Stupid Noob Question of the Week

UMEXT

Original poster
Dec 16, 2011
110
Now, keep in mind, I work for the Blue Oval and have had training in this subject, but the training left out what I think is one important fact. Keep also in mind that I've stared into my engine compartment, and though I think I'm fairly bright most of the time, still can't figure it out. So, here it is:

I understand that the 4.2 is a direct ignition engine (coil-on-plug), and I understand that this means there are no spark plug wires. So, how on God's green Earth does the electricity get from the battery to the coils?:undecided:
 

kardain

Member
Dec 16, 2011
557
If you understand how coil packs work, it's a similar concept...

At least on the 2.2 i4 ECOtec (not quite sure yet how the 4.2 is layed out... haven't de-plasticked my engine to look), which also uses coil on plug, there is an ignition control that runs the length of the cam cover. On one end is a plug in for power and spark control. The spark control is commanded by the ECM.... It's basically one big coil pack for all 4 cylinders.

I'm posting from my phone so if i left out anything, that's why.
 

UMEXT

Original poster
Dec 16, 2011
110
kardain said:
If you understand how coil packs work, it's a similar concept...

At least on the 2.2 i4 ECOtec (not quite sure yet how the 4.2 is layed out... haven't de-plasticked my engine to look), which also uses coil on plug, there is an ignition control that runs the length of the cam cover. On one end is a plug in for power and spark control. The spark control is commanded by the ECM.... It's basically one big coil pack for all 4 cylinders.

I'm posting from my phone so if i left out anything, that's why.

That takes me one step backward, but still doesn't answer the question, so let me ask it slightly differently. How does the electricity get from the battery to the PCM, or the crankshaft position sensor, or whatever device controls the spark?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
UMEXT said:
That takes me one step backward, but still doesn't answer the question, so let me ask it slightly differently. How does the electricity get from the battery to the PCM, or the crankshaft position sensor, or whatever device controls the spark?

Wires and magic. :wink:
 

kardain

Member
Dec 16, 2011
557
Yeah, I figured my explanation would leave some questions.... this might help


"GM Service Manual" said:
Electronic Ignition (EI) System Description

The electronic ignition (EI) system is responsible for producing and controlling a high energy secondary spark. This spark is used to ignite the compressed air/fuel mixture at precisely the correct time. This provides optimal performance, fuel economy, and control of exhaust emissions. This ignition system consists of a separate ignition coil connected directly to each spark plug, known as coil on plug. These coil assemblies are located in the center of the camshaft cover. The driver modules within each coil assembly are commanded ON/OFF by the powertrain control module (PCM). The PCM primarily uses engine speed and position information from the crankshaft and camshaft position sensors to control the sequence, dwell, and timing of the spark. The EI system consists of the following components:

Crankshaft Position (CKP) Sensor

The crankshaft position (CKP) sensor is a permanent magnet generator, known as a variable reluctance sensor. The magnetic field of the sensor is altered by a crankshaft mounted reluctor wheel that has seven machined slots, six of which are equally spaced 60 degrees apart. The seventh slot is spaced 10 degrees after one of the 60 degree slots. The CKP sensor produces seven pulses for each revolution of the crankshaft. The pulse from the 10 degree slot is known as the sync pulse. The sync pulse is used to synchronize the coil firing sequence with the crankshaft position. The CKP sensor is connected to the PCM by a signal circuit and a low reference circuit.

Camshaft Position (CMP) Sensor

The camshaft position (CMP) sensor is triggered by a notched reluctor wheel built into the exhaust camshaft sprocket. The CMP sensor provides six signal pulses every camshaft revolution. Each notch, or feature of the reluctor wheel is of a different size for individual cylinder identification. This means the CMP and CKP signals are pulse width encoded to enable the PCM to constantly monitor their relationship. This relationship is used to determine camshaft actuator position and control its phasing at the correct value. The PCM also uses this signal to identify the compression stroke of each cylinder, and for sequential fuel injection. The CMP sensor is connected to the PCM by a 12-volt, low reference, and signal circuit.

Ignition Coils

Each ignition coil has an ignition 1 feed and a ground. The PCM supplies an ignition control (IC) circuit. Each ignition coil contains a solid state driver module as its primary element. The PCM signals the coil driver to initiate a firing event by applying the IC circuit voltage for the appropriate time or dwell. When the voltage is removed, the coil fires the spark plug. The coils are current-limited to prevent overloading if the IC current is held high too long. The spark plugs are tipped with platinum for long wear and higher efficiency.

Powertrain Control Module (PCM)

The PCM controls all ignition system functions, and constantly corrects the spark timing. The PCM monitors information from various sensor inputs that include the following:

The throttle position (TP) sensor
The engine coolant temperature (ECT) sensor
The mass air flow (MAF) sensor
The intake air temperature (IAT) sensor
The vehicle speed sensor (VSS)
The transmission gear position or range information sensors
The engine knock sensors (KS)
Modes of Operation

During normal operation the PCM controls all ignition functions. If either the CKP or CMP sensor signal is lost, the engine will continue to run because the PCM will default to a limp home mode using the remaining sensor input. As mentioned above, each coil is internally protected against damage from excessive voltage. If one or more coils were to fail in this manner, a misfiring condition would result. Diagnostic trouble codes are available to accurately diagnose the ignition system with a scan tool.

So the power goes to the ignition control module, the various sensor inputs tells the PCM when to send the necessary spark signal, the PCM sends that to the ICM to fire the proper plug.
 

