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I have here a 03 Durango 4.7L V8 with a COP ignition system. Each coil pack has 2 wires which according to FSM one wire feeds all injectors and coil packs from a relay and one is a "signal" from the "coil driver" for Its cylinder. When I use my DVOM I get a voltage from the supply wire only if the other probe is touching the battery positive terminal. I'm guessing this means that it is shorted to ground. The voltage read is slightly less than the battery voltage. Truck runs but very rough, only pulls about 15inHg vacuum at idle, 18 ish at 3000 rpm. Also just finished repairing a vacuum leak on back of intake, a leaky fuel injector (fuel pressure was dropping KOEO), new plugs, and MAP. Pulled o2 to check for clogged cat no change. All coil packs had 1Ω resistance. Have spark at coil packs but it appears weak. Is the signal wire supposed to be hot and PCM supplies ground, meaning the supply is shorted to ground? If so, what's the correct method to hunt down where the fault is? Or is it the other way around?

  • If the coil supply voltage wire were shorted to ground the engine wouldn’t run. I’ll look at a diagram when i have time but iirc they pull power from the asd relay which isn’t on koeo and likely you’re getting your ground from another component in the circuit. – Ben Oct 1 '18 at 13:54
  • BTW - Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Oct 1 '18 at 17:30
  • Yes the ASD relay supplies the power to all 8 injectors and coil packs. So you're saying I should be getting voltage with the engine running only... Ok. I'm at a dead end here all 8 injectors replaced fuel pump assembly was replaced 3 months. Fuel pressure is good, all new plugs, coil packs ohm out okay. Compression is low but even across all cylinders (125psi). New MAP, intake is sealed no vacuum leaks (repaired already). Truck runs terrible almost like the cats are clogged, i pulled the o2 and it still ran the same. vacuum bounces between 13-15inHg at idle holds steady at 18 at 3000 RPM. Idk – Mike P Oct 2 '18 at 0:52
  • Actively misfiring? Any counts per cylinder? What are fuel trims? What’s running compression? Can we get a datalog with the major obd2 pids? – Ben Oct 2 '18 at 3:00
  • Its my father's truck so I can get the data just not right away. O2 stays rich around .8, MAP stays around 7-9psi, LTFT and STFT both pegged at -33%. No misfire codes, no codes at all actually. I don't think the truck is missing I just think it is not getting complete combustion because the fuel is so low, but the computer is right because i tried enriching with propane and it stalls. – Mike P Oct 2 '18 at 11:45
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When you use a multimeter to measure the voltage, one end connects to the battery negative (black probe) and the other to the wire to be tested.

Do note that a multimeter may not show all that is going on - the signal may be too small to register on a multimeter - an oscilloscpe would be better.

Also, a meter can be used to measure the voltage drop between two points so the black probe may not always be connected to battery negative.

  • Yes I know that, but my point is if I only get voltage when connected to the wire and battery positive, doesn't that mean the wire is shorted to ground ? – Mike P Oct 1 '18 at 11:37
  • Consider the meter's internal resistance... – Solar Mike Oct 1 '18 at 12:20

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