The Throttle Position Sensor (TPS) is used to report various throttle positions to the DME. A very common failure item on the 944 is the "closed-position" micro-switch. Whether by heat, oil intrusion, or usage wear, this switch seems to fail every 100k miles or earlier.
The most predominant Indication of failure is a wavering idle from 800-1200 RPM.
Before replacing the TPS, you might just try adjusting it.
There are two Phillips screws holding the TPS to the throttle. The mounting tabs on the TPS are oval, which allows for some fine adjustment of the switch's "break-point". The "break-point" is the position at which the throttle is closed, and the micro-switch within the TPS is closed also. The adjustment is entirely by feel. You want the micro-switch to just engage as the throttle is completely closed and against its' stops. Adjusted too high, and the car will buck during low speed driving because the DME shuts the fuel down every time the closed position is reported during deceleration (above ~1600RPM). Adjusted too low, and the engine will not idle properly or consistently (wavering idle from 800-1200 RPM).
At this point, it is also important to be sure the throttle door is adjusted properly. Unless the throttle is not original or suspect, no adjustment will be needed. If the throttle door does not close far enough, it may be the cause of poor idle due to a vacuum leak. If the throttle door closes too far, it can jam shut or just be sticky (this is rare, and usually indicates another problem within the throttle.)
NOTE: Some cars have the TPS mounted under the throttle. It is necessary to remove the throttle from the car to properly install and adjust the TPS.
Troubleshooting: (using an ohmmeter or multimeter)
[For normally aspirated cars, pin 18 is common/ground] On the TPS, harness disconnected:
- pin2 to pin18 = 0 ohms with throttle closed, infinity with throttle open (slightly). In you get infinity with throttle closed, the microswitch is failed or the break point is misadjusted (worthwhile to test TPS completely off of throttle.
- pin3 to pin18 = 0 ohms with throttle wide open (starts just before WOT, you should be able to feel the indent if the TPS is removed from the throttle), infinity at anything less than full throttle and at throttle closed.
NOTE: Normally step 2 will read open if the TPS is failed. This an indication of a failed micro-switch
Note: Some cars have the TPS mounted under the throttle. It is necessary to remove the throttle from the car to properly install and adjust the TPS. The throttle is attached to the head intake by four Allen headed bolts, and to the air intake by a hose clamp. The throttle cable does not need to be removed from the cam, but it's probably a good idea. Turbo's and S2's have it mounted on an accessible side of the throttle (in fact, the throttle is actually mounted sideways).
- Unplug the connector to the TPS.
- Remove the Phillips screws.
- The TPS will now slide off the throttle body.
- It is impossible to install it the wrong way due to a sectioned mounting spindle.
- Adjust the new micro-switch as prescribed above.
Tools: Phillips screwdriver, Ohmmeter or Multimeter (if troubleshooting), 6mm Allen key or driver
Credits: Skip Grehan
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> Control Arm Bushing Upgrade - Weltmeister
> Control Arm, Front - Info
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