hi guys,
i'm actually the guy from the other thread, who bought a HDS to diagnose the auto engine idle stop problem on the MDX. having it always not working and just showing "unavailable". i had to get down to the bottom of it without the help of the dealerships.
what i really found out with the HDS, is that the triggering factor (of it not working and showing "unavailable") was with the BMS (battery management system) which checks only 2 things (according to HDS info): 1. battery temperature and 2. state of charge. i checked the battery temperature from the hds reading/parameter and is at 27-28C (and i ran it a 2nd time, battery temp was at 37C) which is in normal range, but "idle stop inhibit (BMS)" is still red, so that means the auto start/stop feature isn't working because of "battery state of charge" reason. meaning the bms module does think that the battery is not fully charged. especially if there was a lot of drain on the battery while the engine was off for a long time (e.g. #1 if the idle stop was activated until the battery drained to the point that the engine restarted by itself before you got to a green light, #2 if the car was parked for a long time ("park&fly" sort of thing)with something draining the battery down) then the BmS module would think there isn't enough stored charge on the battery so it gives you "unavailable" on the dash when you come to a complete stop.
also, the alternator on these mdx's (2016 & up) are always (for most of the time) running in their "Low power output" unless you turn on (i forgot if full headlights on or full headlights on + fog lights) triggers the "high output charge" on the alternator. they somehow output just around 12.3v while you are driving with the low output mode. (i guess fuel efficiency design for honda/acura to achieve more mpg). less drag for the alternator on the engine when operating in the low power output. so the battery doesn't get charged up to the point the BmS would allow it to go to Idle stop mode. hence the "unavailable" message on the dash. charging a low charge battery at 12.3v doesn't really get it charged up quickly even if it is an AGM battery. it needs more voltage & current to get a depleted battery up quickly. high output mode usually in the mdx is around 14.something volts. and does replenish the charge between traffic light stops...
what i have done, is i have left the headlight switch on "ON" all the time + the fog lights "on" and it always triggers the high output mode of the alternator. the down side is driving with your full low beams on even in the daytime. also don't worry leaving your headlight switch to the "on" position, as when you get off the mdx (it will chime to remind you the switch is still "on") and park it, the bms module will shut the headlights off after 1 minute of locking the doors.
after leaving the headlight switch on the "on" position all the time (24/7), the idle stop always works 100% of the time.
i even went the length of plugging in the battery when i get home to a smart charger just to make sure in the morning when i wake up, the AGM battery is fully topped off. but you don't have to go that route. too overkill in my opinion.
so the short solution to this "unavailable" problem is to keep your headlight switch on the ON position i guess.
so let us know if that works for you,
we'd like to hear feedback
thanks,
hope this info helps
jeffrey