2. Throttle Position Sensor
As the name suggests this lets the ECU know how far open the throttle is. Although this is not the primary load sensing device, it can upset things pretty badly if it's faulty or not adjusted correctly. It consists of a potentiometer (just like a volume knob) that rotates as the throttle is opened and closed. It has 3 pins. The ECU feeds 5V to one, 0V to the other and a voltage somewhere in between is picked off and sent back to the ECU.
Adjustment : This can either be done manually (with a multimeter) or by plugging into the Consult Port with a laptop and reading it straight off the screen. If using a multimeter it must be done with IGN ON (engine does not need to be running) and the connector fitted to the TPS. You'll need nice sharp multimeter probes to pierce the insulation on the wires - it often pays to give them a quick sharpen with a file/grinder etc. Carefully probe the 0V wire (Black) and the signal wire (White). Don't use chassis GND. With throttle closed you should be seeing 0.5V. The spec is 0.35V to 0.65V but it's good to have it right in the middle. Adjustment is done by loosening the 2 screws that mount the TPS - NOT by using the throttle stop screw.
TPS Closed :Check that the ECU is actually seeing "TPS Closed". It monitors the TPS position and when it's in a certain range (normally 0.35 to 0.65V) it assumes the throttle is closed. This triggers a flag in the ECU which makes it go into idle mode. You can monitor this via the Consult port using Datascan. If your throttle is closed but the ECU doesn't see it, then stable idle will be impossible to achieve. Some ECU's will learn where TPS Closed is by checking the value at startup. After a few starts (with throttle closed) the ECU will assume that the voltage it's seeing is TPS Closed and the TPS Closed flag will be set.
Check TPS function : While you're playing with the TPS it's a good idea to check it across the range. Slowly open the throttle - you should see the voltage increase smoothly to just over 4V. If you see any sudden changes (to 0V or 5V) then the TPS must be replaced. The Consult Port is extra handy for doing this check as you can log the TPS value while you drive the car - and then check the trace for any drop-outs later.
Autos : These use a different TPS. It has 6 wires. Half of it still functions the same as the manual 3 pin TPS - so all of the above still applies (wire colours are different but you still need to measure between the Black and White wires). The other half has switches (for throttle fully closed and fully open) which go to the auto trans to determine shift points. The TPS in auto vehicles is notoriously unreliable. When the auto trans half goes faulty the trans will not shift correctly. Unfortunately the main TPS part also goes bad very quickly. They suffer from moisture ingress. Once this happens a replacement TPS is required. If you've converted to a manual trans then it pays to also convert to a manual TPS for reliability's sake. They bolt up but you'll need to replace the 6 pin TPS connector and replace it with a 3 pin manual TPS connector. You only need the White (signal), Light Green/Black (5V power) and Black (return) wires. Snip the others off.