TEST 2: Making Sure The TPS Is Getting 5 Volts And Ground
If you've reached this point then you have confirmed that the TP sensor did not pass TEST 1.
Generally this will indicate that the TPS is bad, but to be sure we need to check that it's getting 5 Volts and Ground. This we can easily do by doing a simple multimeter voltage test.
The wire that feeds 5 Volts to the TPS is the brown with white stripe (BRN/WHT) wire. This BRN/WHT wire connects to the TPS terminal labeled with the number 3 in the illustration above.
The wire they feeds Ground to the TPS is the grey with red stripe (GRY/RED) wire. This GRY/RED wire connect to the TPS terminal labeled with the number 1 in the illustration above.
NOTE: The fuel injection computer on your 3.8L V6 Ford Taurus (Mercury Sable) is a component that feeds the TPS with Ground. For this reason, you need to be careful not to short the Ground wire to battery voltage, or you will fry the computer. The multimeter voltage test indicated in the instructions below is a safe way of testing the Ground circuit.
These are the test steps:
Check the BRN/WHT wire for power with the Key On Engine Off (KOEO). The BRN/WHT wire is the one that connects to terminal number 3
Connect the red multimeter test lead to the BRN/WHT wire and connect the black lead to the battery negative (-) post.
The multimeter should register 4.5 to 5 Volts DC.
Check the GRY/RED wire for Ground with the Key On Engine Off (KOEO) (see illustration above). The GRY/RED wire is the one that connects to terminal number 1
Connect the black multimeter test lead to the GRY/RED wire and connect the red lead to the battery positive (+) post.
The multimeter should register battery voltage (12+ Volts).
Let's interpret your test results:
CASE 1: The TPS is getting 5 Volts and Ground. This is the correct test result.
Taking into account that you've reached this point because the TPS did not pass the test in TEST 1 and that it is getting 5 Volts and Ground, you can now correctly conclude that the TPS is defective. Your next step is to replace the throttle position sensor.
CASE 2: The TPS is not getting 5 Volts or Ground. Without 5 Volts or Ground the throttle position sensor will not work.
The next step is to find out why power or Ground is missing and resolve the issue. I can tell you from personal experience that usually what causes this to happen is an open circuit in the wires between the fuel injection computer and the TPS connector.
TPS Trouble Code Won't Go Away
I've seen a few cases where the fuel injection computer accuses the TPS of malfunctioning by setting a TPS trouble code yet the sensor is functioning correctly.
In these cases it's usually some other mechanical issue that's fooling the fuel injection computer into thinking the TPS is defective. So, if you're experiencing a similar situation, I think the following suggestions may help you to save sometime, frustration, and money:
- Check that the TP sensor's wiring harness connector for damage.
- It's a common problem for the locking tab, of the connector, to break and cause a false contact condition as you drive the vehicle on the road.
- Check that the throttle plate idle stop screw isn't misadjusted.
- What I've seen quite a bit, is that the idle stop screw has been adjusted to increase the engine's idle RPM to mask a rough idle condition. But doing this usually causes the computer to set a TPS trouble code.
- Check the continuity of the wires between the fuel injection and the throttle position sensor.
- The idea behind this test is to make sure that there isn't an open circuit in the wiring between the TPS and the fuel injection computer.
- Check that the computer isn't defective. This can seem like quite a challenge but it's possible. This is how you can conclude that you have a defective fuel injection computer on your hands:
- Eliminate the throttle position sensor as defective first.
- Make sure that the wiring between the TPS and the computer has continuity. In other words there aren't any short circuits or open circuits in the wiring between these two.
- Confirm that the fuel injection computer's Ground wires are actually feeding it Ground. The best way to do this is to do a voltage drop test on each fuel injection computer's Ground wires.
- Confirm that the fuel injection computer has power in each of its power wires.
- After eliminating the TPS itself, after verifying the continuity of the TP sensors 3 wires (between its connector and the fuel injection computer), after verifying that the fuel injection has good Ground and power; then and only then can you conclude that there might be a problem with the computer internally.
More Ford 3.8L Tutorials
You can find a complete list of 3.8L Ford tutorials in the following two indexes:
Here's a sample of the tutorials you will find in the indexes:
- How To Clean the Ford Mass Air Flow Sensor.
- Coolant Leaking From Intake Gaskets (Ford 3.8L).
- How To Diagnose Misfire Codes (Ford 3.0L, 3.8L).
- How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L) (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!