TPS & ISC replacement/adjustment
1980's cars

If the diagnostics are run after a Service Soon error is shown, and errors relating to the Throttle Position Sensor (TPS) and/or Idle Speed Control (ISC) are 21, 22, 26, 27and/or 30, it is likely that the TPS is worn.

A worn TPS will give an intermittent throttle position readout to the ECM, and can cause the idle speed to fluctuate and cause the above errors. The TPS is used to send the throttle position to the ECM, indicating to the computer the angle of throttle. It senses 0-90 degrees of throttle angle.

The ISC is controlled by the ECM and adjusts the idle speed dynamically according to load when the engine is at idle.

The airconditioning compressor cycling or moving the gear selector from Park/Neutral to Forward and Reverse positions will change the load on the engine, which the ISC compensates for. If either the TPS or ISC are replaced, they need to be adjusted together.

Factory Manual section 6D3-12

http://www.ht4100.com/Diags/HT4100ISC.pdf