Blazer 4.3 Ignition Coil Test Procedures

The Vortec series of engines were something of a high point for the old small block; think of a caveman armed with a jetpack and a laser cannon, and you've got the idea. The final iteration of this classic design still used a distributor and even used a separate "divorced" coil mounted to the manifold. But that coil was actually a two-part unit. Half of it was the actual coil; the other half was the ignition control module. So, when you look at the coil, you'll see two wiring harnesses -- a three-wire harness in the top going to the coil and a four-wire harness in the front going to the module.

Depending on the truck, your terminals may already be lettered on the connector. But if not, on the three-wire connector: the A-terminal (pink wire) corresponds to the 12-volt power circuit, the B-wire (white) goes to the tachometer and the C-wire (black and white) provides the switching signal from the ignition module. On the four-wire plug, from top to bottom: The A-wire (pink) is the 12-volt power wire, B-wire (white) is the ignition control signal, C-wire (black with white stripe) is the engine ground and D-wire (white with black stripe) is the switching signal circuit. Realistically, the three-wire terminal doesn't matter for these testing purposes -- but it's nice to know what all this stuff does.

Start by setting your digital multimeter to read voltage in DC mode and fit a sharp needle-probe to it. With the ignition key in the "on" position, place the black probe lead on the battery's negative terminal, then poke the red lead into the A-wire (pink) -- you should see between 10 to 12 volts. Now, touch the red DMM lead to the battery's positive terminal, and poke the black lead into the C-wire (black with white stripe); again, you should see between 10 to 12 volts. If you didn't get these results, then your problem is the power supply, not the coil/module.

For this test, you'll need an LED-style test light; an incandescent test light won't turn on and off fast enough to give you a result. Start by unplugging the four-wire harness from the module. Connect the red lead on your LED test light to the battery's positive terminal and connect the black lead to the D-wire (white with black stripe) on your harness. Have an assistant try to crank the engine -- you should see the light flicker on and off very quickly. If it flashed on and off while your engine was cranking, then the ignition coil is almost certainly bad. If not, then you've got a problem with the ignition control module or somewhere else.

Plug your connector back in. Pierce the white B-wire with the needle probe on your LED test light's negative wire. If you don't have a needle probe, then poke an actual needle into the wire and clamp the negative wire to the needle. Touch the LED light's positive lead to the battery's positive terminal, then crank the engine again. If the light flashed on and off while you cranked the engine, then the ignition control module is bad. If it didn't, then you've likely got a problem with the computer, the crankshaft position sensor, or a severed wire or loose connector somewhere between the ignition module and the computer.