Throttle Position Reset (1987-1989)
If your car has an erratic idle and the RPM’s either drop or raise up when you are just idling at a stop you need to first address this by Resetting the TPS/ISC. All you need is a $15 multi-meter. Take it one step at a time.
1. Before warming up the engine, pry out the rubber end of the TPS connector so you can stick a voltmeter in there later. Do the same on the ISC motor position sensor (MPS) connector. The ISC connector should be the rear-most 3 wire connectors on the throttle body: yellow+blue, yellow+black, and yellow+red wires on the wiring harness side (NOT the sensor side of the harness) and red, green, and black wires on the end going into MPS on the throttle body. Pry out the MPS end of this connector – it should be the end pointing upwards. The TPS is the black cylinder at the bottom/front of the throttle body.
2. Loosen the accelerator cable. Get lots of free-play in there since the idle climbs as things warm up and this cable tightens too much if not enough slack is present.
3. Warm up the engine. Then shut it back off. The engine must be fully warmed up.
4. Turn the key to ON but do not crank/start. Leave it there for at least 15 seconds. You should hear the ISC motor “zip” in as it commands a drop from fast idle to normal idle. If you want, stick a voltmeter on the motor position sensor (not the TPS) Black wire (-) and Green wire (+) and look for 0.9 volts after the motor zips back to the low idle position. If you don’t get 0.9 volts, there is a problem with the wiring harness between the MPS and the ECU. Some cars seem to settle at 0.93 to 0.95 volts; that’s close enough. Turn the key off, wait a few seconds, and then turn it back on. The ISC will “zip” again and after 15 seconds you should be back at the 0.9volt position. Do this several times and see how consistent the voltage reading is; if it varies more than a tenth of a volt your ISC/MPS is either dying or your throttle linkage is binding. Or your car isn’t warmed up… if the ECU thinks the engine is cool it holds the idle higher and the MPS voltage is correspondingly higher.
5. With the ISC/MPS in the 0.9 volt position, remove the ignition key. Then unplug the ISC servo motor electrical connector. This locks the ISC at the position the ECU thinks will give a perfect idle speed. Run a jumper wire from harness Yellow+Red wire to ground to simulate a closed Idle Position Switch to the ECU. (NOTE: This wire is on the harness side, not the sensor side)
6. Fire it up and look for 850 to 950 RPM. If it’s not there, crank the ISC screw until it is there. This screw is the hex-headed one visible on the linkages if you look from the passenger side of the car. A 7mm socket should fit it. Make the adjustments in the tightening direction (i.e. increasing the RPMs); if you need to lower the RPMs go well below the correct idle speed and then creep back up. This compensates for lash (slop) in the screw.
7. Now crank the old-fashioned idle screw at the base of the throttle body, the Fixed SAS, until the RPMs increase (touch point). Back off one full turn from the initial touch. This is the mechanical min-idle speed backup in case the ECU can’t control the idle speed for some reason.
8. Shut down the engine, then turn the ignition key back to ON. Measure the TPS voltage from the Green+White leads (+) to Green+Black and look for the famous 0.50 volts. Move the TPS potentiometer as needed to adjust this. Get within +/-0.02 volts.
9. Turn the key off. Tighten the accelerator cable but leave a little slack, restore the rubber end caps to the connectors, and plug everything back in. Then unplug the battery for 30 seconds to clear any ECU errors.
Note: during step 4, watch the throttle linkages as the ISC does the “zip” movement. If you notice oscillations, there is binding in your throttle linkage and the ECU can’t control the ISC well enough to do the job.
