Dwell Lab Scope Coil

The two-wire-coil versions can be found on Ford and Chrysler products. The PCM directly controls the pulsed ground dwell to each primary coil. Because of direct coil primary circuit access, these coils provide flexibility in diagnostic testing. Primary ignition waveforms can be accessed at each individual coil or at the PCM, whichever is easier. Dwell time is the length of time the coil primary circuit is being completed, and current is flowing through it. You can’t use an ordinary lab scope! If you are using an ordinary lab scope, you can still perform a secondary ignition measurement but you’ll be observing only one cylinder waveform at a time. Then move the test probe to the.

General description
An ignition system is a system for igniting a air-fuel mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol (gasoline) engines used to power the majority of motor vehicles. Ignition system is divided into two electrical circuits - the primary and secondary circuits. The primary circuit carries low voltage. This circuit operates only on battery current and is controlled by the breaker points and the ignition switch.
Principle of operation of the primary ignition circuit
The coil is the heart of the ignition system. Essentially, it is nothing more than a transformer which takes 12 volts from the battery and increases it to a point where it will fire the spark plug as much as 40,000 volts. The term 'coil' is perhaps a misnomer since there are actually two coils of wire wound about an iron core. These coils are insulated from each other and the whole assembly is enclosed in an oil-filled case. The primary coil, which consists of relatively few turns of heavy wire, is connected to the two primary terminals located on top of the coil. The secondary coil consists of many turns of fine wire. It is connected to the high-tension connection on top of the coil (the tower into which the coil wire from the distributor is plugged).

Ignition systems can be divided into the following types:

  1. Figure 2: Lab scope measurement of COP ignition. Channel 1 (red) in figure 2 shows the signal from a 4 wire COP ignition measured with the Coil-on-Plug probe TP-COP750. The signal shows the typical waveform of an ignition system. On the left side it starts with the module switching on the coil to build up a magnetic field.
  2. A better, safer way is to attach a spark plug simulator to the coil secondary lead or to an individual plug lead. Just clamp the simulator to a good ground and attach the coil wire or plug lead to its terminal. If you’re troubleshooting DIS, install the simulator on one or more coils to check for spark to several cylinders.
  3. The command pulse is worth checking using a lab scope. The majority of vehicles use a 5-volt pulse; however, a few use full charging voltage. If a coil is unplugged, thereby eliminating electrical load, the signal will be full value, typically 5 volts. However, on a connected and loaded circuit, the voltage is closer to 4 volts.
  • Distributor Ignition System
  • Direct Ignition System (DI)
  • Coil-on-Plug (COP) type – individual coil for each cylinder and the coil pack is mounted directly over the spark plugs.
  • Individual coil for each cylinder with separate HT (high tension) leads.
  • DIS-Wasted Spark Ignition – separate coil for each two cylinders.
    Synchronous ignition with two secondary winding coil terminals.

Distributor Ignition
The distributor ignition system is the most common ignition system for early model year vehicles. Distributor ignition systems use one coil that fires one spark plug at a time on the compression stroke only. Viewing the primary ignition pattern requires that you have to monitor the voltage signal on the negative side of the coil’s primary circuit and to identify the trigger cylinder by using the RPM probe.
The classical or conventional ignition system consists of the following components: ignition coil, distributor, spark plugs, high-voltage wires and some means of controlling the primary ignition circuit. The primary circuit of the ignition coil can contain: points, points controlling a transistor, the transistor being controlled by some other means (breaker less) or electronic ignition. In point-type ignition systems the current in the primary circuit is controlled by a mechanical switch (or breaker). The mechanical points may control a switching transistor which opens and closes the primary circuit of the ignition coil. In breaker less transistor and electronic ignition a Hall effect, VRS (Variable Reluctance Sensor) or an optical sensor may be used to control the switching transistor.
Current flows from the positive terminal of the battery, through the ignition switch and/or relay, through a fuse and on to the positive terminal of the ignition coil. The current returns to the battery through the negative terminal of the ignition coil, on through the switching device (points or a transistor) through the vehicle chassis, and to the negative terminal of the battery. While current is flowing in the primary circuit a magnetic field builds up in the ignition coil. Due to the inductance of the ignition coil it takes some time (1-6 mS, depending on design) for the primary current to reach its nominal value. When the primary current flow is interrupted, the magnetic field collapses rapidly (in about 20µS) and a high voltage is induced in the primary winding (CEMF Counter electro motive Force). This voltage is transformed in to a very high voltage in the secondary winding. The amplitude of this voltage depends on the turns ratio (commonly 100:1). A 300V primary voltage, therefore, will be 30 000V in the secondary winding. The voltage will only build until the break down voltage of the spark gap is reached - the firing voltage of the spark plug.

