The throttle position sensor on a 1993 Toyota Corolla is a linear potentiometer (variable resister). The sensor signals the computer the amount of throttle open angle relating to the throttle plate as seen looking into the throttle body; this is the large plate in the opening. It also signals the closed throttle position. The computer uses these signals for power enrichment, deceleration fuel cutoff, idle stability and spark advance corrections. If the car has an automatic transmission, the TPS controls the shift points.
Locate the throttle position sensor on the opposite side of the throttle body throttle linkage. It will have a four-wire connector. When viewing the electrical plug wires, notice they are in a vertical line. Starting from top down, the first wire is the computer 5-volt power wire (VC), second is the TP signal to computer in terms of the degree of throttle plate opening called variable throttle angle (VTA), third is the closed throttle position (CTP) and fourth is the computer ground designated (E2). If there is a dashpot on the throttle body, it must have a vacuum before testing the TPS. Its job is to slow the throttle closure to prevent backfiring. Pull the vacuum hose off and apply a vacuum to this port on the dashpot -- just enough to extend the pin on the top so it touches the throttle linkage.
Connect the voltmeter black lead to the negative terminal on the battery. Use the sharp pointed probe on the red lead to pierce the TP signal -- the second wire from the top. Turn the ignition key on without starting the engine.
Check the voltage. It should be 0.5 volt with the throttle closed. Using the throttle linkage, slowly and steadily sweep the throttle to wide open. The voltage should rise smoothly to about 4 volts with no drop-outs or glitches. A drop-out is when no voltage is displayed in one particular spot in the sweep; a glitch might be a sudden spike up or down in the voltage. There isn’t anything can be done about the wide open voltage. If it isn’t in parameters, the TP sensor must be replaced. If the wide open voltage is very close but the closed throttle voltage is high or low by a small amount, the sensor can be easily adjusted. Loosen the two retaining bolts and twist the sensor in either direction as permitted by the slots in the sensor, until the throttle closed voltage is correct. Tighten the two bolts.
Move the red probe to the CTP wire (third wire down). There should be no voltage with the throttle closed. Slowly open the throttle, and with very little movement, the voltage should jump to 5 or 12 volts, depending on the make of sensor. This signal is sent to the transmission control unit, which signals the transmission to squat. On a Corolla, the transmission goes into third gear first to reduce the shock on the transmission when first going into gear. It drops into first gear after the initial shock. There are two types of TPS sensors used on the '93 Corolla. One is direct to the transmission control unit, and the other is indirect -- hence the reason for the differences in voltage on CTP signal. Replace the sensor if the voltage does not jump up when the throttle is opened. If the TPS is way out of range or has no signal at all, proceed to the next step.
Move the probe to the last wire on the bottom, the computer ground (E2). Voltage should be below 100mV. If it is higher, there's a short in the wire to the computer. Repair the short and retest.
Move the probe all the way to the top wire (VC). This is the 5-volt computer wire. Voltage must be 4 to 6 volts. If there is no voltage or it is too low, there's a problem with this wire to the computer. Inspect, repair the problem and retest.
Turn the ignition key off. Pull the plug out of the TPS. Place the voltmeter on the ohms scale. Connect alligator clips to both probes. Connect the red clip to the second wire from the top, which is the VTA, and the black clip to the bottom computer ground (E2).
Observe the resistance displayed. The amount of resistance is unimportant as long as there's continuity (ohmmeter displays a value). What you are looking for is a base value with extremely smooth consistent ascent to wide-open throttle and descent to closed throttle with no abnormalities, spikes or drop-outs in the values at any point. Pay particular attention to mid throttle, This is where most TP sensors fail. It is at cruise where the TP sensor is used the most and wears out. You may move the throttle open, and right at this point you see the resistance drop off dramatically or completely and them come back on either side of this spot. Smoothly open the throttle all the way and close it again keeping this in mind. Replace the sensor if there are any problems.
Move the red alligator clip down to the third wire (CTP.) This is the closed throttle position wire. This signals the beginning of valve opening for the transmission and engine computer so the engine management computer will begin to advance the timing. Resistance should initially be infinite (no value displayed). As soon as the throttle is opened a small amount, resistance should be displayed, and when closed again, the resistance should go infinite. If not, replace the TP sensor.
