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 is usually mounted externally on the throttle shaft as is the case on most late model fuel injection throttle bodies, but on older vehicles with electronic feedback carburetors, the TPS sensor was mounted internally (such as the Rochester Varajet, Dualjet and Quadrajet).
The TPS sensor 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.
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.
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.
First, check for the presence of any fault codes. OBD II 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.
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.
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.
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.
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