Tech Tips On Pro Saw Ignition Systems
Modern electronics have revolutionized the world in many ways. Even tools like chain saws have been affected by this technology. Today’s pro saws are light and powerful. Electronic ignition systems are one of the most significant factors in this evolution. The following information provides pro saw users with working knowledge on this important component.
Pro Saw Ignition Systems
Not too many years ago, ignition systems used in pro saws were simple points triggered magnetos. They were heavy, complicated, expensive, and prone to all sorts of problems - especially on wet days.
Even when saws “had spark,” they were often difficult to start. Some barely produced enough energy to fire the plug at cranking speed. In other cases, points-wear, vibration, or poor maintenance would cause the saw to get “out of time.” Having a saw “backfire” and rip the starter handle from the operator’s hand was not uncommon. Sometimes after a good “back pop,” the saw would actually start, but be running backwards.
“Breakerless" transistorized electronic ignitions revolutionized these early ignition systems. This eliminated the mechanical “switch,” which profoundly improved performance. Even though these early electronic systems were bulky and prone to failure, the stage was set for today’s sophisticated systems.
Today's systems are small, lightweight, and durable. Compared to early ignitions systems they are almost trouble-free. It is common for a pro saw to run its whole working life with no ignition problems. While improvements in ignition systems have been dramatic, occasionally they can be problematic. Often these problems are easy to remedy and don’t require a trip to the shop for an expensive repair.
What to Look For When You Have Ignition Problems
When having ignition problems, one of the first items to inspect is the spark plug. Look for damage to the ceramic insulator and damage or wear on the electrode. If the spark plug has a removable terminal, make sure it is crimped tight.
Next, look for bare or broken wires. Engine vibration may cause wires to rub against each other or to the side of another component. In time, even tough insulation will wear through. A kill switch wire that has rubbed against the case long enough to wear through the insulation has the same effect as a kill switch that is turned off.
Wires vibrate and their insulation can wear through causing a short circuit. This occurs most often where they turn and are routed through a metal component or where they rub against another wire.
Other problems occur when wires pass between parts that are isolated by the saw's anti-vibration system. Constant flexing or stretching will eventually cause them to break. Sometimes these breaks are not obvious because they occur within the insulation. A wire that is broken on the inside causes an "open." These can be difficult to find because the wire may look perfect on the outside.
The spark plug wire should also be inspected. Sometimes a cut or wear on its insulation will cause problems. Another thing to look for is a split in the spark plug boot. A visible sign of problems is a brown or burnt residue on the outside of the spark plug ceramic. This is a sign that spark is traveling along the outside of the spark plug. While a saw may still run in this condition, it may be a ready source for a nasty electrical shock.
Check the spark plug boot for cracks or splits. In some saw models, these boots are exposed. If this boot becomes damaged, the saw may still run, but can give the operator a nasty shock if he gets close to it. In the photo above, follow the wire to the boot down to where it is taped. On this saw, the insulation had been damaged and repaired by the operator with tape. Tape is not a permanent solution for a plug wire repair. This wire needs to be replaced.
It is time for a short story. A few years ago a fellow came in the shop hopping mad. He was running a pro saw that had an exposed spark plug boot on the top of the saw. He said he knew the boot had a split in it, but chose to run it anyway. He told us he had the saw in a big undercut running at peak speed when he leaned against the top shroud and... well... by the time he got to the shop he was still walking funny. It was clear he took some voltage in a sensitive area.
Bad Switches
If the wires check out, next disconnect the kill switch wire and test for spark. Sometimes a switch will fail, causing it to be permanently ”off.” If you have spark after disconnecting the switch, replace it and test the saw.
The sheath on this wire is frayed indicating a wear spot on this saw engine. All wires are prone to damage from vibration and should be inspected occasionally.
Problems with Modules
Ignition modules and coils occasionally fail. When everything else checks out, suspect these. When they fail, one of two things usually occur. The obvious symptom is no spark. About the only thing you can do is replace the module with a new one - or better yet - swap it for a used one you know is good. If the saw has spark after you change modules, you fixed it. If it doesn't, you missed something.
Replacing a module is the only sure way to test it. We don't have a tester that tells us with certainty if a module is good or bad. The only certain test is to install a new one and test run the saw.
