I recently (when we had the bad weather) decided I would take the 916 off the road for the winter so I disconnected the battery for a full charge and re-fitted it about 4 days later........turned the key expecting the roar of the v6, only to discover it would crank but not fire.
Looked at the dash warning lights to see engine fault/injector light on but no imobiliser light (on turning the key).......ok time to check fuses. Couldn't find fuses where hand book said they were so posted on forum for help.
Stixy came to my rescue and provided me with chapter and verse as it were and even still replied to my emails when I was less than understanding (bordering on thick) about somethings..............Anyway found the 10a and 7a supply fuses intact. the code box imobiliser is fed by both a constant 12v supply and a switched (ignition) 12 v supply so the next step was to access the code box/imobiliser and check for continuity. I found when I disconnected the multipin connector from the code box that I indeed had 12v to each supply pins (3&8) and terminal 4 which is earth. Now I was getting worried about new codebox/ecu required so decided the next phase was to remove the code box for identification (not an easy job) and take it from there.
When I got the box off I decided to open it in the hope of finding a dry joint or something fixable but alas all seemed ok.
the box itself contains one single p.c.b. which slips out of its casing without any problems. I noted soldered on to the board was a relay and wondered if indeed the relay coil had failed so I plugged the now naked p.c.b. board back into the multipin plugs and when I then measured the voltages on the pins I got 12v ignition supply but only 7v on constant supply. So I bridged the two supply pins together as a temp measure and sure enough code light on then off which when cranked restored the Busso roar
So the moral of the story is even though you may have 12v showing as a supply you may not have 12v actually under load.
I still aint found where I'm getting me voltdrop but I can always run a new supply if all else fails. Any guideance as to where know joint boxes or dodgy connectors are to be found I'm all ears.................On searching this forum for a similiar problem experienced and resolved I did not find an answer. A few threads started but you never got to the answer so I hope this write up helps someone else who begins to think ..oh no me codebox is u/s
Once again many thanks to stixy