So, you just changed your ignition key switch out for a new one (or a new-to-you one) and now your main fuse blows whenever you turn your key to Park (P). What gives?
Unfortunately, this is most likely to be a symtom of a keyswitch which doesn’t suit your model. One example of the cause of this is the above picture – on the left is a keyswitch from a 1980 CX500, on the right a new replacement from eBay which is supposedly for the same year of CX500. Note the difference….:
The pinout is 6 vs. 5 pins… and while the matching 6-pin connector on the wiring harness fits, you’ll blow the fuse when you turn it to park. The ignition switch on the right is for later/different models of Honda.
Can’t return it? Well, if you have an intact plastic section containing the electrical parts on your old keyswitch and the new one, you can combine the two to create one which works… see this post!