2018 Mercury 90 CT

by Pete
(Australia)

- Rough idle (engine shakes)

- Engine picks up speed or surges occasionally without throttle input when idling in gear.

- Vessel View 403 tachometer appears to be erratic at idle and low revs

These symptoms have appeared after the battery isolator switch was inadvertantly turned off while the engine was running at idle on the muffs. The engine stumbled initially when this occured and then resumed idling fairly consistently. After this occured the Vessel View 403 lost all of its presets and had to be set up again. The rough idle manifested itself on the next outing. The motor appears to run OK at higher revs and volts do increase with the motor running so I assume the charging circuit is working to some extent, however I am concerned I may have damaged the voltage regulator or stator. I have unplugged the stator from the regulator and checked the resistance across each winding and they appear pretty similar.

Can I start the engine with the stator disconnected to check it's voltage output? If so how many volts should I expect and will this create any error codes that need to be cleared?

I figure the engine should be running on just the fully charged battery when I do this as the charging circuit will be disconnected. I am thinking if it runs smoothly and the stator output is OK then it points to the regulator being faulty.
Is this a reasonable approach or should I be looking at other things first?