1999 Ford Expedition: One coil not firing—PCM issue?

Ford Models Forum Expedition
1

Hi all, I have a 1999 Ford Expedition, and I’m facing a frustrating issue where one ignition coil isn’t firing. Here’s what’s been done so far:

  • The faulty coil has been replaced twice, but the problem persists.
  • Two skilled mechanics have suggested the PCM might be at fault, but it seems unlikely since the other 7 coils are firing correctly.

Here are my questions:

  1. Could the PCM really cause an issue with just one coil, or is it more likely a wiring or connector issue?
  2. Is there a way to troubleshoot the wiring harness from the PCM to the coil before considering a PCM replacement?
  3. Has anyone encountered this issue before, and if so, how was it resolved?

Additional info:

  • I’ve been advised to inspect the grounds, ignition relays, crank and cam position sensors, and perform a bench test on the PCM.
  • I’m looking into replacing the wiring harness for the affected coil. Does anyone know how to identify the correct part number for this?

Any insights or suggestions would be greatly appreciated!

2

Yes, the PCM can cause a single coil to fail since each coil is controlled by individual pins on the PCM. If the pin for that specific coil is damaged, it won’t fire.

3

Zuri said:
Yes, the PCM can cause a single coil to fail since each coil is controlled by individual pins on the PCM. If the pin for that specific coil is damaged, it won’t fire.

Exactly. I had a similar issue, and it turned out to be a damaged PCM pin. Testing the output voltage at the PCM pin can confirm this.

4

Before replacing the PCM, check the wiring between the PCM and the coil. A loose or damaged wire can cause the same issue. A pinout diagram will help trace the connection.

5

Nicole said:
Before replacing the PCM, check the wiring between the PCM and the coil. A loose or damaged wire can cause the same issue. A pinout diagram will help trace the connection.

Here’s a link to an ignition coil wiring kit: Amazon link. It might be what you need if the wiring is the issue.

6

If you decide to replace the PCM, check for a refurbished or used one. New ones can be hard to find for a 1999 Expedition.

7

Oliver said:
If you decide to replace the PCM, check for a refurbished or used one. New ones can be hard to find for a 1999 Expedition.

Also, make sure the replacement PCM is properly programmed for your vehicle. Some vendors offer pre-programmed options.

8

Testing the wiring harness with a multimeter might save you a lot of guesswork. If the harness checks out fine, the PCM is likely the culprit.

9

I had the same problem with a misfiring coil. Turned out to be a loose ground near the PCM. Double-check all grounds before replacing expensive parts.