Bad PCM Causing MAP Sensor Code (1993-1994 GM 4.3L, 5.0L, 5.7L)

TEST 4: PCM Power And Ground Tests

The very last thing that we have to do, before condemning the PCM as bad... is to make sure it's getting both power and Ground.

Ground is supplied by terminals A-1, A-2, and A-12 of the red PCM connector. We'll check these Grounds by doing a continuity test between the terminal and a good clean metal spot on the engine (or intake manifold).

Power is supplied to the PCM by terminals E-15, E-16, and F-15 of the blue PCM connector. Power is in the form of battery voltage (12 Volts) and we'll check with a multimeter voltage test between the terminal and Ground.

NOTE: The following PCM pin out chart may come in handy: 1993 PCM Pin Out Chart (GM 4.3L, 5.0L, 5.7L GAS w/ 4L60-E).

These are the test steps:

1. 1

   Disconnect the battery negative cable and unplug the PCM from its two connectors. The PCM will remain disconnected from its two connectors for the remainder of the tests.

2. 2

   Test continuity between terminals A-1, A-2, and A-12 (of the red connector) and Ground with your multimeter in Ohms (Ω) mode. See image 1 of 2 in the image viewer.
   
   Your multimeter should register continuity (.5 Ohms or less) if these circuits are feed the PCM a good Ground.
   
   IMPORTANT: Don't probe the front of the PCM connector. You need to back-probe the connector or use a wire-piercing probe on the wire that connects to the terminal to be tested.

3. 3

   Re-connect the battery negative cable BUT leave the PCM unplugged from its two connectors. This is so that we can have power available for the next round of tests.

4. 4

   Test terminals E-15, E-16, and F-15 (of the blue connector) for voltage (10 to 12 Volts) with your multimeter in Volts DC mode. See image 2 of 2 in the image viewer.
   
   Terminals E-15 and F-15 will have power only with the key in the On position. Terminal E-16 is hot all of the time.
   
   IMPORTANT: Don't probe the front of the PCM connector. You need to back-probe the connector or use a wire-piercing probe on the wire that connects to the terminal to be tested.

Let's take a look at what your test results mean:

CASE 1: The PCM is being fed power and Ground where indicated in the test steps above. This is the correct and expected test result.

These test results tell you, with about 95% accuracy, that the PCM is bad only if you have:

1. Eliminated the MAP sensor (itself), a stuck-open EGR valve, the fuel pump, engine compression, and vacuum leaks as the source of the MAP sensor code.
2. Confirmed all 3 MAP sensor wires have continuity between the PCM and MAP sensor (TEST 1).
3. Confirmed that the MAP sensor wires are not shorted together (TEST 2).
4. Confirmed that the MAP sensor wires are not shorted to Ground (TEST 3).

CASE 2: The PCM is missing power or Ground. You'll need to solve the missing power or Ground issue and retest.

After restoring the missing power or Ground you'll need to road-test the vehicle and see if it solved the MAP sensor trouble code issue.

More GM 4.3L, 5.0L, 5.7L Tutorials

You can find a complete list of tutorial in this index: GM 4.3L, 5.0L, 5.7L Index Of Articles.

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

• How To Test The Blower Resistor (1988-1993 Pickups).
• How To Test The Blower Motor (GM 4.3L, 5.0L, 5.7L).
• How Can I Tell If My 4L60-E Transmission Is Bad?
• Shift Solenoid A And B Tests (GM 4.3L, 5.0L, 5.7L).
• How To Test The Engine Compression (GM 4.3L, 5.0L, 5.7L).
• How To Test The GM Distributor Mounted Ignition Module (at: easyautodiagnostics.com).