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1996 Mustang 3.8L P0351 Fault: Coil Pack Tower 1 & 5 Not Firing

Model: 96 mustang 3.8L Fault Code: P0351 Posted: 2015-03-03 17:37

I recently replaced the coil pack on my 1996 Mustang 3.8L, but Towers 1 and 5 are not producing spark. I used a spark tester and confirmed that all other towers are firing properly. The diagnostic trouble code displayed is P0351 — indicating a misfire in cylinder 1 or 5. After replacing the coil pack, I’ve tested the wiring harness to the coil and found that all power wires show voltage when the engine is running. However, the signal wire for Towers 1 and 5 does not flash — it remains constant, while the other towers’ control wires flash as expected. This suggests a potential issue with the signal circuit from the Powertrain Control Module (PCM) to the coil pack. I’m seeking advice on how to properly diagnose whether the problem lies in wiring, ground connection, or a faulty PCM.

Related fault codes
P0351
Comments (7)
Anonymous 2015-03-04 08:40

Make sure you're testing for spark at the coil pack terminals, not the spark plug ends. This helps rule out issues with ignition wires or plugs. If no spark is detected at the coil, it may point to a faulty signal from the PCM. Use a digital multimeter (DVOM) with a piercing probe to check for switching signals in the wiring harness.

Anonymous 2015-03-04 13:58

I have already tested the ignition wires directly at the coil pack and confirmed they are receiving power — the test light shows proper voltage when the engine is running. This rules out a blown wire or faulty plug connection.

Anonymous 2015-03-04 14:03

The signal wire for cylinders 1 and 5 must be checked for proper switching from the PCM. Use a z-test lamp connected to battery positive — probe the signal wire at the PCM connector. When the engine is running, the lamp should flash in sync with engine operation. If it doesn’t flash, there’s likely an issue with the signal circuit.

Anonymous 2015-03-04 14:34

I retested the harness wires to the coil pack and confirmed that the signal wire for cylinders 1 and 5 remains constant (always hot), while the other towers’ control wires flash as expected. The battery-positive wire is also consistently powered, which further indicates a problem with the signal path from the PCM.

Anonymous 2015-03-04 17:00

The signal for cylinders 1 and 5 is constant — not pulsing. Possible causes: (1) A ground connection between the coil pack and PCM has been lost, or (2) the PCM itself has failed with a driver circuit malfunction. To test this, unplug the PCM connector and check if the signal wire for cylinder 1/5 remains grounded. If it’s still grounded when disconnected, the PCM is likely faulty. Also, was the misfire issue in cylinder 1 or 5 left unpaired for an extended period? This could have contributed to a failure.

Anonymous 2015-03-04 23:33

I want to make sure I understand correctly — as a beginner in this area, I’m learning quickly. Should I disconnect the harness from the coil pack and check if the signal wire for cylinders 1 and 5 is grounded? If so, how do I perform that test? Thank you for your detailed guidance.

Anonymous 2015-03-05 09:38

Unplug the PCM connector to remove its internal ground. Then use a digital multimeter (DVOM) to probe the signal wire from cylinder 1 and 5 at the PCM terminal. Check continuity between the coil pack and the PCM terminal, and verify for any grounding path. If the circuit is intact but still shows no pulsing when reconnected, it indicates that the PCM has failed — specifically in its driver circuit.