Powertrain Controls - ECM/PCM

Code 21 Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION:

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to the throttle opening. Signal voltage will vary from about 0.4 volt at idle to about 5.0 volts at wide open throttle (WOT).

The TPS signal is one of the most important inputs used by the ECM for fuel control and for most of the ECM control outputs.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 21 will set if:

^ TPS voltage is greater than 4.9 volts at any time or if:
^ Engine is running and air now is less than 15 gm/sec.
^ TPS signal voltage is greater than 2.5 volts.
^ Code 34 not present.
^ All conditions met for 5 seconds.
^ With closed throttle, ignition "ON," or at idle, voltage at "YD13" should be 0.33-0.46 volts. If not adjust TPS. See TPS ADJUSTMENT under ADJUSTMENT PROCEDURES.

2. With the TPS disconnected, the TPS voltage should go low and a Code 22 will set. Therefore, the ECM and wiring are OK.

3. Probing circuit 452 with a test light checks the TPS ground circuit. A faulty sensor ground circuit will cause a Code 21.

NOTE: If a code 21 is set, the ECM will use a default value for TPS of about 0.5 volt.

DIAGNOSTIC AIDS:

^ Scan tool reads throttle position in volts. With closed throttle, ignition "ON" or at idle, voltage should be 0.33-0.46 volts. If not adjust TPS. See TPS ADJUSTMENT under ADJUSTMENT PROCEDURES.

^ An open in CKT 452 will result in a Code 21. See DIAGNOSIS BY SYMPTOM.

^ If circuit 417 is open or grounded when the vehicle engine is started, a high idle may result. A "Scan" tool may read the following with circuit 417 open or grounded:

^ TPS =.04 volt.
^ RPM > 1000 in park.
^ Desired idle = 725.
^ IAC Counts > 40.
^ "SERVICE ENGINE SOON" light "ON," Code 22 set.

If the intermittent is repaired without cycling the key "OFF," the "Scan" tool may read the following:

^ TPS = 0.42 volts.
^ RPM > 1500 in park.
^ Desired idle = 725.
^ IAC Counts > 55.
^ "SERVICE ENGINE SOON" light "OFF."

Check For:

^ Poor Connection or Damaged Harness Inspect ECM harness connectors for backed out terminal "YD13," improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent Test. If connections and harness check OK, monitor TPS voltage while moving related connectors and wiring harness. If the failure is induced, the display will change. This may help to isolate the location of the malfunction

^ TPS Scaling. Observe TPS voltage display while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from closed throttle TPS voltage when throttle was closed, to over 4.0 volts (4000 mV) when throttle is held at wide open throttle position. Typically, accelerator pedal travel only yields 4.1 or 4.2 volts maximum, but full throttle rotation at the throttle body should be about 5.1 volts.