View Full Version : Scag Ignition Problems

07-18-2006, 12:29 AM
I have a late '80s scag with a Kohler MV18 engine. After hitting a tree branch and stalling the motor, the starter would turn the engine over, but it would not start. I checked the 0.010" air gap disconnected the kill wire from the ignition module and it still would not start. I also checked the resistance of the coils and determined it was bad so I replaced the ignition module and the keyswitch. Upon installation of both items, it started right up. After shutting it off with the keyswitch, it won't start again. I measured the resistance of the ignition module and it appears to have gone bad during that one time running. What would be causing it to go bad and how do I go about beginning to diagnose it? The manual for the engine the ignition module has a diode which protects it from 12V being applied to the kill terminal. Is there any way to fix this ignition module? I have a feeling it was the act of shutting it off with the keyswitch that killed it. I was able to successfully shut it off before by grounding the kill terminal.

07-18-2006, 12:51 AM
did you simply check for spark? did you check the flywheel key?

07-18-2006, 07:36 AM
I don't have any spark. That's why I think I killed the ignition module again.

07-18-2006, 08:03 AM
Did you check the diode ? Did you compare the terminal letters on the new switch with the old one ? The way I have seen a new coil blow is from 12 vdc getting applied to the kill wire lead with the wrong switch installed or the battery cables being hooked up backwards.

07-18-2006, 02:37 PM
How do I test the diode? The entire ignition module consists of two high voltage wires which go to the spark plugs and a spade connector which accepts the kill wire from the engine. The other space connector is soldered to the ground of the module. I before I hooked up the kill wire, I tested it with a multimeter and it was always ground or floating, never 12V. I wonder if something funny happened in the ignition switch when I turned the key to the off position that would cause a temperary short.

07-18-2006, 08:34 PM
How do I test the diode?

I am assuming the diode is in the kill wire lead for this test.
Remove the kill lead from the coil, Remove the harness plug off the ignition switch. Set your multimeter on ohm's, Put the red lead on the coil end of the kill wire and the black on the switch end. You should get a ohm reading, Reverse the leads black on the coil end and red on the switch end and you should have no ohm reading. If you have a reading in both directions the diode is bad and most likely let a small voltage spike go to the coil burning it out.

The fix, Replace the diode and install another new coil.

Again Check the letters on the switch terminals making sure you were not sold the wrong switch.