I just got a 2021 155mhev, and when the engine turns off via stop-start, the interior fan speed drops right down. None of the lights on the HVAC change, it just doesn't blow much. As soon as the car starts back up, the fan ramps back up.

I only use the HVAC in "manual" mode, where I just set a temperature and a fan speed like a dinosaur - I've no idea what happens in auto. AC can be on or off - it's not a factor. 

I've never experienced this in other stop start cars.

Is this normal behaviour?

Normal behaviour to save power.

If it's very noticeable then suspect the main 12v battery has had it

How noticeable should it be? 

It's noticeable to the point that I'm concerned enough to ask! If say that, if I'm running the fan at "4" out of the 7 speeds, then it drops to "1". 

That's exactly how it was on my 2021 MHEV.  Sounds normal, unless mine was also faulty, owned from new.

It feels like deliberate behaviour, but it's also a bit like, "every car I've ever had managed to keep the interior fan running when the engine wasn't running", so it's slightly puzzling too. In going to hybrid and adding battery capacity, we're also removing the ability to use it! 

But if that's the way it is, that's the way it is. 

1 hour ago, DaveT70 said:

If it's very noticeable then suspect the main 12v battery has had it

 

It should drop, but if it's almost dropping off totally then the vehicle is preserving power so it can restart the engine.

Turn off Stop/Start if it's getting too much

5 minutes ago, Johnnybriefs said:

In going to hybrid and adding battery capacity, we're also removing the ability to use it! 

The MHEV system and battery is totally separate to the 12v system

2 minutes ago, DaveT70 said:

It should drop, but if it's almost dropping off totally then the vehicle is preserving power so it can restart the engine.

Turn off Stop/Start if it's getting too much

It's restarting using the hybrid battery though right? Although I suppose spark etc is 12V.. I can have the battery checked I suppose, but it's hard to know what too low fan speed is without another to compare too. If it sits for a couple of minutes at standstill it will disable stop start and start the engine, so I'm surprised it'd actively save power until that point. 

40 minutes ago, Johnnybriefs said:

If it sits for a couple of minutes at standstill it will disable stop start and start the engine, so I'm surprised it'd actively save power until that point. 

It will do that if you have A/C on anyway

I'm really not sure which system starts the engine on the Fords.

Is it the 12v using the alternator to start or the 48v??

The 12v definitely does the initial start up with the starter motor

But the 12v runs absolutely everything else and if it senses low power it will conserve as much as it can.

I would be tempted to either fully charge your battery OFF the vehicle or fit a fully charged new one

And definitely deactivate Stop/Start

And that's, of course, if your 48v system is working correctly.

Have you thought about taking it in and seeing if there are any software updates available?

Might be worth the spend on an MHEV car

3 hours ago, DaveT70 said:

It will do that if you have A/C on anyway

I'm really not sure which system starts the engine on the Fords.

Is it the 12v using the alternator to start or the 48v??

The 12v definitely does the initial start up with the starter motor

But the 12v runs absolutely everything else and if it senses low power it will conserve as much as it can.

I would be tempted to either fully charge your battery OFF the vehicle or fit a fully charged new one

And definitely deactivate Stop/Start

I think there's a standard starter motor for initial startup, using the 12V battery, but all stop start following that has to be via the hybrid motor - it's virtually silent.

3 hours ago, DaveT70 said:

And that's, of course, if your 48v system is working correctly.

Have you thought about taking it in and seeing if there are any software updates available?

Might be worth the spend on an MHEV car

I'm sure it's working just fine, and I'm sure it'd be shouting at me if it wasnt. I'm going to have my local garage give it a once over anyway, but in hybrid world all seems well. It's been well used, so I feel quite confident about the drivetrain (aside from regular wear). 

12v battery with starter motor starts a cold engine.

48v battery with generator restarts a warm engine from stop/start phase.

There is no alternator on these, it's a 48v generator which is converted to 12v to recharge the starter battery.

Was it in Eco Mode, if Hybrid cars have Eco Mode?