I've owned my truck for a 1 and 1/2 months now and have tried to pay attention to when the auto stop/start is activated. Engine temp has to be one parameter monitored, but there are others. This evening I was sitting in a drive-thru ordering dinner. I pulled up and auto s/s engaged shutting off the engine. I kept my foot on the brake and didn't flinch at all. As the guy in front of me drove ahead the truck started to my surprise. I think the collision warning sensor detected the movement of the vehicle in front and started the engine in response so I think that's another thing it monitors. I know a lot don't like auto s/s. Sometimes it is annoying, but mostly I like it. I'm more annoyed when it doesn't stop when there seems no reason.

 

Engine Temperature, steering wheel angle, incline/decline angle, battery charge status, A/C / Heat demand compared to interior temperature are among others. You can also prevent it from engaging by lightly pressing the brake pedal, as opposed to pressing it firmly at a stop.

 

The auto stop start is controlled by the battery management system (BMS). While the engine is off the battery isn't recharging from the alternator. Most likely a coincidence.

 

So when voltage drops enough it auto starts. That makes sense. When the guy drove away in front of me at the Chick-Fil-A the truck might have just started by coincidence triggered by the voltage drop. I was surprised about the incline/decline being a trigger.

 

Battery voltage is certainly one of the reasons if you have things turned on like AC, your radio, ect. and of course your break lights and running lights. The manual lists a bunch of things that enable or disable the start stop function all controlled by the BMS for the sake of the battery. No way I can tell you of what exactly turned the engine back on.

When I was not so acute to the auto start stop function being new to the feature I mistakenly thought my truck had stalled at a stop light. I put my truck in neutral to restart my engine and to my surprise it started right up by itself. The auto start stop will disable in neutral. Since then, I've gotten into the habit of turning it off and began a study about the BMS. The BMS is what you really need to be familiar with because it will disable things to your surprise including the radio all for saving the battery, to assure driveability and the power for vital functions.

 

I've collected some in driving my 2018 Ford F150:

1. Steering Wheel Manuevering
2. Battery Charging
3. Cabin Climate unsatisfactory
4. Ambient Climate isn't Moderate
5. Selected Gear (4x4)
6. Selected Mode (Sport, Baja, Mud)
7. Windshield Defrosting
8. Primary Seatbelt not fastened
9. Door Ajar
10. Incline / Decline too Steep
11. Elevation exceeds 11,000 ft
12. Insufficient Brake Vacuum
13. Normal Operation (if you stop, then proceed without exceeding 3MPH then Auto Stop Start is still disabled)
14. Engine Temperature
15. Trailer Connected
16. Selected Manual Transmission
17. Hill Decent Control Active

There may be more even, but judging by my prior experience and the instruction booklet this should be most of them.

 

As noted in the post above, there are a lot of things that factor into whether the Auto Start Stop will activate. While the BMS is a part of those decisions, it is not the only controlling factor, and in fact isnt even the majority of them...

If you really, really want to know, get forscan, and look at all of the parameters that are listed. You can monitor them and see exactly why Auto Start Stop engages or not.

 

Do an Internet search for the following Ford Doc.
.
General Service Bulletin, Auto Start-Stop Operation, Knowledge & Technical Information.
.
You will find 31 different parameters listed and there are more.
Advise to print or save these sources when you find them for future references.
The vehicle is 99% controlled by computer such that there are no simple answers any longer but understanding that fact.
You can't go back from Electric Lights to Oil Lamps and keep up with the rest of the world.
Good luck.

 

Yeah and bms isn't driving start stop system. But it's a one of the sets of parameters the ecu uses

If the newer car has that autop stop and go feature. The brakes to stop and will go on its own system. I think that's 2019 and up. That system will trigger engine on and off. Not all models have it.

 

Straight from the Workshop Manual:

Auto-Start-Stop System

The Auto-start-stop system helps reduce fuel consumption by automatically shutting off the vehicle’s engine while the vehicle is at a complete stop and restarting the engine when the brake pedal is released. The system can be disabled through the auto-start-stop control switch on the instrument panel centerstack. The Auto-start-stop system is automatically enabled whenever the ignition is turned on.

The engine automatically restarts when:

• the brake pedal is released.
• the Auto-start-stop system is disabled through the auto-start-stop control switch on the instrument panel centerstack.
• the battery has a low state of charge.
• it is necessary to maintain interior comfort.
• the blower fan speed is increased or the climate control temperature is changed.
• an electrical accessory is turned on or plugged in.
• there is low brake vacuum.

The Auto-start-stop system may not turn the engine off under these conditions:

• the HVAC system is in A/C, heat or defrost modes.
• the rear defroster is on.
• the battery has a low state of charge.
• the battery temperature is below 5°C (41°F) or above 60°C (140°F).
• the engine temperature is below 46°C (115°F).
• the engine temperature is below 60°C (140°F) and the HVAC system is in heat mode.
• the steering wheel is turned rapidly or is at a sharp angle.
• vehicle speed of greater than 4 km/h (2.5 mph) for more than 2 seconds has not occurred.
• the vehicle is on a steep road grade.
• elevation is approximately above 3,048 meters (10,000 feet).