I have a Whirlpool microwave that shuts off after 3 secs of operation. I spent several days looking on the net and found several posts specifying replacement of the magnetron. Well, I just replaced the magnetron and it is still shutting off after 3 secs. Anyone have any ideas? I am now $60 into a new magnetron and it didnt fix it so now I really kind of feel obligated to fix it......

Thanks in advance!

The fuse is good because it will run so the door switchs are ok because it will run so It must be a part that clears the way for the high power to kick in. I could not see a triac on the parts list but I did see a thermister.
http://www.abt.com/images/produ..._Files/gmh5184_parts.pdf
What if the thermister is shorted and the brain of this space shuttle cannot get a heat reading inside the oven? That part is 20 bucks and a lot of them are crappy. Try to find out what the resistance is supposed to be of the thermister and see what that one is reading. That is what I would do.