456M strange instrument backlighting problem

Gents,

The instrument backlighting on my 456M works periodically, as in very rarely. I have determined it's not the rheostat as when the backlighting isn't working, there is no power at the fuse on the fuse board and when the lighting is working, twiddling the rheostat thumbwheel increases and decreases the brightness with no problems or "dark spots". Sometimes however, the backlighting will come on when I switch the lights on and will work for 10-15 minutes then go dark, other times it won't work at all. I don't see that there is a relay for the instrument lighting (the same fuse however also handles license plate lamps, fuse #13 if I'm not mistaken) so unless I'm missing something in referencing the WSM, I'm at a loss as there is no complete circuit diagram anywhere to be seen, only pics of the connectors. I guess what I need to know is what the first power stop in the chain is, whether it be the headlamp switch or elsewhere, I need to know where the heck to start testing. Any help is of course, appreciated.

 

License plate lights go dark too. I feel like the issue is further up the chain.

 

Thanks for the replies, I agree, probably an issue at the fuse box since these were known to have some issues in that department. I'll be doing some investigating hopefully this weekend.

Ryan; was the issue with a pin in a connector or was it at the point on the underside of the fuse board where the fuse slides in?

 

Thank you. I'll hopefully be able to dig into it shortly. Also suspect a bad ground somewhere as my fuel gauge has a tendency to occasionally dance around. I have to say, the 1990s Ferrari WSMs are among the worst I've ever seen, which of course comes as no shocker, but the old 308 and Testarossa books are far more detailed, IMHO. The '90s WSMs are more like enlarged owners handbooks!