Hi, first post here on the Alfaowner forums!
Got myself a 147 last week and set about solving some of the niggles with it, one of which is boot light not working.
Changed the bulb. Nothing
Changed the complete light unit (happens to be exactly the same unit as the main interior light on a Fiat Seicento, which is my other car!) Nothing.
Tested the voltage on the 2 wires on the plug. Sometimes 12V, sometimes 7 or 8V, sometimes 3V. Weird. Assumed loose connection or bad earth at this point.
Went on eLearn for the 147 and downloaded this wiring diagram.
Where G40 is the boot light, and I11 is the switch (inside the boot latch) which switches depending on the condition of the latch (closed or open)
So I first tested continuity on the black and grey striped wire between boot light and switch. All good.
I then checked there was continuity to earth from pin 2 of the switch. The easiest way to do this is to check between this pin and the earth for the rear lights - just pull the connector off the left rear light, the black wire at one end of the connector is earth. This was okay.
Then I tested the operation of the switch - The wiring diagram suggests it is supposed to short all 3 pins together when the boot is open, and isolate all three from each other when it's closed.
Mine is only connecting two pins (the pins 1 and 3, between the boot light and body computer) together. Neither are being connected to pin 2,the earth connection.
So new latch unit needed.
This is one example where the switch is knackered but you still get voltage at the boot light bulb (presumably milliamp currents to and from the body computer, via the failed boot latch switch.)
Of course, I could be talking tosh if I go and buy a new latch tomorrow and it still doesn't work