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Troubleshooting Ignition Systems - Two Approaches
By Roy Bourke, MAAC 204L
Three things in life that are absolutely certain are death, taxes, and problems with spark ignition systems. Most of the traditional problems with spark ignition systems have been significantly reduced or eliminated with the transistor trigger systems we use today. However, problems still occur so it helps a great deal in finding and solving these problems when we can isolate them to one part of the system.
Figure 1 shows the circuit of a typical transistorized ignition system.
I separate the system into three modules that are connected by inline connectors. I use a kill switch mounted on the engine mount assembly, so the points and kill switch form one module. The coil, transistor and arming switch form an ignition module, and the battery is the power module. The connector to the points and kill switch (I use Deans 2-pin) is accessible in the engine compartment and makes for easy disconnect of the modules for engine removal or for troubleshooting the system. To troubleshoot I use two simple gadgets that are equipped with the same type of connectors.
The test light circuit in Figure 2 will test the points of the engine in isolation from the ignition circuit. (It can also be used to measure the timing and dwell of the points). The tester has a socket S that will mate with the plug P (Fig. 1) when disconnected from the ignition circuit. Also I equip the test circuit with a plug that will mate with the charge jack for the radio system, so the radio battery already in the aircraft will provide power for the test light.
The micro switch circuit in Figure 3 is used to test the ignition circuit when disconnected from the engine module. (Plug P mates with the socket S in Fig.1). The micro switch provides a much cleaner simulated point contact than simply shorting the points with a screwdriver, a common practice which sometimes results in damage to the resistor in the high tension lead. The clip on the tester can be used to hold a spark plug, or to provide a ground path for the spark from the H/T lead (since the engine ground is disconnected for the test).
The micro switch circuit in Figure 3 is used to test the ignition circuit when disconnected from the engine module. (Plug P mates with the socket S in Fig.1). The micro switch provides a much cleaner simulated point contact than simply shorting the points with a screwdriver, a common practice which sometimes results in damage to the resistor in the high tension lead. The clip on the tester can be used to hold a spark plug, or to provide a ground path for the spark from the H/T lead (since the engine ground is disconnected for the test).
I have been running sparkers for about 67 years and still can't stand to test the spark like Sal Taibi does (with his thumb across the plug and HT lead!). These days I have an excuse; the $280,000 spark sensitive device in my chest! I use 3 checks: 1st--With switch on & piston at BDC ( EXH port open), short the timer point conn. to ground, intermittently,with a screwdriver and listen for spark popping in the exhaust; 2nd-- Remove plug and lay with base on cyl, turn switch on and short the open timer points as in 1st try, this time looking for spark at the plug points.; 3rd--Test as in 2 but this time turn the prop slowly, looking for a spark at the plug points. Test 1 checks batteries, wiring, coil & condenser. Test 2 repeats test 1 but adds plug points. ( you can get an internal spark which will not fire fuel) Test 3 adds the timer points to the check. One of the aggravating things about the spark system is that many pieces are in series so any error in any one piece causes the whole thing to fail. Use of one of the transistor controlled devices will eliminate one of the most common problems, dirty timer points, but it won't fix bad batts., switch, flight timer, coil,plug or wiring, or timer points which don't "make and break " properly. I also have a booster battery with a volt meter. A double throw switch shows whether or not most of the system is working, an idea by one of my British modeling friends.
Thermals to all, Charlie Bruce