Jspreezy

About five days ago check engine light came on, scanned and P0353 showed. Ignition coil C primary/ secondary crkt. Replaced the coil and the problem still persisted so i then changed the plugs and plug wires( this is a single coil setup on a V6) the plug gap was set correctly(.054). The problem still persits. I swapped once more with a new coil and it continued to do the same(3 coils, including the origional had same symptoms).
So, I began pulling wires whil the car was running and the only two cylinders that werent firing were the two wires connected to the center terminals of the ignition coil. It would seem to be a coil issue but I have tried multiple with the exact same problem continuing. Could this be a computer issue? has to be from the coil back because those two terminals arents even sparking, the other four are. Any help is well appreciated.

Stevo86

Well the definitive way to rule out the coil pack is with an ohm meter. There is actually 3 coils contained in that pack. Each coil fires 2 cylinders at the same time. If you take an ohm reading across the pack for each of the three coils the readings should be very close to one another. Within .2 ohms. With enough googling you can probably find the exact tolerance.

If that checks out good which I suspect it will, examine the connector. There are four pins. One for each pack and another for power. Power is supplied at all times (you know this is good because the other cylinders fire) and the packs fire when the computer opens the ground circuit. You need to know which cylinders aren't firing to know exactly which wire you need to look at. If the connector looks good you need to ohm the wire from that connector back to the pcm harness. Don't be afraid to give the wires a light tug, if one pops out you have figured it out right there.

I highly recommend going to pick up a Haynes or Chiltons manual so you know which wires to probe. Good luck.

Jspreezy

Ok, put the original coil on and a separate new and both coils did the same thing, no spark from the center two plug wire terminals. Cylinders 2 & 6. Checked voltage at wires at pin connector, each read 14v at one point and then they all went sporadic. They all felt secure in the plug. Nothing has happened to the car other than the p0353 code and me changing the plugs, wires and coil(tried three in total). Dark and cold out, I'll check for resistance between the pin connector at the coil and at the PCM. Thanks again and any help is appreciated

petrock

If you don’t know the ohm spec for the circuit/component you are ohm’ing, an ohm test is virtually useless. All circuits have some resistance. Even a 1 inch piece of wire will have a very small amount of resistance. So unless you have the spec there is no way to know what is a good ohm reading and what is a bad one. The other problem is that you have to disconnect whatever circuit you are testing in order to ohm it. Power can’t be running through it. A voltage drop test is easier and way more accurate. You don’t need to know any specs for the circuit, nor have to disconnect anything. All you need to know is the voltage going through the circuit which is easily measurable. The only requirement is that there is current flowing through the circuit. An ohm test shows you the amount of resistance in a circuit/component. A voltage drop test shows you the effect of that resistance on the circuit.

To give a little background on coils, so my explanation below hopefully makes more sense, V6 coil packs contain 3 coils. Each coil has two windings of wire (primary & secondary windings). The primary windings get power & ground through the 4 wire connector on the coil pack. Power is through the ignition circuit and ground comes from the PCM. The secondary windings get power from the primary windings through electromagnetic induction and ground through the coil pack tower -> spark plug wire -> spark plug -> motor. The PCM connects the ground for the primary winding to allow the primary winding time to ‘charge’ and disconnects the ground when it wants to fire the spark plug.

So, as you can imagine, the PCM will connect/disconnect the ground very very fast. Even expensive volt-meter/multi-meter are too slow to accurately measure these voltage changes. The sample rate is too slow. An oscilloscope is much better suited for this task, but most people don’t have access to one. So instead watch the voltage readings on the meter for about 30 seconds to a minute to get a good ball-park guestimate of the minimum & maximum voltage readings.

With all that said, since the problem reproduces with multiple new coil packs I doubt the problem is the coil pack itself. My theory is that it is the ground wire for the primary winding for the cylinder 2 & 6 coil. It is either broken, shorted to ground or has high resistance somewhere between the coil pack and the PCM. A short to ground will leave the coil constantly charging so it will never fire the spark plug. High resistance on the wire won’t let enough current flow through the primary winding to build up enough of a charge to induce the needed voltage in the secondary winding in order to create a spark.

