After three weeks of nights and weekends tearing apart my wifes 99 astro I finally found the problem. As I mentioned, the van kept blowing the 40 amp main fuse that controls the windows, fan, wipers, and radio.(The ignition 'b' main fuse). After replacing the ignition switch and coincidentally having the alternator go out during the process, putting it all back together several times and still having the problem. My wife says, "it seems to happen whenever I open the window(mostly)". So having tried everything else, and I have wiring schematics that I got from my mechanic buddy, I decided to tear into the door and see if anything is out of whack in there. I didn't do this at first because I figured if there was a problem with the window wiring, it would blow the individual window fuse located under the dash, which it never did. Turns out sure enough, the wires in the door were getting mangled by the operation of the window arm that moves the window up and down. The harness inside the door runs right along side the moving parts in there and the gear arm cut right into three wires in the harness. But still why wasn't it blowing the window fuse? Turns out that the window fuse is a 30 amp breaker type fuse and was stuck closed and would not trip! That one threw me for a loop for 3 weeks! That's why it was blowing the main fuse.
Sounds like that might be colts problem too, as I notice he had a new window motor put in.
I searched through the whole wiring harness under the dash and the wiring block that penetrates the firewall, over and over before finding this, and that was extremely difficult, you can't even see it let alone get to it to work on it. At least it was worth it cause I found a couple other small wiring problems while I was at it, now everything is back to normal. Actually better than normal, when we first bought the vehicle, it had electric problems since day one that an entire shop full of mechanics could not figure out. It kept randomly blowing the electric lock fuse, all these years I figured if a shop full of mechanics couldn't fix it, I wouldn't be able to. But in the process of working on those other problems, I found a taped up factory splice that the wires were poking thru the tape and arcing to some metal part above the steering column so I fixed it as well. Now it's all good.
Sorry this post is so long, but hopefully it will save some folks with similar problems a lot of grief. I couldn't ignore this problem, cause we need that vehicle as we both work full time and have a six year old daughter. I had serious doubts that any shop mechanic would find the problem or if they did it would cost a small fortune in labor time to find it, so I made up my mind to tear into it myself. I am a union electrician, but I knew before hand how tough electrical problems in a car can be, mainly because access is so limited without tearing the whole dash out. Which I was almost ready to do.
Good luck, and if you have time, let me know if this helped at all.
Thursday, June 19th, 2008 AT 7:07 PM