I have an 2008 F-450 6.4 diesel . Put the truck in neutral to slow down and the engine died. Now the wrench light is on and it won't start .

Tun off fuel ..drain c arbs . There is a screw at bottom .. fill bowl with carb cleaner
Let it soak awhile

governor pressure to high,
possible causes
governor is sticking
high vacuum at modulator
bad modulator
engine rpm to high
miss aligned lincage

sounds like a fuel problem, how many miles on vehicle? as a quick fix it would be worth using a fuel addative , you haven't said if petrol or diesel, if petrol get addative that covers the catalytic converter ,having a blocked exhaust system can cause engine to stall also get the OBD codes read as this will tell you what if any sensors/switch's are faulty

I assume there's a problem with alternator. Check the grounding to engine, check continuity of a positive wire between alternator and battery. the energizing voltage should be checked as well. Might be the bad connection between battery and vehicle body grounding. Diesel cars are sensitive even in a little loss of connection and rust in the connection points.

fuel pressure,IAC system,MAF or MAP, is there any engine code. check engine light on or off

Do an upper intake clean that will clean the throttle body and the Idle Air Control valve. Also clean the mass air flow sensor

Too much trans fluid would not prevent it from going into gear. Do you have a check engine light on? This is probably going to be expensive. Get it to a transmission shop.

How long has it been since the fuel filter was changed?

Hello, I read your problem, and a lot of the symptoms you are describing seem like itmis possible, that you have a bad 02 sensor, this is part of your emissions, relating to the cadilac converter, it mostly acts up at lower speeds, like coming to a stop or slowing to make a corner. Sometimes also a bad fuel filter could cause this when rpms are down such as slowing or on corners. That is the two things I would check. Thanks mike

when was the last time you changed fuel filter. and are you running bio or reclaimed fuel?