This tutorial will help to test the throttle position sensor (TPS) circuits between the PCM and TPS connector to see if they are shorted together or 'open' (or not).
More importantly, this tutorial will help you to come to an informed decision as to whether the PCM has fried and is causing the TPS trouble code or not.
IMPORTANT: Although the basic testing method described in this tutorial could be used with any vehicle, the illustrations, PCM pin identification, and throttle position sensor (TPS) pin identification APPLY ONLY to 1993 and 1994 4.3L, 5.0L, and 5.7L GM equipped pickups equipped with throttle body fuel injection (TBI) and the 4L60-E automatic transmission.
Contents of this tutorial:
Can A Bad PCM Cause A False TPS Code?
It doesn't happen very often that a PCM goes bad and causes a false throttle position sensor (TPS) code, but it does happen. When it happens, the most effective way to diagnose/troubleshoot this hypothesis is to make sure that the TPS is good and that there ISN'T a problem in the wiring between the PCM and the throttle position sensor (TPS).
So, in a nutshell, testing for a bad PCM involves a process of elimination that involves checking these specific things:
- Making sure that the old (or new sensor you just bought and installed) is working correctly and producing a variable throttle angle voltage signal when the throttle plate opens and closes.
- Making sure there is continuity in the wiring between the PCM and the throttle position sensor (TPS).
- Making sure that the throttle position sensor (TPS) wires are not shorted to each other.
- Making sure that the throttle position sensor (TPS) wires are not shorted to Ground.
- That the PCM is getting power and Ground.
All of the above circuit tests aren't hard and I'll show you how in this tutorial. Remember, you have to first start by verifying the sensor is OK. The following tutorial will help to test it:
- How To Test The Throttle Position Sensor (GM 4.3L, 5.0L, 5.7L) (at: easyautodiagnostics.com).
OK, before we get started, it's important that you know that the PCM covered by this tutorial has two connectors. One PCM connector is red and the other is blue in color.
Each PCM connector has 32 slots and are identified by a number preceded by a letter prefix (e.g. B-4, B-13, E-14). The test steps, in this tutorial, will point out what specific PCM circuits you have to test in a detailed step-by-step way and with an illustration to boot.
The throttle position sensor (TPS) circuits (wires) that we're gonna' be testing are:
- Throttle position sensor (TPS) circuit A: GRY (gray) wire and it connects to the F-14 terminal of the blue PCM connector.
- Throttle position sensor (TPS) circuit B: DK BLU (dark blue) wire and it connects to the A-15 terminal of the red PCM connector.
- Throttle position sensor (TPS) circuit C: BLK (black) wire and it connects to the B-3 terminal of the red PCM connector.
The PCM circuits we're gonna' be testing are:
- F-14 terminal of the red PCM connector: GRY (gray) wire and it connects to the throttle position sensor (TPS) circuit A.
- A-15 terminal of the red PCM connector: DK BLU (dark blue) wire and it connects to the throttle position sensor (TPS) circuit B.
- B-3 terminal of the blue PCM connector: BLK (black) wire and it connects to the throttle position sensor (TPS) circuit C.
There a few other PCM circuits (for power and Ground) that we'll testing that I did not include in the list above, but you'll see them in TEST 4. To correctly identify the PCM terminals, the following 1993 PCM pin out chart will come in handy:
TEST 1: TPS Circuit Continuity Tests
To get this show on the road, the first set of tests will be to check that continuity does exist between the PCM and TPS connectors.
These continuity tests are simple resistance tests done with a multimeter in Ohms (Ω) mode.
In the test steps below I'll show you which PCM and TPS circuits you'll need to test.
IMPORTANT: Do not probe the front of the PCM connector to test for continuity between it and the throttle position sensor or you will cause permanent damage to the metal terminal (inside the connector).
These are the test steps:
- 1
Disconnect the battery negative (-) terminal. All tests in this section must be done with the battery disconnected from the battery negative cable.
- 2
Disconnect the engine coolant temperature (EC) sensor from its connector. Don't skip this step because the throttle position sensor (TPS) PPL wire also connects to the automatic transmission and this circuit needs to be completely isolated.
- 3
Disconnect the PCM and throttle position sensor (TPS) from their connectors. Check the connectors for any obvious problems (like damage) and repair if necessary.
IMPORTANT: Before disconnecting the PCM, touch any unpainted/clean metal surface on your vehicle to discharge any static electricity you may have stored on you. Static electricity can fry the PCM. - 4
Check continuity between throttle position sensor (TPS) terminal A (GRY wire) and terminal F-14 of the blue PCM connector using the appropriate tools (see image 1 of 3).
Continuity should be present (.5 Ohms or less). - 5
Check continuity between throttle position sensor (TPS) terminal B (DK BLU wire) and terminal A-15 of the red PCM connector using the appropriate tools (see image 2 of 3).
Continuity should be present (.5 Ohms or less). - 6
Check continuity between throttle position sensor (TPS) terminal C (BLK wire) and terminal B-3 of the blue PCM connector using the appropriate tools (see image 3 of 3).
Continuity should be present (.5 Ohms or less).
Let's take a look at what your test results mean:
CASE 1: Continuity was present in all throttle position sensor (TPS) circuits. This tells you that the wires (that connect the PCM to the TPS) are OK and are not 'open'.
The next step is to make sure that they are not shorted to Ground or each other. For this test, go to: TEST 2: Circuits Shorted To Each Other.
CASE 2: Continuity was not present in one or more circuits tested. Double check that you're testing the correct circuit(s) and repeat the tests.
If your multimeter still does not register continuity in the indicated circuit, then that throttle position sensor (TPS) wire has a problem that needs to be solved and tells you that the problem IS NOT due to a bad PCM.
Repairing the 'open' in the wire that did not show continuity should solve the throttle position sensor (TPS) problem and trouble code.