How To Test The GM Accelerator Pedal Position (APP) Sensor 1

TEST 4: Testing The APP Sensor 1 Signal Circuit

So far in your troubleshooting, you have:

• Resistance tested Accelerator Pedal Position (APP) Sensor 1 (TEST 1).
• Checked for the 5 Volt Reference signal (TEST 2).
• Checked and confirmed that APP Sensor 1 is getting Ground (TEST 3).

The next step, and the last one of this tutorial, is to check the voltage output of Accelerator Pedal Position (APP) Sensor 1 with your multimeter in Volts DC with the Key On and Engine Off (KOEO). When the accelerator pedal is at rest (not depressed), it should output about 1.0 Volt DC. When the pedal is fully depressed, the voltage signal should go up to about 2.0 Volts.

NOTE: This test has to be done with the Accelerator Pedal Position Assembly back in its place (in the vehicle) and connected to its electrical connector.

OK, to get this pot boiling, this is what you need to do:

1. If you haven't already, install the APP Sensor Assembly and bolt it down.
2. Reconnect the APP Sensor to its connector and reconnect the battery negative cable to the battery negative post.
3. Turn the key to the ON position but don't crank or start the engine.
4. Set your multimeter in Volts DC mode.
5. With the APP Sensor Assembly connected to its connector, probe the Dark Blue wire.
   • This is the wire identified with the letter F in the photo of the connector above.
   • This is where a wire piercing probe comes in handy since it'll let you easily access the wire inside the insulation (to see what this tool looks like, click here: Wire Piercing Tool).
6. Ground the multimeter's black test lead directly on the battery negative (-) terminal.
7. Observe your multimeter's voltage DC reading.
8. You should see about 1.0 Volts DC.
9. Now, manually push down the accelerator pedal till it bottoms out.
   • Your multimeter should register about 2.1 Volts DC when the pedal is fully depressed.
   • NOTE: If all is OK, your multimeter should register a smooth voltage increase with no gaps in the voltage numbers.
10. After pushing the pedal all the way to the floor, slowly release it as you watch the numbers on your multimeter.
    • If all is OK, your multimeter should register a smooth voltage decrease with no gaps in the voltage numbers.

Let's examine your test results:

CASE 1: Your multimeter registered the indicated voltages with NO gaps. This is the correct and expected test result and lets you know that APP Sensor 1 is OK.

Now, if you have performed all of the tests (in this tutorial) and they have all passed with flying colors, YET you still have an APP Sensor 1 diagnostic trouble code, there's a good chance that:

1. There's an open-circuit problem in this wire between the APP Sensor Assembly's connector and the Throttle Actuator Control (TAC) Module.
2. The Throttle Actuator Control (TAC) Module is bad.

It's beyond the scope of this article to troubleshoot these specific issues, but you have at least eliminated APP Sensor 1 as being bad.

CASE 2: Your multimeter DID NOT register the indicated voltage and/or showed gaps in the voltage numbers. This is not good and tells you that APP Sensor 1 is malfunctioning.

You can confidently replace the APP Sensor Assembly if the APP Sensor Assembly failed this test and TEST 1.

Some Final Thoughts On APP Sensor 1

Testing GM's electronic throttle body system can seem difficult, especially if you only have a generic scan tool (compared to a $5 K Professional level scan tool used in the auto repair shops), but it doesn't have to.

The key, to successfully diagnosing an Accelerator Pedal Position (APP) sensor problem, is remembering that the APP Sensor(s) inside the Accelerator Pedal Position Assembly are just potentiometers (like the old style throttle position sensors on fuel injection systems with a throttle cable).

Since they are just potentiometers, they can be resistance tested with a multimeter (like 'TEST 1: Resistance Test Of APP Sensor 1' shows) or voltage tested (like 'TEST 4: Testing The APP Sensor 1 Signal Circuit' showed).