Direct Ignition System (DI)

COP systems use one individual coil for each spark plug. Each coil is located directly on top of its spark plug and does not use any external spark plug wires. Each coil pack also has an independent primary circuit which must be tested individually.
The individual ignition coil by one running cycle of the engine generates one ignition spark. Therefore, in individual ignition systems is required synchronization of coils work with position of a camshaft.
At submission of the voltage to the primary coil, the current starts to flow by a primary coil and because of that in the core of the coil changes the value of the magnetic flux. Change of the magnetic flux value in the core of the coil leads to occurrence of the voltage of positive polarity on a secondary coil. Because the speed of increasing of the current in the primary coil is slow, the voltage arising on a secondary coil is small – according 1…2 kV. But in the certain conditions the voltage value can be sufficient for untimely occurrence of the spark between electrodes of a spark plug and as consequence, too early ignition of the air/fuel mixture. In order to prevent possible damages of the engine due to untimely occurrence of the spark, formation of the spark between electrodes of a spark plug at submission of a voltage to a primary coil should be excluded. In the individual ignition systems, occurrence of this spark is prevented by means of built-in diode EFU to the ignition coil switched consistently in a circuit of a secondary coil.
At the moment of closing of the output ignition cascade, current in the primary circuit sharply interrupts, and the magnetic flux promptly decreases. This fast change of the magnetic flux value is causes to occurrence of the high voltage on a secondary coil of the ignition coil (under certain conditions, the voltage on a secondary coil of the ignition coil can achieve 40…50 kV). When this voltage achieves the value providing formation of the spark between electrodes of a spark plug, the compressed in the cylinder air/fuel mixture is ignited from the spark between electrodes of a spark plug.
In some systems coils are not located directly on top of each spark plug and external spark plug HT leads are used. Each coil pack also has an independent primary circuit which must be tested individually.

Dwell Lab Scope Coil Mounts

DIS-Wasted Spark Ignition

DIS ignition systems use one coil for every two cylinders, also called “waste-spark” systems. A waste-spark system fires one coil for each pair of cylinders that are at top dead center (TDC) at the same time. These cylinder pairs are called “running mates.” One cylinder is at TDC on the compression stroke, while the other is at TDC on the exhaust stroke. The spark in the cylinder at TDC on the compression stroke ignites the air-fuel mixture to produce power. The spark in the cylinder at TDC on the exhaust stroke is “wasted,” hence the name “waste-spark.” Each waste-spark DIS coil is hooked in series with its two spark plugs. As the coil fires, secondary current creates a high voltage spark across the gaps of both plugs. One plug fires with the traditional forward polarity of an ignition system: negative (—) to positive (+) The other plug fires with opposite polarity: positive (+) to negative (—) Thus, one plug always fires with what has always been called “reversed polarity.” The voltage capacity of a DIS coil is high enough, however, to ensure that the available voltage is always high enough to fire the plug with reversed polarity when it’s on the compression stroke.

Fig.1 Primary ignition waveform

1. The ECU internal switch closes. Current rushes into the coil and begins to build, which is why voltage drops close to ground and essentially remains there until the firing spark.
2. The coil is now saturated with electricity, as indicated by the jump in voltage.
The coil is no longer charging up thanks to the ECU.
3. The ECU switch opens, unleashing all the built-up current. Amps drop like a rock and voltage skyrockets.
4. The spark line indicates the length of the spark event at the plug.
5. When not enough power is left for the spark, remaining power is rung out and the event begins all over again.

Procedure to verify functionality of the primary ignition circuit
— Ohmmeter and voltmeter measurements of the ignition coil primary winding

  • Measure the coil’s primary winding resistance with ohmmeter. Normal resistance must be less than 1Ω.
  • Switch ignition on but do not start the engine.
  • Use a voltmeter to check whether battery voltage is applied to the coil’s positive terminal (usually “2”) and chassis ground.
Dwell lab scope coil placement
To perform a diagnosis of primary voltage of ignition systems, it is necessary to monitor the ignition coils primary winding charge waveform by inserting probe(s) to (each of) the primary circuit coil(s) negative terminal(s). If the ignition module (ECU power switch) is not combined into one unit with the coil primary winding, it is possible to observe both primary voltage, and primary current.

1. Measuring the primary voltage
- Connect the active test lead to the ignition coil negative terminal (usually “1”) and the ground lead to the chassis ground.
Important note: To measure the primary voltage input voltage range of the oscilloscope should be set to ± 400V.

Dwell Lab Scope Coil Tester

Geforce nvidia drivers update. 2. Measuring the primary current
- Connect an AC current clamp to the other oscilloscope channel. Range ±20A.
- Start the engine and left it idling.
- Compare result with the waveform in fig. 2.