According to the electrical diagram for my wife’s 98 3.8L Mustang, the primary windings of the coil pack get constant power through the red/light green wire in the 4 pin connector. We’ll call it the power wire for this discussion.. The coil for cyl 2 & 6 gets ground through the tan/light green wire in the 4 pin connector. We’ll call it the ground wire for this discussion.

Here’s how to test my theory:

• Start the car
• Set your volt/multi-meter to read as close to 15 volts as possible without going under
• Touch the negative lead of the meter to the negative battery post
• Touch the positive lead of the meter to the positive battery post

Write this voltage reading down and call it “battery voltage”. It should be somewhere between 13 & 14 volts with the car running.

• Take a small safety pin and back probe the power wire in the 4 pin connector
• Touch the negative lead of the meter to the negative battery post
• Touch the positive lead of the meter to the safety pin

Write this voltage reading down and call it “power wire voltage”. It should be no less then 0.5 volts from battery voltage. So if battery voltage is 14 volts, the reading should be no less then 13.5 volts. If it is less, then there is resistance on the power wire between the coil pack & the positive battery post which could be, or could be contributing to, your problem.

• Take a small safety pin and back probe the ground wire in the 4 pin connector
• Touch the negative lead of the meter to the negative battery post
• Touch the positive lead of the meter to the safety pin

The voltage should be jumping all over the place, with a minimum of between 0.5 volts & 0.0001 volts, and a maximum of power wire voltage. If the voltage reading is steady at power wire voltage then there is a break in the ground wire. If the voltage reading is around power wire voltage, but fluctuates a little, then there is a short to ground. If the voltage reading has a max of power wire voltage, but does not drop below 0.5 volts, then there is resistance on the ground wire. Either a break, short or resistance will cause your problem.

To find resistance on a wire, keep the negative lead of the meter on the negative battery post and walk up the wire from the coil to the battery with the positive lead until you get a reading you expect. The resistance will be between the last point you got a bad reading and the first point you got a good one.

To find a break or short to ground on a wire, keep the negative lead of the meter on the negative battery post and walk up the wire from the coil to the battery with the positive lead until you don’t read any voltage. The break or short is between the last point you read voltage and the first point you don’t.

Avoid poking wholes in the wires. That will just create a vector for moisture to get in the wire and corrode it from the inside out over time. Back probe connectors instead. But if you must poke a whole make sure to seal it back up with some Liquid Electrical Tape. Normal electrical tape won’t seal, so use the liquid stuff. Clear nail polish will also work, but doesn’t hold up to heat or flexing as well.

Do not back probe two wires at the same time. If they accidentally touch you can cause a short. In this scenario, since the PCM is in the circuit, you could fry the PCM.

Hope that makes sense and helps... Good luck...

Stevo86

I can't disagree with that one bit. To shortcut a step or two though, if you disconnect the pcm harness and use an ohm meter from the connector to ground and get anything other than OFL (or whatever your particular tool uses for lots of resistance) you can bet you have a short and then continue what petrock said.

Derf00

Petrock, I agree with you but you can get a good idea of the expected ohm range for the coils by testing the resistance of all of them. If 1,2,4, and 5 all read close enough to each other (within 10%) and 3 and 6 are way off (15% or more) then you can at least narrow it down to those two cylinders.

http://repairguide.autozone.com/znet...52800a891b.gif

Measure coil primary resistance between ignition coil pin 4 (B+) and pins 1 (coil 2), 2 (coil 3) and 3 (coil 1).

Resistance should be 0.3-1.0 ohms. If resistance is out of specification, the coilpack is bad. If resistance is within specifications, test the secondary windings.

Measure coil secondary resistance between the corresponding spark plug wire towers on the coil.

Resistance should be 12.8-13.1 kilohms (1,000 ohm scale). If secondary resistance is not within that specific, The coil pack is bad.

On a side note, a lot of specs out there (in repair books) state 6.5K to 11K is the expected resistance of a good Mustang V6 coil pack. That info is incorrect as just about every posting I've ever seen and including my own experience on my old 2000 V6. The expected values are closer to the range of 12.5 to 13.2 Kohms.

Here is just one example.

https://mustangforums.com/forum/v6-1...de-0303-a.html

As far as the wiring harness. If the coil checks out fine, tracing out the wire like you described above is going to be a PITA but just about the only way to do it with a basic VOHM meter.