Powertrain Controls - ECM/PCM

Code 21 Diagnostic Chart:

TPS, CTS, & IAT Sensors (Circuit Diagram):

CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle blade angle. Signal voltage will vary from about 0.4 at idle to about 5 volts at wide open throttle. The TPS signal is one of the most important inputs used by the PCM for fuel control and for most of the PCM control outputs.

Code 21 will set if:

^ TPS voltage is greater than 4.8 volts at any time.

OR

^ Engine is running and air flow is less than 15 gm/sec.
^ TPS signal voltage is greater than 0.80 volts.
^ Code 34 not present.
^ All conditions met for 5 seconds



TEST DESCRIPTION
Numbers below refer to circled numbers on diagnostic chart.
1. With closed throttle, ignition "ON," or at idle, voltage at BB10 should be 0.33-0.46 volt. If not adjust TPS. See ADJUSTMENTS.
2. When the TPS sensor is disconnected, the TPS voltage will go low and a Code 22 will set, indicating the PCM and wiring are OK.
3. Probing CKT 452 with a test light checks the sensor ground. A faulty sensor ground circuit will cause a Code 21.

NOTE: If a Code 21 is set, the PCM will not allow 4th gear or TCC operation.

DIAGNOSTIC AIDS:
^ Scan tool reads throttle position in volts. With closed throttle, ignition "ON" or at idle, voltage should be 0.20-0.74 volts. If not adjust TPS. See ADJUSTMENTS.
^ Also some "Scan" tools will read throttle angle as a percentage. 0% = closed throttle, 100% = WOT. An open in CKT 452 will result in a Codes 15 and 21.

If circuit 417 is open or grounded when the vehicle engine is started, a high idle may result


Check For:

^ Poor Connection or Damaged Harness Inspect PCM harness connectors for backed out terminal "BB10," 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 (0.33 - 0.46) when throttle was closed, to over 4.0 volts when throttle is wide open. Typically, accelerator pedal fully depressed should yield approx. 4.1 - 4.2 volts from the sensor. If voltage is greater than 4.8 volts at any time while operating throttle with accelerator pedal, replace the TPS.