Basic Idle Switch and Throttle Position switch testing (from the Bentley)
 

For the Idle Switch (or Closed Throttle Position (CTP) switch):

From pg J 24-64 Bentley Vol 1:

- Disconnect Throttle position (TP) sensor connector.

- Set Multimeter to resistance range

- where Pin 1 of the TP sensor is at the top and pin 6 is at the bottom, connect the multimeter between pins 4 and 6

- Resistance must be 0 ohms (continuity)

- Open throttle valve slightly

- Resistance must be infinite ohms (open).

Checking voltage supply:

Disconnect the harness connector from the TP sensor

- Switch ignition ON

- Connect digital multimeter set to 20 VDC, in turn, between

- Terminals 1 and 2 in harness connector where, with the connector wire "bail" (spring) up, terminal 1 is on the left and 6 is one the right, voltage should be between 4.5 and 5.5 VDC

- Terminals 2 and 3, voltage should be between 4.5 and 5.5 VDC

- If no, check ECU (ECM) and ECU wiring
- If yes, check the resistance of the TP sensor

Checking the TP sensor resistance (from Bentley Vol 1, pg. J24-72):

Where pin 1 on the TP sensor is at the top and pin 6 is at the bottom, with the digital multimeter set to resistance,

- Connect between terminals 1 and 2:

- Must be 1500 to 2600 ohms

- Connect between terminals 2 and 3:

- Must be 750 to 1300 ohms

Measure resistance again while bringing the throttle lever slowly to Wide Open Throttle (WOT) position:

- Must increase to 3600 ohms maximum

If NO (for one or both circuits) - replace TP sensor (PN 078 133 154 - common to many Audi's of the same vintage).

Hope that helps