Breaks and 12V system

For voltage to go from "good" (i.e. over 12.8 VDC)to so low that neither the engine nor the electric brake boost would work doesn't sound like an alternator to me.

That assumes you were monitoring voltage.

More likely, there was a connection/fuse issue near the chassis battery. That would be more likely to result in voltage going from OK to doesn't work at all.

More insight on whether this WAS a sudden loss of 12 VDC or a gradual loss until voltage got so low that nothing running on 12 VDC would work would be enlightening.

As long as you are rolling, steering usually isn't a big issue.

The brakes still work but you now have to provide all the power. Biggest issue is recovering from the surprise. You may be used to supplying 5-10lb of pressure to the brake pad and suddenly, you need to apply 50-100lb of pressure. If you are slow to adjust, it may be out of control before you have a chance to react.

One question: Before the engine died, did the battery light come on or some other warning? Normally, you would have some warning if the alternator quit entirely but before the engine cuts out.

Per opnspaces
“ I have no idea if it would work. But if the engine quits you might try shifting the automatic transmission down to a lower gear. Worst case nothing happens as the transmission is freewheeling. Best case is you get some engine compression braking to help with keeping everything under control. ”

A very interesting question, hopefully someone with real experience will chime in
With all the electronic systems in our vehicles, engine control modules, transmission modules , how much will actually work without and 12v power. Hopefully there is enough manual connection between the gear shift lever, the transmission and the rear wheels to assist in slowing down.

Ken

Not familiar with an F53 but do you have a mechanical emergency brake that you can use to slow things down?

Go ahead and have alt replaced. You have to do it anyway.

On a long downhill it's not going to be fun. A braking system holds only one maybe two pumps on the brakes in reserve when the engine quits. After those two pumps you're down to non assisted braking. As stated above you might need both feet on the pedal at that point, but the brakes will still work.

I have no idea if it would work. But if the engine quits you might try shifting the automatic transmission down to a lower gear. Worst case nothing happens as the transmission is freewheeling. Best case is you get some engine compression braking to help with keeping everything under control.

Your options in that situation are downshifting and using both feet on the brake pedal to get enough stopping pressure. When the engine died, the power steering pump also stopped, dropping the hydraulic boost pressure for the brakes and the power steering. Oh, and on a steep down hill, pray... ;)