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This article will give information and diagnostics on the GM TPS (throttle position sensor) This article will work for any GM vehicle using a standard 3wire resistance based TPS


What it does: The TPS provides the computer with information of the position of the throttle blades. The sensor is a 3-wire design. A 5-volt reference, a ground and a computer signal are contained in the system. The TPS acts as a potentiometer to send the computer the resisted signal. The sensor is mounted to the Throttle body and usually attaches to the throttle blades via an arm, which rotates the center of the sensor.

The voltage sent to the computer will give the system the information needed top allow for correct operation. TPS voltage is used for establishing "clear flood mode", engine load and throttle angle for use of the computer systems as well and transmition operation


A faulty TPS could result in a no start, start and stall, erratic performance or transmition shifting problem depending on the type of faulty. Generally a faulty TPS will see the following codes:
OBDI codes
21 Throttle Position (TP) sensor circuit (signal voltage high)
22 Throttle Position (TP) sensor circuit (signal voltage low)

OBDII codes
P0121 Throttle Position (TP) sensor circuit insufficient signal activity
P0122 Throttle Position (TP) sensor circuit low voltage
P0123 Throttle Position (TP) sensor circuit high voltage


A faulty TPS will not always set a code. Sometimes a glitch in the sensor will allow it to function correctly except for at certain spots in the resistance bored. In this case it will usually not cause a code to set. For this reason a "sweep test" is best for finding a faulty TPS sensor.

Most GM TPS sensors use this system of wiring.
Grey: shared 5 volt reference from the computer
Black: shared sensor ground from the computer
Dark blue: input signal to the computer.



Testing and diagnostics

In order to test the TPS you will need: a multimeter for circuit testing and a multimeter or scanner for sweep testing.





circuit testing

Turn the key to the on position. With your multimeter on volts scale probe the black wire. Do you have full system ground?

If yes . next step
If no, check condition of black wire back to PCM and repair as needed


Next test for 5 volts at the grey wire. Do you have full 5 volts?
if yes continue to sweep test.
if no, check condition of wire back to PCM and repair as needed




Sweep testing

Probe the dark blue signal wire with your multimeter with the key on. you should see anywhere between 0.20V and 0.80V . that is an acceptable range however 0.60-0.80 is considered the best range.

if you are not in that range then you have to do 1 of 2 things.
if your TPS is adjustable (bolt holes slotted for movement) then you need to adjust the reading to above before continuing the test.
If you have a non adjustable (auto zeroing) TPS then you are in need of a new sensor.


Sweep test: Now you need to probe the signal wire with the key on while very very slowly opening the throttle. AS you give more throttle the voltage will start to increase. You need to watch carefully for a smooth, glitch free raise in voltage. If at anytime the voltage make a large jump, a large dip, drops to zero or jumps to above 4 volts the sensor is bad. Continue the sweep test until you have reached wide-open throttle at which time the voltage should read over 4 volts.




Some other notes and thoughts.
It’s common on fbodys to replace the stock Throttle body to a larger TB. In most cases this can through off the TPS voltage and some modifying will need to be done to get idle, IAC counts and TPS voltage back into range.

First set the idle speed with the idle set screw while watching RPM on a scanner until you’re at 800rpm for M6, 550rpm for A4 in neutral and the IAC counts are between 20-40. Now at this point you can look at the TPS voltage. you want it at aprox 0.75V. If you are not near this range you will have to remove the TPS and take a dremal or similar and slot the bolt holes so that you can rotate the sensor. Once you are in spec you are good to go




This article writtcn by Harlan Hynes. All content is protected. The reproduction of this article is expressly forbidden and is only to be used for personal use. If you wish to quote or use a part or all of this article on another site, please email hbhracing@yahoo.com
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Harlan Hynes
HBHRacing.com