Secondary Diagnosis Most technicians have diagnosed secondary ignition systems using the large console-type analog scopes, but there are times when it just isn t practical to use an analyzer that is that large. When presented with a problem that needs to be checked quickly, or in a remote location, a hand-held digital storage oscilloscope (DSO) may be a better choice. Now that many technicians have purchased DSOs, its time to learn how to set up a DSO to view an ignition pattern. Some DSOs have presets that will allow a secondary pattern to be displayed without knowing how to adjust time per division or voltage levels. However, many DSOs must be adjusted manually to view the pattern. Even if yours has presets, you may find it necessary to readjust the scope to analyze a specific problem. At first, viewing ignition patterns on a DSO may appear differently than on an ignition scope, but by adjusting the DSO you can bring the pattern to a more recognizable picture. With a little practice you may find the DSO can be used in place of a larger, more expensive console-mounted scope. By Randy Bernklau 29
Secondary Diagnosis Your DSO configuration may vary, but many of the items I will discuss can be used on all scopes. The first thing to consider are the leads. The capacitance lead is used for reading secondary ignition. It is clamped around the coil wire and connected to channel 1 of your scope. Check with your supplier for a secondary lead. With the lead connected, adjust the scope. Most of us are familiar with viewing secondary ignition patterns with the voltage spike going up. This is actually upside down. To get this view, select the invert function on the scope. Without this function, you must view the pattern upside down. Alternate depending on the coil side for DIS. The volts per division setting will vary depending on your lead s voltage ratio. My lead reduces the voltage by 100 times, so I set the scope to 10 or 20 volts per division depending on secondary voltage. A normal voltage of 6,500 volts will be displayed as 65 volts. This fits nicely on a 20 volt scale. A lower voltage scale helps provide detail.
The next step is to supply a trigger to stabilize the pattern. You can trigger on the coil wire but using a separate lead on number 1 plug wire is best. Use an inductive lead and connect number 1 to external trigger or Channel 2. When using Channel 2, adjust the trigger point, then turn Channel 2 off. This helps clean up the screen. Setting up the time scale is a little more difficult. It depends on what you are trying to examine. Lets start with 500 microseconds per division. This provides a good view of the firing line and burn time. Depending on the trigger source you are using, the screen will show all patterns or just number 1. This can be used to your advantage.
Secondary Diagnosis Using the inductive trigger and the scope set in Normal mode, the DSO draws a trace when a trigger is present. This means you know the pattern is only showing the number 1 cylinder. By moving the trigger lead, you can display any other cylinder. Most DSOs screens are small. This method give you the best detail for any single cylinder firing event. Changing the trigger to the capacitance lead allows you to view all cylinders. Detail is still retained but when a problem is seen, it is impossible to tell which cylinder is causing the problem. This method is best for cursory checks or to find the overall burn time. It is also good for coil, wire and rotor gap checks. At times it may be more important to view all firings in a parade view. In most cases you will need peak detect. Failure to do this will result in missing firing lines. Make sure you are triggering off the inductive lead on the number 1 plug wire. To get parade mode, adjust the time per division to a higher setting. 10 ms is a good place to start. How can we tell which cylinder is which in parade view? Since we are triggering from external trigger, we know that number 1 cylinder is the one at the trigger point. In this example the trigger point is to the left side and number 1 is the first firing event. If you are using normal mode and the pattern is freezing, you are losing the trigger. 33
Secondary Diagnosis When you lose the trigger, the pattern stops. This means you are missing an event. Switch to auto mode to see why. It may be the trigger level is set to high or number 1 cylinder may be misfiring. This pattern shows a fouled plug which causes a lower firing line. The result can be an intermittent loss of trigger when the plug misfires. Now that we know how to set up the DSO, let s look at a simple ignition problem. This Toyota Pickup came in with a misfire under load that felt like an ignition problem. I connected the DSO and set it to 10 ms and 20 volts. I soon realized the pattern was off the screen and readjusted the volts to 50. I left peak detect off. You can see the firing voltage is too high on one cylinder. The first pattern in this image is 21,000 volts at idle. High firing voltage indicates high resistance. In this case it was caused by an open plug wire. You can see why peak detect should be on. The normal wires appear to lack any voltage kick. With peak detect on they would look normal. This diagnosis was easy. With a new set of wires this truck is ready to go. The tough problems to solve are the intermittents. Getting to know at a glance what is normal and what isn t will help when you are trying to catch a random misfire problem. Secondary ignition analysis is an important skill that takes time to master. 34
This is a normal pattern after the wires were replaced. The KV range is good and burn time appears normal. Also the slant of the burn time is normal. When looking at the pattern live, the KV lines will bounce up and down slightly. Excessive bounce is a good indication of a lean mixture or possible excessive EGR flow. Now it s time for something new. I started looking at this several months ago and haven t worked out all the bugs yet. Spread the pattern out to 5 to 20 microseconds per division and observe the rising edge of the voltage spike. Two things come into play. The first is the double peak that you see. This is caused by the voltage jumping two gaps. As you see in this waveform, the double peak is gone. This is because it is taken from a distributorless ignition system. The double peak in the previous example was caused by the spark jumping the rotor gap and the spark plug. This allows you to measure the actual rotor gap voltage. This is a quick way to check for rotor condition. Checking the rising edge of the secondary signal is the second part of this analysis. I have noticed when the edge is hashy or rough, it means the spark is jumping multiple gaps. Check for loose plug wires or corrosion on the coil wire. Try this method of secondary analysis using your DSO. I would be glad to hear how it works for you. 35