looking for a little help with a 2009 F350. 340,000 kms
here is what happens.
start it in the morning and everything is working correctly
15-20 minutes after starting the alternator light will come on, its still charging, i verified it with ohm meter and amp clamp. 14.3 volts and 40-50 amps after glow plugs are off and truck is warmed up.
then the gauges will start acting up
speedo and tach will go to 0 and back to current speed and rpm
fuel gauge will go empty to full and back.
Warning on the info screen come up:
check brake system
check charging system
check air bag system
it does some other warnings to just cannot remember right now.
this also is affected by speed, driving 80-100 kph it does it all the time
as i slow down and stop the frequency slows down also.
I've changed all the relays for the dash and interior power.
checked all fuses
removed and cleaned then reinstalled all ground and power wires.
we did have the resistor for the climate control burn out and that has been changed.

 

Even though it's charging when you checked it, it sure does sound like the alternator. Acting up at speed is usually a brush problem inside the alternator.

I know alternators are not cheap, and this is just a guess. See if you can do some research on it and get some more verification. I do know these later trucks are sensitive to voltage problems, and the gauges acting wacky are usually a sign of voltage problems out around the alternator and the batteries. You did good checking all the connections. The only thing that triggered my response was it doing it worse at higher rpms.

 

Alternator has been replaced. it was rebuild by a local shop that has a certified test bench.
I've known these guys for years, nothing leaves the shop without a piece of paper showing amperage output and voltage from low idle to wot.
the truck does have 37" tires with aggressive tread, there is some vibration at speed,not out of balance but tread vibration. I had a 2007 F550 do this once and it was a relay behind the dash to the right of the cluster but a can't find those on this truck.