The A/C Control Unit (ACU) sends a signal to the ICU whenever the A/C compressor is on. When the car is at idle, the ICU is supposed to increase the idle speed to compensate for the extra load placed on the engine by the compressor. Without this compensation, normal idle speed would drop below the recommended limit.

The ICU only needs to make this adjustment at idle and not at normal driving RPMs. How does the the ICU make this determination? It does this by sensing the position of the TPS. If the TPS malfunctions, then the ICU won't increase RPMs in response to the A/C compressor. This is why I suggested performing the A/C test.

The ICU also uses the TPS to trim the amount of fuel delivered by the injectors by way of the EHA (Electro-Hydraulic Actuator) based on the position of this switch. The ICU is supposed to reduce the amount of fuel injected when it senses the TPS in the "Idle" position. This is why when you pull off the 3-prong connector, the idle speed should increase since the ICU no longer senses an idle condition.

To Test the TPS:

When you pull off the 3-prong connector, idle should increase if the TPS is working correctly. Another way to test is to use a continunity tester on the male (stationary) end between the middle pin and the pin furthest from the CSI. At idle or when the engine is off, the meter should show a short between the pins. FYI, the middle pin on the male side of the connector is not grounded but is simply one of the switch contacts. On the female cable side of the cable, the middle pin IS grounded. This is where the TPS gets its ground connection from and uses it to ground the idle sense input pin of the ICU. Make sure the middle pin ground connection on the female cable side is good since this could also cause a problem.

T.C. Mann (aka TechMann)