Throttle Potentiometers
& Idle Switches

Pre-March 1990 Production

The early 1990 cars have a separate idle switch and throttle potentiometer, but the pot actually has an internal switch that acts just like the idle switch. The idle switch (and pins 3 & 4 of the throttle pot) should be closed about 1.5 degrees before the throttle plates close, and should be open at all other throttle positions. If you operate the throttle with your hand, you should be able to see the switch as it is activated when the throttle is closed. Use a multimeter to verify that the switch is closing properly and that resistance isn't too high  (< 10 ohms) when it is closed. A replacement early-style switch is P/N 054 133 093 A.

The throttle pot is supplied 5 V to pin 4 by the ECU, the output back to the ECU is on pin 3, and pin 2 is ground. Here are some tests to perform on the throttle pot with a multimeter. Please note that the numbers in the following table correspond to the numbers in the picture above. There are numbers molded into the plastic connector that will be opposite from these.

A replacement throttle potentiometer for the early 1990 cars is P/N 054 133 154 C.

Throttle Potentiometer Removal/Replacement
(Wylie Bean)

1. Consider removing the battery, which will provide much needed room to get at the bottom left screw mounting the throttle body to the intake.
2. Remove the hose that connects the ISV to the intake hose.
3. Loosen clamps and remove intake hose. The metal section of the crankcase breather feeds into the bottom--just pull straight up to disconnect the intake hose from the metal tube.
4. Unplug the throttle switch (mounted at top of throttle body) and remove the two screws for the mounting bracket.
5. There is a vaccuum tube (metal) attached to the right side of the throttle body via a right angle rubber section. Carefully pull it loose...the rubber section is probably dry and could break.
6. There is also a vaccuum hose attached to the rear of the throttle body just below the mounting point for the idle switch. Remove it too.
7. The throttle cable is attached to the top of the throttle body in a grooved-sleeve. Remove the two clips holding it in place, and push down on the cable to release it. There will be some slack, so just push it into a groove at the adjusting point to keep it out of the way.
8. The throttle body is mounted to the intake with four 7mm hex head screws. They are tough to get off with an allen wrench. Consider using a hex head socket and some penetrating lubricant (WD-40) on them, and maybe a few taps with a hammer to break the seal.
9. Once the screws are out, the throttle body just pulls straight off the intake manifold. Be careful if your throttle pot is loose--it will fall off!
10. Reattach your throttle pot securely, or put on the new one if replacing. Adjust so that the idle switch (and pins 1 & 2 of the throttle pot) are closed about 1.5 degrees before the throttle plates close. You may want to check the readings for the potentiometer to make sure everything is lined up properly. You don't want to have to go through all this again just to readjust the potentiometer position.
11. Check the gasket between the throttle body and intake manifold and consider replacement (P/N 037 133 073 A).
12. You might take this opportunity to thoroughly clean the throttle body with carb cleaner before putting it back on.
13. Getting the metal breather tube back into the underside of the intake hose is tricky, and takes some coaxing. You can try a thin coat of lubricant (silicone free!) on the mating surfaces to get them together.

Post-March 1990 Production

The late 1990 cars and all 1991 cars have a single unit that functions as both an idle switch and throttle potentiometer. The electrical connector is in roughly the same location, though.  The tests are the same: the idle switch should close about 1.5 degrees before the throttle plates close, and the resistance shouldn't be too high (< 10 ohms) when closed.

The throttle pot is supplied 5 V to pin 1 by the ECU. The output can be measured across pins 3 (output signal) and 2 (ground). You should see 5 V with the throttle closed and a linear decrease as the throttle is opened until it reaches 0 V at WOT. Here are some tests to perform on the throttle pot with a multimeter:

A replacement throttle pot/idle switch for the later cars is P/N 051 133 154 A.

Replacement procedure should be similar to the above instructions, but the ISV is mounted directly to the intake hose so adjust step #2 as necessary.

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