UMEXT

Original poster
Dec 16, 2011
110
Wooluf1952 said:
Wires and magic. :wink:

So the claim that there are no spark plug wires is actually a lie?

I'm not trying to be a jackass, I just need to understand how this system works, and I know the 12 volts don't just magically beam themselves to the coils. Yet no article on the internet seems to think important to say how it actually happens.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
kardain said:
If you understand how coil packs work, it's a similar concept...

At least on the 2.2 i4 ECOtec (not quite sure yet how the 4.2 is layed out... haven't de-plasticked my engine to look), which also uses coil on plug, there is an ignition control that runs the length of the cam cover. On one end is a plug in for power and spark control. The spark control is commanded by the ECM.... It's basically one big coil pack for all 4 cylinders.

I'm posting from my phone so if i left out anything, that's why.

Not really. On the 4.2 I6, there are six separate coils. The spark timing is controlled by the PCM.
 

UMEXT

Original poster
Dec 16, 2011
110
kardain said:
Yeah, I figured my explanation would leave some questions.... this might help




So the power goes to the ignition control module, the various sensor inputs tells the PCM when to send the necessary spark signal, the PCM sends that to the ICM to fire the proper plug.

Thanks, but still not at all helpful. Where does the PCM get its power from?
 

kardain

Member
Dec 16, 2011
557
UMEXT said:
Thanks, but still not at all helpful. Where does the PCM get its power from?

There are several 12v+ constant wires that go from the battery to the fuse panels then straight to the PCM.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
UMEXT said:
So the claim that there are no spark plug wires is actually a lie?

I'm not trying to be a jackass, I just need to understand how this system works, and I know the 12 volts don't just magically beam themselves to the coils. Yet no article on the internet seems to think important to say how it actually happens.

There are no spark plug wires, but there is a spring that connects the coil to the plug.
Have you removed the intake resonator and looked at the coils?
 

UMEXT

Original poster
Dec 16, 2011
110
kardain said:
There are several 12v+ constant wires that go from the battery to the fuse panels then straight to the PCM.

So, basically, there are spark plug wires, they're just in a different place in the circuit. Do these go bad like traditional spark plug wires and cause misfires, or are they reasonably permanent?

And thanks for the answer.
 

kardain

Member
Dec 16, 2011
557
UMEXT said:
So, basically, there are spark plug wires, they're just in a different place in the circuit. Do these go bad like traditional spark plug wires and cause misfires, or are they reasonably permanent?

And thanks for the answer.

I guess you could call them spark plug wires... the 12v+ feeds are not just for spark control, but also for other purposes. Misfires are not usually caused by those going bad unless you have a breakage in the main engine harness.... at which point, there is a bigger problem on your hands than a misfire.

Misfires with coil on plug are mainly going to be attributed to either 1) a bad spark plug, b) a bad spring between the spark plug and ignition coil, or 3) a bad ignition coil.

Less common occurances would be a damaged crankshaft reluctor wheel (this will throw off the PCM signals causing it to fire late or early), MAP sensor failure, timing chain stretching or breakage, and worn camshaft or valve lash adjusters.
 

gmcman

Member
Dec 12, 2011
4,658
I'm not 100% on these but this the way I understand them and they are very similar to most styles. There are no big plug wires since the coils only need low voltage (12) to energize them, hence the lack of large wires. The coil is grounded to the head through the bolts and the 2 wires going to the coil are the 12V and the signal lead which is used to collapse the field thus producing the high voltage to fire the plug. The ECM determines when the signal is energized to collapse the coil to fire the plug.

Coils are connected to the plug via the spring replacing the plug wire and "wire" distance is kept to a minimum this way.

The ignition coils on most older systems needed only a small hot lead to energize the coil as well, the large wire fed the distributor rotor.
 

million-miles

Member
Jan 10, 2012
189
Spark works by electro magnetic force. The 12V wire that goes into and out of the coil is going to be a small dia wire. What happens the 12V wire is coiled around another coil and when the 12v is cut off the magnetic field is transferd to the other coil and multiplied many times then goes to the spark plug to fire.

The voltage going into the coil pack doesnt carry man amps so it doesnt need a big thick wire from the batt to the coil like the old systems.

Im sure the roadie can explain it better than i can

http://auto.howstuffworks.com/ignition-system3.htm
This might help. Intead of one coil like the old system it has 6 coils. and directly over the plug
 

DJones

Member
Jan 21, 2012
701
St. Petersburg, Florida
Somewhat like a flash tube. In place of a xenon gas discharge flash tube, the tube is a cylinder filled with gasoline.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Here's a visual. The plugs are hidden inside the plug wells, and the coils mount right on top of them vertically. There's no high voltage wires because of this piggy-back arrangement.

View attachment 19111

The control wiring is in a harness to EACH coil separately, so there is no distributor in the old fashioned sense. The main engine computer is called the PCM, and is mounted on the driver's side of the intake manifold with three huge connectors because of the numerous sensors and items the PCM controls. Like the six spark plug coils.
 

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Sparky

Member
Dec 4, 2011
12,927
I think we have a terminology problem.

Spark plug wires ALWAYS refer to the thick, heavily insulated, high-voltage wires that go from the coil that generates the high voltage to the plug. Always.

Since the coil-on-plug design eliminates those high voltage wires, it is 100% correct that these engines (and others that use coil-on-plug setups) do not have any spark plug wires.

The wires that feed the coils are not spark plug wires as they are low voltage only. They are permanently part of the wiring harness and not typically a serviceable part, but they also don't break down and go bad with time and use like spark plug wires would.
 

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