#4 Fuse Blowing

[MustangSally44]

I've got a 1973 351. As soon as the key is turned to the ON position, #4 fuse blows.  This fuse is 14A for the "Idiot" lights.  I've disconnected the (semi-circle) "directional signal connector' and the (square black) 'Ignition switch connector' from the steering column.  I have no continuity from those connectors to ground - with the exception of the ground pin (black wire).  Therefore the wiring in steering column is okay. (?)  I have removed the instrument cluster and disconnected the headlight switch.  When i test the fuse holder, with the fuse removed, i get continuity from one sde to thei other.  Is this right?  Is there an obvious cause to the fuse blowing?   I have inspected the wires as much as I could and see nothing obvious. (bare, loose, corroded wires).  What am i missing??  Everything else seems to work just fine!  I'm not sure if it's relevant, but the motor has after market carborator and there are a few wires from the harness...that are not connected to anything.  My diagram says that #4 is also for "Emission control solenoid", 'Throttle solenoid positioner', and 'Electronic spark control' ......none of which I'm even sure are still there (Holly Carb).  Any help would be appreciated

 

[midlife]

It would help to know what year your Mustang is. In addition to what the manual states, that fused RUN-only power is also used in a seat belt relay system that runs down to the transmission for switching. I'll bet there's a pinched wire down there...

 

[1sostatic]

Hi 

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if that is the 3/4"  14Amp ... on mine that also serves the horn circuit...

​

 

[Vinnie]

I had a blowing nr 4 a while ago. It was due to a bad ignition coil. With the coil disconnected the short went away. With a new coil everything worked again. I played the elimination game to figure this out. Might be worth a try?

Good luck!

Vincent.

 

[MustangSally44]

Thanks for your input. You guys ROCK. Solved the problem (sort of) Rigged up a blade type fuse holder from the fuse box and mounted it to the steering wheel for easy viewing.  Blew fuse after fuse so I unplugged all connectors coming out of the firewall.  Still blowing the fuse but now we know its inside the car, not under the hood.  Started unplugging connectors under dash until fuse stopped blowing.  I think its the connector to the fuel tank sending unit, but not sure.  It is located above the glove box and you can follow the wires down behind the passenger kick panel but then it disappears into the body.  I cant seem to find these wire colours on the wiring diagram.  Im not even sure what you would call these colours.  Anybody recognize this connector?   Once again- you guys rock

 

[71fast]

I think that plug is for the ambient temperature sensor switch.  

1973_full.pdf on page 12

640 red/yel hash

369 brn/ora hash

connector c-1202

 

[MustangSally44]

I think that plug is for the ambient temperature sensor switch.  

1973_full.pdf on page 12

640 red/yel hash

369 brn/ora hash

connector c

Thanks 71fast  I think you nailed it. My PDF stopped at page 11 so I never would have found that information. I couldnt figure out why I would have an ambient temperature sensor switch in the first place, with no AC or computer but, after some reserch , turns out the sensor tells the transmission controled emmission stuff to turn on but only above 58 degrees F. I think all that stuff was removed years ago so if I leave the connector unplugged and put the vacuum advance hose into the intake manifold I should be golden. Thanks again to everybody who chimed in.