***"1. How can the engine run with a coil that tests open in the secondary circuit?"*** To understand how that can be, you must remember that the secondary circuit of an ignition coil produces voltage ranging from over 20,000 volts to maybe 30,000 volts or more. Voltage like this is called "High Tension" or "High Potential" voltage. Lightning (extremely true) falls into this category). Voltage this high can very easily jump the gap between the broken ends of wiring in the secondary windings of an ignition coil and will produce a somewhat weaker and less defined spark across the plug gap. But the ohmeter you use to check the resistance of the coil might only have a few volts worth of potential (from the meter batttery) and that is not sufficient to jump across the gap between the broken ends of the wire in the secondary coil circuit. You do not have to use an Onan branded coil. The main criteria that must be met is the resistance value of the primary winding of the coil, and whether the ignition circuit of the engine utilizes an external ballast resistor, or a resistor inside the coil itself. You do not want to use a coil having an internal resistor on an engine that already has an external ballast resistor because the engine will be hard to get started, and if old schoool contact points control the ignition timing, the life of the points will be decreased. If electronic ignition controls the spark timing, the electronic ignition will be overloaded and fail. Likewise, if you use a coil intended for use with a ballast resistor (non resistor coil), but there is no external ballast resistor in the engine circuit, the life of the coil will be shortened (coil will burn out). If there are any automobile electric specialty shops/stores in your area, they should be able to cross ref a replacement coil for around $50 or less. Take the old coil with you.

Talked to a guy today same as you. Told him to clean the vent in his fuel cap and got a call back w/ a thank you. Check it out.

Man, could we use a good wiring diagram for this one? The "no spark until the starter is de-energized" sounds like what happens if a "battery & coil ignition" features a "ballast resistor" in the power wire that feeds the ignition coil, and the "ballast bypass provision" is not working. Ignition systems that feature a ballast resistor are designed that way to help minimize a weak spark during cranking when the amp draw of the starter might pull the electrical system voltage to low to make a good spark. The ballast resistor systems work like this. These systems use an ignition coil that is rated at about one half the voltage level of the vehicle electrical system. The ballast resistor is wired into the feed circuit to the coil and under normal operating conditions, cuts the system voltage in half to keep from damaging the ignition coil. Or, look at it this way. The battery voltage is 12VDC and the voltage rating of the ignition coil is 6VDC. Under normal operating conditions, the ballast resistor drops the 12VDC of the battery to 6VDC for use by the coil. But, under cranking conditions, there is a featured way to "bypass the ballast resistor" so that full electrical system voltage is fed to the coil, resulting in a "hot spark" for easy starting. The "bypass provision" might be built into the ignition switch as an extra post and wire to feed the coil during cranking only, or the bypass provision might be built into the starter or starter solenoid.

Colby, If you have a Cummins dealer near you thats where you'll find your onan parts. Cummins owns Onan but you can also find onan parts at your local John Deere dealer if there is not a Cummins close to you. John Deere used Onan engines in many of their l&g tractors in the 80's and early 90's. I would try a Cummins dealer first as they would be able to track down your needed parts. Good luck!