Another symptom that indicates a bad electronic module or coil is spark that becomes weak when the saw gets hot. If a saw won’t start after refueling, chances are engine heat has weakened or eliminated the spark. Weak ignition systems sometimes make just enough spark to operate the saw when it is cool, but at operating temperature, the resistance goes up and the weak spark fails to fire the plug. Symptoms are, the saw will start and run good in the shop, but on the job, the saw dies and won't restart until it cools off. This can be difficult for some shops to diagnose because the saw will run well in the shop. It may even perform well in several test cuts.
This is a load simulator we use to diagnose saw engines with ignition systems that have heat-related problems. It is an adjustable air brake, so while the engine thinks it is cutting wood, it is actually pumping air. We joke that it simulates the world's longest cut. With this tool, we can bring a saw engine up to (or passed) working temperature in a very short time. Since the load is constant and fairly quiet, it is also easy to monitor the engine for misfires or other operating defects that would be hard to observe if the saw was actually cutting wood.
This symptom is also similar to what happens when the fuel vent on the tank is plugged, so diagnosing heat related ignition problems can be tricky. When trying to restart a saw you suspect has a heat related ignition problem, run the saw until the problem occurs, then open the fuel cap just enough to allow air into the tank, then reseal it. If the saw restarts immediately after you vented the tank, the ignition may be fine and you have a vent to repair.
Replacing Ignition Components
Some early electronic ignition systems had two parts - a trigger module and a coil. Today, they are combined into one part. On saws with two piece ignition systems, the trigger module can be changed without changing the coil. This is an easy job, as the trigger is usually mounted with a couple screws. When changing this part, simply remove the old module and mount a new one in its place. Just make sure the new module has a good ground. If you notice any corrosion on the mounting posts or if the screws were not tight, this could be your ignition problem. Modules need to make a good connection with the case.
Changing the coil is a little more difficult. This is the part the spark plug wire comes out of and is mounted in the fan housing just over the flywheel. This part, along with the magnets in the flywheel, is what produces the energy for spark and provides a reference for ignition timing. When changing the coil, remove the old coil from its mounts and inspect it. Look for scrape marks or for any indication it has been hit by the flywheel. Contact between the coil and flywheel can be caused by loose mounting screws or a worn main bearing on the crankshaft.
Even if you don’t see any damage, check for “end play” on the crank shaft. If you feel movement in this bearing, contact between the flywheel and the new coil could occur. This could damage the new coil and in some cases, the flywheel, too. If you find a loose bearing, replace it. If everything looks good, install the new coil. The base of the coil should be mounted about .012" above the magnets on the flywheel. If you don't have a gauge, try using a business card or matchbook cover as a spacer. Turn the flywheel to where the magnets are just under the coil. With the spacer in place, tighten the mounting screws. Rotate the flywheel to carefully remove the spacer.
When replacing the coil the air gap must be set to insure the flywheel can not contact the coil at any point in its rotation. If you are replacing this component in the field and lack the proper tool, a thick cardboard back from a notebook suffices as an air-gap gauge in many cases.
Features of Modern Electronic Ignition Systems
At the beginning of this information, we stated early ignition systems used simple points triggered magnetos. These early ignitions were statically timed, so the saw's idle timing was the same as it was at full throttle. This was also true on early electronic ignition systems.
Today's ignition systems have changed that. Some now have features that change the timing based on the speed of the saw. This timing “curve” is preprogrammed into the “chip” and helps the saw produce more power by changing when the combustion charge is ignited. This gives it more burn time so more of the energy in the fuel can be utilized. In addition to making more power, some ignition systems now retard for easier starting, then advance for a smooth idle. Others have “rev” limiting technology built in.
At the same time ignition systems are doing more and more, the components themselves are getting smaller and more durable. Today’s one piece ignition systems consist of a coil and control module about as big as a fingernail. These compact units are so durable, it is not uncommon to see saws run their entire lifetime without a problem.
Ignition systems have come a long way in the past ten years, but engineers say emerging technology will drive even more improvements in coming years.
Our Advice
Hopefully, this information will help you diagnose and fix a problem with your saw’s ignition system. You have learned that ignition problems are frequently wiring problems and easy to repair. If you have a more a serious problem such as a coil failure, you have learned that even they are not difficult to replace.
Just remember, if you need help, contact us. Our technicians are factory trained and certified. They also have years of experience and special tools for dealing with difficult problems... That's why we're the Pro's Choice.
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