i had a fuse to the alternator blow today, local backyard mechanic said some times fuses just get old and blow. alternator is working fine. put in new fuse and drove 8 miles home no problems. is this a sign of alternator going bad? 97 f150 4x4 ext cab 4.9L 118,000 original alternator any help? thx note: lost all guages and drove about 5 miles before engine died.

 

i've had fuses blow, and years go by without a repeat problem. i think heat, vibration, slight corrosion or whatever will sometimes kill a fuse.

 

Alternators can become less efficient because the rectifiers (diodes) within them loose their ability to pass electrons with age. They become resistive. Fuses blow if the product of the current through them and the time that current goes through them exceeds a given amount. Alternators have to draw more electrons as they get older to overcome this lack of efficiency. This leads to a shortening of the life of the fuse. The fuse is doing its job (letting you know you have a well seasoned alt) but I wouldn't rush out to replace the alt untill the fuse blowing becomes overbearing. I agree with the privious post, just keep some extra fuses on hand.
As a side note, If you are blowing a fuse say every month or so, do not install a new battery without first installing a new alternator. Inefficient alternators can have a hard time reaching a high enough voltage to keep the lead plates free of sulfate crystalls. If these crystalls build up on a battery there will be a lesser surface area exposed on the plates effectively lowering the voltage the battery can charge up to. A bad alternator can shorten the life of a perfectly good new battery. Installing a new alternator and leaving the old battery is ok, but very old batteries can overdrive an alternator also, which will effectively shorten the alternators life.
Don't assume the alternator is the reason the fuse blew however, an old battery can overdrive the alt which may be the reason. With todays automotive electronics, most can start and run with pretty crummy batteries so the problem can be somewhat elusive.
My opinion, most likely you have a bad/old battery with the ripple effect being you alternator fuse blowing. Could also be some other post ignition relay electrical problem. I don't have all the facts so I can't realy call it for sure. Are your eyes glazed over yet, I've got tons more info on electrical systems if you want it. Good luck, sorry for the long answer.