest hundredth of a volt, so if the base TPS voltage reading is not within .05 volts of the specified value, adjustment may be needed (if it is adjustable). If it is not adjustable and the reading is out of specifications, replace the sensor.

The third check is for the proper voltage change as the throttle opens and closes. Voltage should rise smoothly from about 1 volt to a maximum of 5 volts at wide open throttle. No voltage rise or skips in the reading means the sensor needs to be replaced. Observing the sensor's output signal as a trace on an oscilloscope can be a real time-save here because it is easy to see any deviations in the voltage curve.

tps waveform displayed on a scope

TPS ADJUSTMENT

Under normal circumstances, a TPS should not require adjustment. But if your diagnosis reveals a problem with the TPS voltage setting, if the TPS is defective and must be replaced or if the carburetor or throttle body is replaced, then adjustment may be required. Note: This applies to older vehicles only. On most late model vehicles, the TPS is self-calibrating. The engine computer uses the base voltage reading at idle as representing 0% throttle opening.

NOTE: The TPS on most remanufactured carburetors is preset at the factory to an "average" setting for the majority of applications the carb fits. Even so, the TPS should be reset to the specific application upon which it is installed.

Through 1982, all GM throttle position sensors were adjustable. But on newer applications, many sensors are not adjustable. Starting in 1984, for example, GM went to a nonadjustable TPS on the 1.8 and 2.5L Pontiac engines. Likewise, Chevy switched to a nonadjustable TPS starting in '85 on the 2.0L engine. On engines with a nonadjustable TPS, the ECM uses whatever idle reading it gets from the TPS as the base voltage reference point.

With adjustable TPS sensors, the adjustment procedure will vary depending on the application. On Rochester carburetors with an internal TPS, it involves removing an antitamper plug on top of the carburetor. On some fuel injected applications, the throttle body must be removed to drill out welds that hold the TPS screws. With externally mounted throttle position sensors, the sensor is adjusted by loosening the mounting screws (or drilling out the mounting rivets) and rotating the sensor slightly one way or the other until the desired voltage reading is obtained.

The basic adjustment procedures goes as follows:

1. Remove the anti-tampering plug (if applicable) or loosen the mounting screws or remove the rivets holding the TPS.

2. Refer to the electrical diagram in a manual to determine which connectors are used to make the TPS reading. On the Rochester carbs, for example, use the TPS center terminal "B" and bottom terminal "C." If the vehicle provides access to TPS data stream, use a scan tool to read the sensor's output by plugging into the diagnostic connector.

  1. Turn the ignition on. Adjust the TPS with the throttle in the specified position (idle, high step of fast idle cam or resting against the throttle stop screw with the ISC plunger fully retracted) until the proper voltage reading is obtained.


News Update: June 2011

New Non-Contact TPS Sensors For Replacing Worn Original Equipment Sensors

Airtex Engine Management has introduced a line of advanced non-contact-design throttle position sensors that eliminate the premature wear and common drivability issues experienced with conventional Throttle Position Sensors. The new Airtex throttle position sensors are now available for many mid-1980s through 2007 Dodge, Ford, General Motors and Mazda applications.

Conventional original equipment and replacement throttle position sensors have metal contact fingers that sweep across a printed resistor board to indicate throttle position. Repetitive motion and vehicle vibration can cause these fingers to wear holes in the board, causing dead spots that result in engine hesitation and other drivability problems.

The new Airtex sensors utilize advanced Hall Effect integrated circuitry that eliminates wear-intensive contact with the circuit board. This technology until now has not been widely available in the aftermarket, in spite of its significant advantages over conventional throttle position sensor designs.

For more information, visit Airtex Engine Management.





More Engine Sensor Articles:

Throttle-By-Wire systems (Electronic Throttle Control)
Diagnose Cruise Control
Making Sense of Engine Sensors
Air Temperature Sensors
Coolant Sensors
Crankshaft Position CKP Sensors
MAP sensors
Mass Airflow MAF Sensors
Vane Airflow VAF Sensors
Oxygen Sensors
Wide Ratio Air Fuel (WRAF) Sensors
Understanding Engine Management Systems
Powertrain control modules (PCMs)
Flash Reprogramming PCMs
All About Onboard Diagnostics II (OBD II)
Zeroing in on OBD II Diagnostics
Controller Area Network (CAN) Diagnostics
OEM Automotive Service Information Websites & Access Fees

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