1999 F250 7.3 super fuse #20 blows when trying to start
 

I have read through previous forums and have seen this problem commonly caused by the dual shock mount. This wasn't the case in my truck. I have had a similar problem before though. The truck starter and radio would magically turn on and off after the first rain, it was hard on the starter and I had to replace the starter and the cause of that problem was the vacuum line coming from the brake booster running over the top of the wires in the 4 plugs attached to the driver side fender. It rubbed through exposing the wires and got wet connecting them. I cleaned them and put the electrical tape silicone and then it has been working fine for a couple months.

Now every time I try to start fuse #20 blows. So far I have traced the wire that goes through fuse 20 through the firewall being the blue wire with a orange stripe on it it goes through the top plug upper left most wire. From there I followed it through the 4 wheel fender plugs the furthest plug from the firewall still being the blue with orange stripe. If I unplug that plug and try to start the truck it won't start but the fuse does not blow. I then followed that wire all the way to the neutral safety switch. This is where I would like some advise. Anything helps. I hate tracing wires. (About 8 hours so far). I am thinking neutral safety switch. Also another thing I am thinking is weird is that blue wire with the orange stripe is grounded when the safety switch is plugged into the transmission and if it is unplugged is is not grounded. And I don't think it is supposed to be grounded because that is the hot wire when the key is in the start position.

Details about the truck:
99 f250 diesel 7.3 super duty automatic 4x4 dated 4/99
I have taken pictures of the plugs and wires I was going to post it but don't remember how.

Hi scrowley,

Looking up the owner's guide for your truck fuse 20 goes to your starter solenoid. Obviously unplugging the fuse prevents power from getting there and it's not gonna start.

I'm not a 7.3L expert, but on most trucks the solenoid is on the starter. I would take the starter off and have it tested by your local auto parts store. If you have a short in the solenoid that would cause your fuse to blow and it not to start. Just a guess.

Thanks for the advise. It's a new starter, I can start the truck by putting the key in the run position and jump the small wire on the starter solenoid located on the starter. But that wire that goes through the neutral switch I am sure it winds up at the starter somehow. I just didn't want to try and trace it through the truck any further. It is showing that the wire is grounded either in the natural switch or beyond on its way to the starter.

Sounds like you're on the right track. I wish I could find some wiring diagrams for your truck, but they're kind of tough to find unless you pay for them. I would test for continuity at the neutral safety switch harness. Of course that's hard to do without a diagram of what's what. :-huh

http://img27.imageshack.us/img27/9742/oyjc.jpg
I put a link to the image of the DTR (digital transmission range) sensor that is the problem? Pin 10, circuit 395, starter relay enable. This is the wire
I was talking about. Only When the key is put in the "start position" the dark blue/orange wire is hot (tested with the sensor unplugged) but as soon se I plug that sensor back in it grounds that wire and pops the fuse. I am not sure where I should be testing from here or how to test the DTR sensor, nut it just makes sense that it shouldn't have a ground in a 12v wire.
I also found a wiring diagram of fuse #20 it might help me tomorrow in following that wire past the sensor.
http://img829.imageshack.us/img829/1039/5yav.jpg

Any recent transmission or exhaust work?

No recent work. Just that starter issue where it would randomly start cause of the wire harness fender problem that killed my starter. I put a new starter made in japan a few days ago. But this problem was there before I changed it.

Ok so here are the results...I followed the wire all the way to the solenoid switch bypassing the DTR by connecting pin 10&12 together and the fuse still blew so I ruled that out. I looked online how to test the solenoid switch cause the DB/O wire to the DTR was fine then changing to T/O wire to the solenoid was fine. So I then checked the resistance in the solenoid. Between the 2 large terminals. Infinity resistance, the 2 large terminals and the small terminal and there was infinity resistance. So that was fine. Then I checked the small terminal to the solenoid base (ground) and this is where I was confused I tried to find info on what it was supposed to be and couldn't find anything. But I came up with a range of 0.5 to 1 ohm meaning there is little resistance to ground. I went to an auto part store and checked the resistance of a new one and it was 3.6 ohms. So I bought it and replaced it and it's working fine. So basically I just ripped apart my entire wiring harness and my truck for a 40 dollar part. Not to mention 3 days. Hope this helps someone someday. Feel free to ask me any questions anytime.
Here is what originally caused this problem. It ran normally in dry weather but when it rains that hose gets wet and would create a short across those wires from years of rubbing. It would cause the starter to randomly start and the radio was turning on. I think that should have been recalled that area is a problem for most of these trucks. How I kept it from happening is I zip tied the hose out of the way and repaired the wires.
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I had the windows and heat ac radio kicking on like I had a ghost in my truck found out I had a bad window seal leaking on the CPU siliconed the windshield problem solved