TEST 5: Testing The Low Reference Circuit

Testing The Low Reference Circuit. How To Test The COP Coils (GM 4.8L, 5.3L, 6.0L, 8.1L)

In this test section, we're gonna' test the 2nd ground the ignition coil receives. This ground is supplied by the fuel injection computer (PCM).

Testing the ground circuit (known in tech speak as the Low Reference Circuit), that the PCM provides, can be accomplished with the Coil-on-Plug (COP) Coil connected or disconnected to its connectors.

IMPORTANT: Since this ground is provided by the PCM, you need to be very careful not to short this circuit to 12 Volts or you'll fry the PCM.

  1. 1

    Disconnect the ignition coil from its connector and place your multimeter in VOLTS DC mode.

  2. 2

    Probe the circuit labeled with the number 3 (see photo above) with the black multimeter test lead (using an appropriate tool to pierce the wire).

    With the red multimeter test lead probe the battery positive (+) terminal.

  3. 3

    Have your helper turn the key to the ON position.

    You should see 10 to 12 Volts on your multimeter, or if you're using a test light, the test light should light up.

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

CASE 1: If the multimeter registered 10 to 12 Volts (or the test light lit up), then the chassis ground circuit is OK. The next step is to make sure that the PCM is activating the ignition coil, go to: TEST 6: Testing The IC Signal (Triggering Signal).

CASE 2: If the multimeter DID NOT register 10 to 12 Volts (or the test light DID NOT light up), then the Low Reference Circuit has a problem. Recheck all of your multimeter connections and retest. If still the multimeter does NOT register the specified voltage, then this result eliminates the COP ignition coil as the source of the misfire condition. You must find out the cause of this ‘open’ in this circuit.

TEST 6: Testing The IC Signal (Triggering Signal)

Testing The IC Signal (Triggering Signal). How To Test The COP Coils (GM 4.8L, 5.3L, 6.0L, 8.1L)

OK, so far all your tests have confirmed that three out of four circuits DO HAVE the correct signal coursing thru' them. The next step is to verify that that Coil-on-Plug (COP) ignition coil is receiving the Triggering signal from the PCM (Powertrain Control Module=Fuel Injection Computer).

This test is done with a multimeter that can read Hertz Frequency (don't have one?... no problem, go to: TEST 7: Testing The Trigger Signal Without A Multimeter) Testing for the Triggering Signal should be done with the ignition coil connected to its connector.

The method I recommend to use is with them connected to their connectors and with a Wire-Piercing Probe (you can see this tool here: Wire-Piercing Probe).

  1. 1

    Place your multimeter in Hertz (Hz) mode and keep the coil connected to its connector.

  2. 2

    Probe the circuit labeled with the number 2 (see photo above) with the black multimeter test lead (using an appropriate tool to pierce the wire).

    With the red multimeter test lead probe the BATT (+) Positive terminal.

  3. 3

    You should see fluctuating values of 5 to 8 Hertz as the engine starts and runs on your multimeter.

  4. 4

    If you're unsure of your Hertz reading, don't worry, test the adjacent (or any other COP coil) for its Triggering Signal and compare that signal to this one. Both signals should behave the same.

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

CASE 1: If the multimeter registered 5 to 8 Hertz (Hz) as the engine cranked and started. Then this test result tells you that the Triggering Signal is present.

This result, also, indicates that the Coil-on-Plug ignition coil on your GM vehicle is BAD and needs to be replaced only if:

  1. The coil is not firing spark in TEST 1 and 2.
  2. It's getting power (12 Volts DC) in TEST 3.
  3. It's getting chassis ground in TEST 4.
  4. It's getting PCM ground in TEST 5.
  5. It's getting the triggering signal from the PCM (TEST 6).

CASE 2: If the multimeter DID NOT register 5 to 8 Hertz (Hz) as the engine cranked and started. Re-check all of your connections and repeat the test again. If still no Hertz readings, then this result eliminates the ignition coil as the source of the NO SPARK condition/misfire, since without the Triggering Signal the ignition coil will not work.

This lack of Signal indicates one of two things: 1) An open-circuit problem in the circuit between the COP coil and the PCM or 2) a BAD PCM, altho' a BAD PCM is rare, but it happens.