Ignition Coil Ramping

Perhaps the easiest way to test an ignition coil today is by using a low amp probe and an oscilloscope. While many technicians may shy away from this method, thinking it is too complex or too difficult, coil ramping is actually a very simple procedure and the fastest way to diagnose a no-start situation with no spark. In this article I will show the basic techniques needed to view an ignition coil current ramp and how to interpret those results.

Getting a clean signal is important

To get a good coil ramp capture you will need a decent low amp probe and oscilloscope. Getting a clean signal is very important when looking at any waveform via an oscilloscope, and this can be especially difficult when working near high voltage ignition systems. The probe should be connected to the main power feed of the coil, but far enough away from the coil so as not to pick up any noise from the high voltage secondary ignition. Often I will try to find a fuse that feeds the ignition coil(s), at least as long as that fuse does not also supply power to any other device such as a fuel pump. Remove the fuse and install a fused jumper wire in its place. You can then easily connect the low amp probe around the fused jumper wire.

Sometimes the only reasonable place to connect to the coils main power feed is near the coil itself. If this causes interference, try using aluminum foil as a shield for your low amp probe, this should cut down on interference. Also be sure the wire lead for your low amp probe is not lying across any secondary ignition cables or other source interference.

Coil ramp patterns explained

Take a look at the example below as I explain the different parts of a coil ramp pattern.

When the ignition module or igniter first turns the ignition coil on by supplying ground to the negative side of the coil, the primary windings begin to charge up inducing a current flow into the secondary windings of the coil. Since the voltage at this point is not enough to ionize the spark plug gap, current is diminished as coil oscillations into the secondary windings. These oscillations then induce current back into the primary windings and that is what you see as turn on oscillations. If that explanation seems a bit confusing, don t worry about it because all you really need to know as that the turn on oscillations should be there.

Once the oscillations dwindle down, the coil should then charge up at a steady rate. The average coil will charge up at a rate of about 1 to 2 amps per millisecond, but this can vary based on system voltage, engine cranking (lower available voltage), engine running(charging system voltage), or the ignition coil itself.

Many, but not all, coil ramps will have their current limited by the ignition module/igniter unit. The module/igniter will not un-ground the coil, as this would cause it to fire, but will add resistance to the ground circuit once a specified current flow is reached. In the example above, current flow reached just over 6 amps when the module added resistance to the ground circuit. This allows the coil to maintain its charge, but does not allow it to over charge and draw too much current through the ignition module.

Once the module is commanded to fire the coil (usually by the ECM), it will release the ground allowing the field to collapse and produce a high voltage spark from the secondary windings. This is called the turn off section of the coil ramp pattern.

The turn on section is where most diagnosing is done

Most coil failures will be seen in the turn on section of the pattern. A crank, no-start, no-spark will usually have shorted primary windings causing the coil ramp to look like the example below.

Notice in the picture above that the current flow does not ramp up but instead rises vertically from 0 to about 3 amps. This indicates the primary windings are shorted causing excessive current flow as soon as the coil begins to charge. Now look at the example below of the same 95 Toyota with a new coil installed.

On this particular example, the turn on oscillations are VERY noticeable. Note that all coils will not have such pronounced turn on oscillations. The example below is of a perfectly normal coil on a 1991 Honda Accord. Note that the turn on oscillations are much smaller and less pronounced than the previous examples.

The best way to get a good look at the turn on section of a coil is to zoom in by selecting a faster sweep time. Have a closer look at the coil pictured above in the next example.

By zooming in, we are able to see that the coil oscillations are indeed there.

But what if the turn on oscillations are missing? What does that mean?

A lack of turn on oscillations usually means that the secondary windings have an open or at the least a high resistance. Sometimes a coil generating this pattern can still produce a spark, but it may be very weak and cause intermittent no-start conditions. The example below shows a 1988 Camry with an intermittent no-start condition.

The coil above was able to produce a spark, but it was very weak. Using a spark tester, the coil could jump a gap of about .030 of an inch or .75mm. If the gap on the spark tester was opened wider, no spark was produced.

Exception to the rule

As with most rules, there is an exception with turn on oscillations. A multi-strike ignition system, that is one that sparks more than once per cylinder event, will have no turn on oscillations and will also appear to have shorted windings in all patterns AFTER the initial spark. With a standard ignition system, the coil has enough time between firings to dissipate any residual charge after it has fired. With a multi-strike ignition system, the subsequent firing events happen almost immediately after the initial firing of the ignition coil. This means that the coil is still partially charged when the module once again grounds the coil to begin the charge cycle again. Since turn on oscillations are caused by going from no-charge to some-charge , they will not be seen on any but the initial charging of the coil.

Take a look at the example below from a Ford Expedition with a multi-strike ignition system. This particular system would fire the coil 3 times instead of just once per cylinder event.

Even though the initial charging of the coil appears normal, the 2 subsequent patterns have the appearance of shorted windings with no turn on oscillations. This is a normal pattern for a multi-strike ignition system.

Firing the coil

Some ignition modules will limit the amount of current that is allowed to flow to the ignition coil and some do not. Therefore it is normal to have a coil ramp that is flat across the top, or one that comes to a point just before firing. The key to firing the coil is a clean and instant break from ground on the negative side of the coil. The pattern of the turn off section should drop straight down to zero amps. It is normal to have some oscillations after the coil has fired, but these oscillations may or may not be present. If the ground is not broken cleanly, the coil will not fire properly and may produce a weak spark or no spark at all. The example below is from a GM 3.8l DIS ignition system with one coil not firing.

Notice how the ground did not break cleanly and the turn off section does not drop straight down. The transistor in the ignition module that is responsible for turning the ground circuit on and off is not working properly. A new ignition module fixed this vehicle, although the coil was also replaced as a precautionary measure. In most cases, the coil is what damages the ignition module with this ignition system.