Ignition Diagnostics: Think you know ignition diagnostics?

As field training specialist for blue streak-hygrade motor products, Doug Doran has been examining ignition diagnostic issues since 1963. Doran, who travels extensively to keep technicians up-to-date on fuel and ignition technology, sat down with SSGM editor Jim Anderton to dispel some common myths about ignition diagnosis.

SSGM: Are failures more common on the primary or secondary side these days?

Doran: “Most failures today are on the primary side, but the secondary side will give you a view of what’s happening elsewhere in the ignition system. ‘No power’, ‘surging’ and ‘missing under load’ are difficult for technicians to diagnose. ‘No starts’ are easy to find.”

SSGM: Is there a universal playbook for those driveability issues?

Doran: “Determine whether you have an intermittent fuel or ignition issue. I’d use a scan tool first. If you have an ignition misfire, what will the O2 sensor see? I’ve done road tests and measured the O2 sensor at 0.01, 0.02 or 0.03 volts, occasionally even with a fuel injector disconnected. Low O2 voltages suggest an ignition misfire, because the air isn’t being “burned” in the cylinder effectively. The O2 sensor only knows air, not fuel.”

SSGM: What’s the next step?

Doran: “I’d use a ‘scope and test secondary patterns. People think that a scope will only show a bad plug wire. Most technicians look at the voltage spike. If it goes way up, there is an absolute break in the ignition wire. It doesn’t tell you if there’s excess resistance in the wire. That spike is the required voltage to jump the spark plug gap. Then the spike comes back down to the retaining voltage to maintain the spark. There’s million of ohms of resistance across the plug gap. Suppose a 20,000-ohm plug wire has a resistance of 40,000 ohms. Will it show on the retaining voltage line? No.”

SSGM: What can that fire line tell you?

Doran: “Lots. If I see the line slanting down, I know that the plug wires are bad due to external resistance. If it ramps up, it’s due to high internal resistance, often due to lean mixtures. I can tell bad valve guides or lifters from the trace too, because it changes the internal resistance (across the plug gap) because of the fuel mixture. A ‘scope can paint a picture of what’s inside the engine.

If you have a broken coil wire, you’ll have a “ball” at the top or middle of the spike, and you can tell whether the break is at the coil or distributor end of the wire.

SSGM: Do you recommend looking at standard patterns to help the diagnosis?

Doran: “No. You don’t need it or want it. You want to know what each part of the pattern means. Consider an injector pulse. You can see the whole system, including the control side of the computer.”

SSGM: Does distributorless technology make a difference in how you troubleshoot?

Doran: “None whatsoever. The same conditions affect that spark and waveform as in a conventional ignition. Coil-on-plug is the same thing, using a small adapter wire. It’s a whole new world of diagnosis when you use a ‘scope. And low amp probes have opened up another new world for diagnosis. It’s non-invasive. You can just clamp it around a fuel pump wire, for example, and detect a short, a bad motor segment, and by the time base, even figure out the RPM of the pump.

Everyone thought that because we have “black boxes”, they’re a mystery, they’re the problem. Everyone forgot how to watch electricity go through a circuit.”