Re: Oman ignition
You should NOT be getting any voltage to the Regulator unless key is turned on, and then shouldn't do anything until after started. Especially the outboard prongs which come from the Stator that only sends out AC current, not DC readings.
Go to bottom of this info and read. After writing, it was something I thought of that ONAN's are notorious for if it has a oil sending sensor. It's a part of coil/ignition control testing I came across one day when I was having issues with my P220. It's a forgotten electrical item due to it being hidden.
The Regulator takes the Stator's voltage which is AC and it goes through the regulator's internal resistors/diodes and converts it to DC, which then goes back (there's a few detours along the way) to the battery for recharge.
When you start the mower, all power should go to ignition control and coil to fire the plugs. Once started then the Regulator kicks in after the Stator builds up the magnetic field to produce current.
Ignition control(module) red wire goes to positive post. Black wire goes to negative post of coil. Switch them backwards, and YOU WILL burn out the ignition control.
That isolator pad on back of module isn't much, but apparently does more than we think.
You might have a bad ignition switch. Switch should be marked on backside with--B(battery-this is a fused circuit, so check for rusty fuse contacts also), S or ST(start), ACC(accessories low oil alarm), IGN(ignition - goes into main wire loom where it branches to coil, starter or such). From this switch you'll also have branching off to other operating items like oil light, deck engagement switch, amp/hour meter or both, light switch.
Now of course if your switch has more posts, then wires will go to other points and switch will have additional markings.
You should have constant 12v to the battery post of switch(off or on). All other posts won't have current unless switch is On or in start position. Accessory side could have current also, but only in run position for any alarms or low oil pressure lights.
Not knowing your switch, it could be a 4-post(screw on posts) like mine, or it could be up to 6-posts(push on wire harness or individual screw on wire post) depending on how yours is wired.
Ignition switch needs tested to be sure it's not shorted out internally by doing a 12 volt check on all posts. Start in "Off" position first to rule out any stray voltage that shouldn't be there. Good. Now go to "On" position and test again, but this time branch out to any amp, hour meters. You should also get a low pressure/oil alarm or oil light. If not, check wires to find out why.
Almost forgot and this is an unusual one that I happen to find when doing a coil test from one of the forums. You'll need to remove the side tin the oil filter is on. If your ONAN has a oil pressure sending unit(found with (2) wires) on the oil mount block just above the oil filter. It's been known to shut down an engine due to malfunction. Replace it.
Apparently the oil sending unit cuts voltage to the coil, when it goes bad.
1967 Cub Cadet 102-Kohler K241(10hp) Kept as original as possible.
1990 Kut Kwick CL20-60-Onan P220(20hp) Front deck/rear steer commercial mower
2003 Craftsman GT5000-B&S engine(24hp) Manual transaxle