How To Test: Throttle Position Sensor (1988-1995 Ford 4.9L)

Throttle Position Sensor (TPS) Is Good But Code Won't Go Away

If you've tested the TP sensor or you've replaced it with a new one and yet the TP sensor code doesn't go away, then this section may be of help.

The most important thing you've done has been eliminating the TP sensor as the source of the problem (trouble code) itself. Your next steps should be:

1. Checking the adjustment of the throttle plate stop set screw.
   • This screw's adjustment usually has been altered to idle up the engine and mask a rough idle condition. The PCM doesn't like it when this is done and sets a TP sensor code.
2. Check the continuity of the TP sensor's wires (between the PCM and the TP sensor connector).
   • If there's a break in any of the 3 wires, the PCM will set a TP sensor code.
3. Check for a bad fuel injection computer (PCM).
   1. Is there a way to test for a bad PCM? Yes and no. The only way to test for a bad PCM is to test it indirectly. This is what I do to test for a bad PCM (with about a 98% success rate):
      1. Eliminate the TP sensor itself as bad.
      2. Check the continuity of the 3 TP sensor wires (between the PCM and the sensor). The idea here is to make that none of the TP sensor wires are cut (‘open’).
      3. Check all of the PCM Ground wires by doing a voltage drop test on them. This simply involves checking the Ground wire for voltage (not Ohms) with the Key On Engine Off. There should be no voltage on the wire. If there's voltage (2 Volts or more) that specific Ground wire is not providing a good path to Ground and it means you need to clean the Ground terminals and/or check their condition.
      4. Check that the PCM is getting power on the all of it's power circuits.
      5. After eliminating the TP sensor, a problem in the 3 TP sensor wires (between the PCM and sensor), eliminating the PCM's Ground wires, and verifying the PCM is getting power on all its power circuits, then, and only then I can say with confidence that the PCM is bad.

The key to testing all of the above (and either exonerating or blaming the PCM as bad) requires that you have the specific wiring diagram for your particular Ford vehicle.

More ‘How To Test’ Tutorials

You can find a pretty big list of Ford tutorials in this index: Ford 4.9L, 5.0L, 5.8L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

1. Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L).
2. Ignition Coil Test -No Spark No Start Tests (Ford 4.9L, 5.0L, 5.8L).
3. How To Troubleshoot A No Start (Ford 4.9L, 5.0L, 5.8L).
4. Testing A Blown Head Gasket (Ford 4.9L, 5.0L, 5.8L).
5. How To Test Engine Compression (4.9L, 5.0L, 5.8L).
6. How To Test The Ford Mass Air Flow (MAF) Sensor (at: easyautodiagnostics.com).