PCM - ignition coil problem - weak ground

[kalok85]

HI guys.......any help or advice would be amazing

A couple days ago my tuck started miss firing and a couple minutes later my PCM fuse blew out and my truck stalled.

I have 1999 mercury Mountaineer 5.0L

I traced the problem to the passenger side coil pack. Two the poles were not sparking the number 8 and 2 (paired cylinders). I bought a new coil (with a lifetime warrant thank god) and replaced all spark plugs and wires. And when I turn the key to the on position the PCM fuse blows immediately.

I can start the car with out the wiring harness plugged into the coil pack and the PCM fuse does not blow. As soon as its plugged in the fuse blows immediately

The ground wire for ignition coil wiring harness on the passenger side coil pack (same one Im having trouble with) is weak.....only reading 10v.

I don't understand why if I have a wire grounding out before the coil pack, the fuse only blows when the harness is plugged in.

Any thoughts would be awesome and if anyone know has the pinout diagram for my truck the information would be invaluable. And if no pinout diagram if someone knows the pin number for the passenger side coil pack ground wire that information would also be amazing

 

[2000StreetRod]

which fuse blows?

My 2000 wiring diagrams should apply to your 1999 wiring. Which PCM fuse blows when you connect the coilpack connector? Is it Fuse 10 (30 amp) or Fuse 6 (10 amp) in the Battery Junction Box or Fuse 19 (25 amp) in the Central Junction Box? I suspect it is Fuse 19 which provides power to the coilpack.

Sometimes the radio ignition suppressor capacitor fails in a shorted condition which would blow Fuse 19. You should be able to disconnect the capacitor from the harness to see if that's the problem.

You should not be able to start the engine with the wiring harness coilpack connector disconnected since there shouldn't be power to the coilpack. Perhaps your engine will run on only 4 cylinders.

The PCM grounds the low side of each coil in the coilpack to build a magnetic field. It interrupts the flow of current when it is time to fire the plug. The PCM connection for coil 3 is pin 78 (PK/WH wire) and for coil 4 is pin 104 (RD/YE wire). I suspect that you have a short in the coilpack harness connector between +12 volts (RD/LG wire) and one of the two PCM wires.

 

[kalok85]

Yes the fuse blows when the connector is attached to the coil pack..... And yes it is fuse 19 that blows. I suspect a short two but the wire colors don't match your discription my ground is pink and some light color.

Would a short to the ground cause the pcm fuse to blow

And my car does run with the connector unplugged

 

[2000StreetRod]

wire colors

Yes the fuse blows when the connector is attached to the coil pack..... And yes it is fuse 19 that blows. I suspect a short two but the wire colors don't match your discription my ground is pink and some light color.

PK/WH = pink with white stripe
RD/YE = red with yellow stripe
RD/LG = red with light green stripe

Would a short to the ground cause the pcm fuse to blow

A short that bypasses the resistance of the primary coil winding to either of the PCM ground wires may cause the fuse to blow. With the ignition off, pull the PCM power relay in the battery junction box and then disconnect both coilpack connectors. The only thing left connected will be the PATS ignition input which should be high impedance. Measure the resistance to ground for the RD/LG wire. If less than a couple hundred ohms then there is a short to ground somewhere.

 

[kalok85]

Thanks I will do that tonight and get back with you later..... I'm at work and want to go home to work on the car now

 

[kalok85]

Ok so I tested resistance in every wire running to the coils with no short found. So as suggested I unplugged the radio ignition noise suppressor and started the car.

Problem solved - it was the ignition suppressor which is only single wire so I can run my own back I the pcm if I have to.

Thanks for all of your help....... I would have been lost otherwise