[Phil Himsworth]

We took the Ioniq to the West Midlands Safari Park today. An EV with Auto Hold is definitely the way to do it, it was so easy and quiet!

"Auto Hold" was a godsend as it's a fairly hilly route with lots of stopping so it's great to have the car hold itself without really having to do anything. Towards the end though it stopped using Auto Hold and started putting the parking brake on instead, giving a little message in the dash that it was doing so. The effect was the same, but it was a bit more jerky engaging and disengaging.

Does anyone know why it did that? Can it only use auto hold so much before it has to use the parking brake?

On the way home auto hold was back to normal, so I'm curious...

 

[HandyAndy]

I suspect the motor was getting rather warm. To hold you stationary on a slope using only the motor, it has to generate the torque required to hold you, so there's a decent current flowing which will warm the windings. At the same time there's no airflow through any radiator (unless there's something like an auxiliary thermostatic fan fitted), so it's possible the cooling system works less well while you're in Auto Hold.

Incidentally, does Auto Hold work properly if you're on a down-slope and trying to reverse-start going backwards? My Ampy does Auto Hold ok in the forwards direction, but the software engineers totally forgot about the mirror-image scenario, sadly! As I park at home nose-in on a downwards slope for reasons of access & charging, I have this scenario every time I leave!

 

[Ere Maijala]

I believe auto hold keeps the brakes pressurised instead of using motor torque. I could be mistaken, but since in my car the brakes give the distinctive click when engaging and disengaging, I've found them to stay applied when I release the brake pedal. Also when you start moving again you may hear the brakes release.

That said, there is some logic that switches from holding with normal brakes to the electronic parking brake when auto hold has been holding the car stationary for long enough. I recall it could be 10 minutes, but that I'm definitely but sure about. Anyway, maybe the car got somehow confused with this, or needed to let the brake booster pump cool down.

 

[Phil Himsworth]

That's interesting, thanks. I wondered if it was something related to the high number of brake applications compared to the very low distance or speed, such as needing to recharge a pressure vessel or something, but couldn't think of why it couldn't do that; letting something cool down does sound now plausible...

 

[TomH]

That's interesting, thanks. I wondered if it was something related to the high number of brake applications compared to the very low distance or speed, such as needing to recharge a pressure vessel or something, but couldn't think of why it couldn't do that; letting something cool down does sound now plausible...

I'm sure that most autohold systems don't involve a traction motor as you can get them on ICEs too!

The handbook for my Golf GTE warns that the autohold sustains the pressure of the last foot take application that you made. That means if you are in the habit of feathering the brakes to a smooth stop, like I was, there is minimal pressure in the system when it holds. So on a slope you get this creaking noise as the parking brake starts to slip.

I now still feather to a stop but, once stopped, I give the pedal a good shove to make sure that the car stays put.

Have a play around with your system. I'm guessing that the release timing lost its smoothness after a lot of repeats. I doubt that the principle of operation changed