TPS TEST 3: Testing The TP Signal
Up until this point, your tests have confirmed that the throttle position sensor is getting 5 Volts and Ground. The next step is to see if it's producing a good signal that the PCM can use.
If the throttle position sensor (TPS) is working correctly, when you open the throttle plate, the multimeter will register a smooth increase in the DC voltage until the maximum voltage is reached, which is about 4.5 Volts DC.
As you slowly release the throttle plate back to its closed position, the multimeter will display a gradual decrease in voltage till the initial base voltage is achieved (which you recorded in the beginning of the test).
If the TPS is bad, then there will be sudden gaps/loss of voltage as you increase or decrease the throttle plate's to its fully open or fully closed position, especially when you tap on the sensor. Or, there will be no voltage reading at all. OK then, here are the two possible outcomes:
You'll need a helper to assist you in this test step, since he or she will need to lightly tap on the throttle position sensor's body with a screw driver (or other appropriate tool) while you observe the multimeter and manually actuate the throttle. OK, let's start testing:
Select Volts DC mode on your multimeter.
Probe the circuit labeled with the number 2 in the photo with the red multimeter test lead, using an appropriate tool to pierce the wire.
Ground the black multimeter test lead directly on the battery negative (-) terminal.
Turn the Key On with the engine Off.
Your multimeter should register about 0.9 to 1 Volt (this voltage specification might vary a little bit on your vehicle).
Manually rotate the throttle plate by hand as you eye-ball the multimeter's voltage readings.
The multimeter's initial voltage reading should increase smoothly as you open the throttle plate to its Wide Open Position. At Wide Open Throttle, the reading on your multimeter should read about 4.5 to 4.9 Volts DC.
Now, slowly release the throttle plate to its fully closed position, all the while observing the multimeter's reading.
The multimeter's voltage reading should decrease in a smooth and linear fashion back to the exact same voltage you noticed in step 5.
OK, now have your helper lightly tap the TP sensor with the butt of a screw-driver's handle (or something similar).
As your helper taps, slowly and smoothly open the throttle to its open-wide position. The readings on your multimeter should increase and max out at about 4.5 to 4.9 Volts DC, at Wide Open Throttle.
Now, slowly release the throttle back to its closed position. Your multimeter should read the base voltage that you recorded in step 5 of this test.
All the while you've got your eyes glued on the multimeter to see if the tapping affects the voltage readings.
Repeat this (tapping the throttle position sensor) several times to make sure of your results.
Let's interpret your test result:
CASE 1: The multimeter registered a smooth increase or decrease in voltage. This is the correct test result.
You can conclude that the TP sensor is working OK and is not the cause of the TPS fault code issue. Go to: TPS Code Won't Go Away for a few more suggestions as to what could be causing the TPS diagnostic trouble code (DTC).
CASE 2: The multimeter DID NOT register a smooth increase or decrease in voltage. With this test result you can conclude that the throttle position sensor (TPS) is bad if you have:
- Confirmed that the TPS is getting 5 Volts (TEST 1).
- Confirmed that the TPS is getting Ground (TEST 2).
- Confirmed that the TPS is not creating a voltage signal that increases/decreases as you open/close the throttle plate (TEST 3).
TPS Code Won't Go Away
So you've tested your Ford Mustang TPS and according to the test results, the TPS is good. But the check engine light keeps coming back on even after you erased the diagnostic trouble code (DTC) from the computer's (PCM) memory. Well, here are a couple of suggestions that might inspire your next diagnostic move:
- The throttle plate's idle-stop screw's factory adjustment has been altered so that the engine could be idled up and mask a miss/misfire and/or rough idle. This increases the TP Sensor's signal to the PCM. The PCM doesn't like it and lights up the check engine light (CEL).
- The throttle cable is binding and causing the throttle plate to not fully close.
- This can be verified by simply having someone inside the vehicle pushing the accelerator pedal to the floor and releasing it, with the engine OFF, while you visually check that the throttle plate and cable are not getting stuck somewhere in their travel.
- The TPS is failing intermittently. Which means that it works fine most of the time, but every now and then it doesn't:
- I have found that the best way to test these intermittents is to road-test the vehicle with the multimeter hooked up to the TP signal wire with a long wire so that I can comfortably observe the signal going up and down as I or someone else drives.
- The TP sensor's connector is bad, usually the locking tab is broken and the connector has worked itself loose, causing an intermittent false connection.
If this info really saved the day, buy me a beer!