Throttle Position Switch - Information, Troubleshooting, Replacement, and Adjustment

Introduction

A faulty throttle position switch (TPS) can produce a wide range of poor-running symptoms. Problems are often difficult to diagnose because they frequently appear only when the engine and switch are fully warmed up.

Possible symptoms of a faulty TPS:

• Poor idle, particularly a high idle condition
• Engine cuts to idle during acceleration
• Poor power at various RPM / throttle positions
• Surging idle
• Misfire
• Stumble during acceleration
• Intermittent boost problems (turbocharged cars)

Tools

• Multimeter
• Test leads
• Phillips screwdriver

Testing

Note: On 1985.5 and newer cars, the DME control unit is located behind the kick panel in the passenger's footwell. On pre-1985.5 cars, the DME is under the dash on the driver's side.

8V Normally Aspirated Cars

1. Disconnect the electrical connector for the DME.

2. Connect an ohmmeter between terminal 2 on the DME wiring harness plug and ground. This tests the idle speed contact.

   • Throttle closed: 0 ohms
   • Throttle open: ∞ (infinite)
   • The transition must occur as soon as the throttle begins to open (approximately 1°).

3. Connect the ohmmeter between terminal 3 on the DME wiring harness plug and ground. This tests the wide-open throttle (WOT) contact.

   • Throttle closed: ∞ (infinite)
   • Throttle wide open: 0 ohms
   • The transition should occur just before the throttle reaches full open.

4. If readings at the DME plug are unsatisfactory, repeat the test at the switch itself. This requires removing the throttle body. Connect the ohmmeter between the center terminal (ground) and each outer terminal (2 and 3) and check resistance as described above.

16V Normally Aspirated Cars

Note: The 16V N/A cars use the same TPS as the early 8V cars.

1. Disconnect the electrical connector for the DME.

2. Connect an ohmmeter between terminals 52 and 24 on the DME wiring harness plug. This tests the idle speed contact.

   • Throttle closed: 0–10 ohms
   • Throttle open: ∞ (infinite)
   • Transition must occur as soon as the throttle begins to open (approximately 1°).

3. Connect the ohmmeter between terminals 53 and 24 on the DME wiring harness plug. This tests the WOT contact.

   • Throttle closed: ∞ (infinite)
   • Throttle approximately 2/3 open: 0 ohms

4. If readings at the DME plug are unsatisfactory, test at the switch as described for 8V cars.

Turbocharged Cars

1. Disconnect the electrical connector for the DME control unit.

2. Connect an ohmmeter between terminal 2 on the DME wiring harness plug and ground. This checks the idle contact.

   • Throttle closed: 0–10 ohms
   • Throttle open: ∞ (infinite)
   • Transition must occur as soon as the throttle begins to open (approximately 1°).

3. Disconnect the electrical connector for the KLR control unit.

4. Connect an ohmmeter between terminals 22 and 23 on the KLR wiring harness plug.

   • Throttle closed: 320–670 ohms

5. Slowly open the throttle toward wide open. As the throttle opens, resistance should increase continuously without interruption. Full open resistance: 2.7–4.7 kΩ.

Replacement and Adjustment

Normally Aspirated Cars

1. Remove the throttle body from the car.
2. Remove the two Phillips head screws that attach the TPS to the throttle body.
3. Install the new TPS and insert the mounting screws. Do not tighten until adjustment is complete.
4. Hold the throttle in the closed position and turn the TPS until the internal stop is felt, then tighten the mounting screws.
5. Verify adjustment: open the throttle — an audible click should be heard as soon as the throttle starts to move. Slowly close the throttle under spring pressure only (do not force it closed). Just before fully closed, an audible click should again be heard. If the click is absent, readjust.
6. Install the throttle body.

Turbocharged Cars

Tip: The throttle body does not have to be removed from the car on turbocharged models. However, the lower TPS mounting screw is very difficult to access with a Phillips screwdriver. Replacing the Phillips screws with 4 mm Allen head bolts makes future removal and installation much easier, especially with a ball-head Allen driver.

1. Remove the two Phillips head screws that attach the TPS to the throttle body.
2. Install the new TPS and insert the mounting screws. Do not tighten until adjustment is complete.
3. Hold the throttle in the closed position and turn the TPS until the internal stop is felt, then tighten the mounting screws.
4. Verify adjustment as described for normally aspirated cars — both clicks must be present.