Without Speed Density Update

Chart - Code 21 Throttle Position Sensor (Signal Voltage High):

Wiring Diagram for Throttle Position Sensor .:

CODE 21 - THROTTLE POSITION SENSOR (TPS) (SIGNAL VOLTAGE HIGH)


Circuit Description:
The throttle position sensor (TPS) provides a voltage signal that changes relative to the throttle blade. Signal voltage will vary from about .5 at idle to about 5 volts at wide open throttle. 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.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Code 21 will set if:
Engine is running
No Code 33 or 34
MAF less than 12 gm/sec
TPS voltage greater than 4.9 volts.
Above conditions exist for over 5 seconds.
OR
TPS voltage greater than 4.9 volts.
With throttle closed, the TPS should read less than .70 volts. If it doesn't, check TPS adjustment.
2. With the TPS sensor disconnected, the TPS voltage should go low if the ECM and wiring is OK.
3. Probing CKT 452 with a test light checks the 5 volts return circuit, because a faulty 5 volts return will cause a Code 21.


Diagnostic Aids:

A "Scan" tool reads throttle position in volts. Voltage should increase at a steady rate as throttle is moved toward WOT. Also some "Scan" tools will read throttle angle 0% = closed throttle 100% = WOT. An open in CKT 452 will result in a Code 21. Codes 15 and 21 are stored at the same time could be the result of an open CKT 452 which would also turn the temperature warning indicator "ON". "Scan" TPS while depressing accelerator pedal with engine stopped and ignition "ON". Display should vary from below 2500 mV (2.5 volts) when throttle was closed, to over 4500 mV (4.5 volts) when throttle is held at wide open throttle position. Refer to "Intermittents" in "DIAGNOSIS BY SYMPTOM - NO TROUBLE CODE STORED".

***THIS PROCEDURE HAS BEEN ADDED FROM TSB# 886E3 DATED NOV., '87

The ECM has the ability to auto-zero the TSP reference voltage. Auto-zeroing can be described as the ability of the ECM to determine a closed throttle position anytime TPS voltage is within a specific range. As long as the TPS is within that specific range, the system will function properly.
When checking TPS output at closed throttle, there are different specifications for different applications. "Scan" tools having the ability to also read the throttle valve angle should read 0% when the throttle valve is closed. Do not adjust the TPS if the voltage is within the specified range and throttle valve angle reads 0%
ACCEPTABLE CLOSED THROTTLE RANGE (VOLTS) = 0.35 - 0.7 Volts
ADJUSTMENT SPECIFICATION = 0.55 +/- 0.1 Volts