Note: Primary voltage can rise up to 380V and primary current can vary from 8A to around 12A.

Dwell lab scope coil tester

If the ignition module (ECU power switch) is combined into one unit with the coil primary winding, it is impossible to carry out diagnostics of the primary ignition voltage. In this case only the primary current can be observed with a current clamp.

1. Measuringtheprimarycurrent
- Connect an AC current clamp to the other oscilloscope channel. Range ±20A.
- Start the engine and left it idling.
- Compare result with the waveform in fig. 3.
Note: Primary current can vary from 8A to around 12A.


Possible reasons for failure of the primary ignition circuit
» No supply voltage to the ignition coil.
• Make sure ignition is on.
• Check electrical connections to the ignition coil.
• Check for burned fuses and/or wires in the ignition coil circuit.
» Broken insulation between the coil’s primary and the secondary windings
» Bad ignition coil.

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What is Dwell Management and CurrentLimiting?


Dwell Lab Scope Coil Placement

Just what is 'Dwell' when referring to theignition system in an internal combustion engine and why does it matter? Dwellis traditionally referred to as an angle but for this explanation, Dwell can bedefined as the time during an ignition 'cycle' that current isflowing into the ignition coil. The images on the left and right areOscilloscope traces I made of what is going on electrically in a 3.8 Ohmignition coil during the interval between the generation of sparks when runningat 1440 RPM. If you assume that the graphs are representative of a 4 cylinderengine, then the 90 degrees of rotation of the distributor shaft represents theactivity within the coil for one cylinder to receive its spark. On the'blue' current curve of each image, the arrow to the left and thusthe point where the curve starts to go up, is the point at which current isallowed to flow into the primary windings of the coil. This is the beginning ofthe Dwell period. Current continues to flow as you progress to the right untilyou come to the next arrow. At this point current is abruptly stopped and theDwell period ends. Here the coil windings are fully 'saturated' andthe abrupt termination of the flow of current causes the generation of a veryhigh voltage discharge from the secondary winding which is the'spark' that goes to a sparkplug. As you progress further to theright, the voltages in the coil windings decay to a steady state until suchtime as the next 'cycle' starts when current is allowed to flow intothe coil again. By comparing the current curve in the two images, you canreadily see that there is a significant difference in the Dwell period that iscontrolled by the two different electronic control modules. As far as Dwellangle, the XR700 is about 67° and the XR3000 is about 26° at 1440RPM.

In a basic points type ignition system, the Dwell isdetermined by the gap between the contacts on the points. Once the gap isestablished, the dwell is fixed and does not change (except for wear of thecomponents). Essentially this fixed Dwell concept is found in most of theelectronic ignition units such as the Pertronix Ignitor and Crane/FAST XR700units. In the original design of all of these units, the guiding principle ofoperation was that the Dwell period must be sufficiently long such that fullsaturation of the coil will occur at the design maximum RPM. Since the Dwell isfixed, more current than is necessary flows into the coil at all lower RPM andthus adds to the heat generated within the coil. This is especially problematicfor the Atomic 4 engine due to the fact that it is a 4 cylinder engine and alsohas a low operating RPM range. These have been major factors contributing tothe coil heating and many failures that have been experienced over theyears.

The images above are actual Oscilloscope traces made usingthe same coil, trigger mechanism, and voltage source but with anXR700module and anXR3000 module respectively. The XR3000 electronic ignition modulehas a unique Dwell Management feature that adjusts the Dwell as a function ofspeed whereas the XR700 has a very long and fixed Dwell. With the ability toadjust Dwell, the Dwell angle actually becomes Dwell Time. Consider this, for afixed Dwell angle the time that current flows gets shorter the higher the RPM.Ideally with Dwell Management you would like to have the Dwell time constantfrom idle to maximum RPM and thus assure that the coil just gets fullysaturated over the entire RPM range. The XR3000 comes pretty close todoing exactly that and thus prevents essentially all coils from overheatingwith NO BALLAST RESISTOR needed while still providing an excellentspark. Note: The Dwell is NOT user adjustable. Dwell is varied by themicroprocessor ONLY.

Another remarkable feature of the new XR3000electronic ignition module is a function known as 'Current Limiting'.This is an especially useful feature in that it allows the use of a very lowprimary resistance high performance coil (.4 - .5 Ohms) which can saturatequickly and always produce an excellent spark. The 'Current Limit'feature mainly comes into play in the lower RPM ranges where there is plenty oftime for coil saturation even with a reduced dwell angel. A more relevant sidebenefit of 'Current Limiting' is the feature that the module willcompletely shut off current to the coil in the event that the ignition switchis on yet the engine is NOT running! No more burned up coils and/or modules dueto a simple oversight when doing maintenance.