Question about Mercedes-Benz CLK-Class

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Error code P0221, expert on this site told me to get a new Throttle position sensor. Looking for diagrams, instructions on how to replace this.

Posted by jalbert41 on

  • jalbert41 Oct 09, 2009

    My 2001 Mercedes CLK320 stalled out while accelerating on the freeway and the Check Engine code is P0221. An expert at this site told me to replace the Throttle Position Sensor which I have the part but don't know how to install it. Can it be done myself or does an expert need to do this.



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Do not forget to accept the solution, if 100% satisfied with my solution.

Note - I can give you all details. Trust me it's more than enough. You are the best judge to determine, whether you need a repairman or you could do it yourself.


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.
    Error code P0221, expert on - edf422a.gif
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 conditioning compressor.
  • 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 iError code P0221, expert on - 2693374.gif
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.
NOTE 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. Voltmeter Testing
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.
NOTE 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 sensor.
  • 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 throttle.
  • If the TP sensor does not have the specified voltage or if the voltage signal is erratic, replace the sensor.
Ohmmeter Testing
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 sError code P0221, expert on - 5cba743.gif 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 inError code P0221, expert on - a979e21.gifor will not have a smooth transition from high to low.

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 strategy).
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 mountingError code P0221, expert on - 77ed7dc.gifrotate 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 are restaked.

Let me know, if need further assistance.

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Posted on Oct 09, 2009


Rylee Smith

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Trouble code P0221 means Throttle/Petal Position Sensor/Switch B Circuit Range/Performance Problem. You can only check the wiring of the throttle wires. And, if the wires are connected fine then you have to replace the Throttle position sensor, which needs to be programmed by the dealer using the tools. See this link for necessary info.--
Mercedes Troublecode PO221

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Posted on Oct 08, 2009


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