GM Electronic Throttle Body Circuit Descriptions And Testing Tips

Testing The PCM's Grounds Basics

Testing the Ground wires of the PCM needs to be done with a voltage drop test (and in this section I'll show you how to do it). You might be asking yourself if you can just test the continuity of the Ground wires? The answer is yes, you can, but a continuity test will not give you a test result that you can trust 100%.

The PCM has two different connectors and each one has Ground wires coming or going to it (depending on if you see 'the cup as half empty or half full'). How can you tell them apart? Well, one is identified, in the service literature, as connector C1 and when you unplug it from the PCM, the part that houses the metal female terminals is a clear Blue color.

The second connector is known, in the service literature, as connector C2 and the part that houses the female terminals is a clear Green color.

One Connector is known as C1 (Blue) and the Ground circuits are:

• Pin 1: Black with White stripe.
• Pin 24: Black with White stripe.
• Pin 27: Black with White stripe.
• Pin 40: Black with White stripe.
• Pin 64: Black with White stripe.
• Pin 67: Black with White stripe.

One Connector is known as C2 (Green) and the Ground circuits are:

• Pin 1: Black with White stripe.
• Pin 40: Black with White stripe.

TEST 1: Voltage Drop Testing The PCM Grounds

I've found, that when the PCM Grounds are not working, it's not necessary to test every single Ground wire. Why? because they all unite together at a certain point in the wiring harness. So testing one or two PCM Ground wires is enough to let you know their overall health.

IMPORTANT: To test the PCM's Ground wires (circuits) the PCM needs to be connected to both its connectors. If you need to disconnect any of the two PCM connectors, to identify these Ground wires, you need to disconnect the battery's Ground terminal first. When you have identified the wire or wires you want to test, re-connect the PCM's connectors and then re-connect the battery negative (-) terminal.

This is what you need to do:

1. Find the wire that you're gonna' test.
   • Ideally, disconnect the C1 (BLUE) PCM connector and identify the Ground wire (you must disconnect the battery negative (-) terminal first).
     • Pin 1: Black with White stripe.
     • Pin 24: Black with White stripe.
     • Pin 27: Black with White stripe.
     • Pin 40: Black with White stripe.
     • Pin 64: Black with White stripe.
     • Pin 67: Black with White stripe.
   • Once you've found the Ground wires, you're gonna' test, reconnect the PCM connector and re-connect the battery negative (-) terminal.
2. Using a Wire-Piercing Probe, probe the Ground wire you have selected.
   • Pierce the wire with the probe as far away as possible from the PCM connector.
   • To see what this Wire Piercing Probe looks like, go here: Wire Piercing Probe.
3. Connect the red multimeter test lead to the Wire Piercing Probe.
4. Ground the black multimeter test lead directly on the battery negative (-) terminal.
5. Turn the Key but don't crank the engine.
   • You can make sure the Key is in the run position by checking that the instrument cluster's gauges are activated.
6. You'll get one of two results, either you'll see voltage values or none.
   • Specifically, if you see a voltage of 1 Volt to 12 Volts, you have a problem.
   • The normal specification should be 0.100 Volts or less (less being 0.040 Volts).
7. Just for the sake of thoroughness, test one more Ground wire by repeating test steps 1-6 on it.

Let's take a look at your test results:

CASE 1: -Your multimeter registered a high voltage (between 1 to 12 Volts) This indicates that the PCM's Grounds have a high voltage drop.

This voltage drop will interfere with the Electronic Throttle Control System. You can do one of two things to solve this problem:

1. You can clean the Ground terminal itself (this bad boy is located on the rear of the passenger side cylinder head) or..
2. You can add (adapt) a Ground wire to one of the PCM's Ground wires.

Now, if you did indeed have a Ground issue with you GM pickup (or SUV), you're gonna' need to reset the Passlock System to get your truck to stay running. For the Passlock reset info, go to: TEST 2: Passlock Reset.

CASE 2: -Your multimeter registered a 0.100 Volts or less This indicates that the PCM's Grounds are OK and that no voltage drop exists.

How Does The Voltage Drop Test Work?

As mentioned before, checking the Ground wires with a voltage drop test is the most accurate way to find out if there's a problem with the circuit itself and I'll explain why.

In a nutshell, corrosion (or any other issue, like broken strands of copper inside the insulation of the wire) will not allow the full amount of voltage that the PCM uses to return to Ground.

The voltage will then find another way to get to Ground and thus you'll see issues like the ones stated in the section: Symptoms of Electronic Throttle Control System Troubles.

A continuity test (resistance test with the multimeter in Ohms mode) of the circuits will always pass. This is because, the multimeter sends such a small amount of current (and thus voltage) thru' the circuit, that no matter what the amount of corrosion (or broken strands of copper), the current will reach the other end.

With a voltage drop test, the multimeter measures the amount of voltage that's not getting thru' to the battery negative (-) terminal when the Ground circuit is actually at work.

• That's why the test is done with the PCM connected to its two connectors and with the Key On.

That's right, whatever amount of voltage is not passing thru' to Ground (and all Grounds final destination is the battery negative (-) terminal), will now pass thru' your multimeter and will register on your multimeter's screen (in Volts D.C. mode).

If the location of the PCM's Ground terminal itself were in an easy to access location, you probably wouldn't need to voltage drop test it. All you'd have to do, to test for corrosion, would be to remove it, visually inspect it and then clean it (although it would not account for any broken copper strands inside the wire's insulation).