I don't know how your headunit tries to detect an open circuit.
The FIS-Control MMI uses decoupling capacitors in the LVDS lines. For DC current a capacitor looks like an open circuit.
Stefan are you using DEcoupling (cap on signal
to ground) capacitors in the LVDS lines or are you
using COupling (in series with the signal line)
capacitors? I suspect the latter since you you
don't want to burden a digital video line with
extra capacitance (slows switching times AND
deforms the orginal signal form).
Either way, however, a capacitor will act as a dead
short when applying DC to it. At least until the
charge on the capacitor plates has been equalized.
After that it looks like an open circuit until charge
on the plates is no longer equal. This is why you
can use a capacitor as a resistor in AC circuits.
However, I don't think that the ECU is this quick
(cap charge time) in filing a fault code.
I think the ECU simply monitors the current that
the LVDS driver circuit is taking up. If it is too
low than it says open circuit. If it is too high it
As to Dippy's question: I would suggest checking
the four HSD plugs involved (Naviceiver to FC MMI
and FC MMI to Monitor MMI), Maybe one of the
interlocks did not snap in correctly and this is why
you get an open circuit once in a while. If your are
really out of luck it might be that one of the
four pins (in each HSD so a total of 16 fault
possiblities ) isn't crimped correctly. This would
mean cutting the plug off and putting a new one
on. I absolutely detest even actually execrate
crimping. It is fast, but a real man's connection is
done using solder and a hot iron
Audi A6 / BDG 2,5 TDI Bj 2003 // Mj 2004 Bekennender FIS-Control Junky ;ö))
Jetzt auch mit A6 4G Avant Bj 2013 unterwegs // Fzg. *NOCH* ohne Mods ;ö))