Fuse only blows when Car running?

[Jordan]

Hopefully someone can help me with this.  My fuse #7 blows as soon as I turn on my head lights, resulting in my losing the dash lights and rear tail light.  I thought I had a short but I have since found that the fuse will only blow if the car is running.  The car was off, no key in the ignition, and then turned on the head lights only to find the dash lights and tail lights stay on.  I flicked them off and on and no blown fuse.  If I start the car and turn on the headlights the fuse blows immediately.  If it is a short, why is it only happening when the car is running.   Can anyone help?
Thanks

[Jordan]

Somebody must have run across this before, No?  Not sure where to start.  Thanks

[w113dude]

 I haven't come across anything like this (yet) this may be a long shot, but it could be a surge in your electrical system when running.

[Jack Jones]

I dealt with a similar issue years ago where the mechanical voltage regulator had the points stick causing the alternator to overcharge. took out fuses and bulbs whenever the car was started but no issue when turned off.

[Jordan]

Thanks for the comments.  None of my other fuses are affected when the car is running.  If the alternator is overcharging would it not affect other systems?  I don't know.  Is it possible to get a surge only when I turn on the lights?  How can I test the alternator?  Lots of questions and thanks for the help.  I need to sort this out or no night driving.

[w113dude]

Did you look at the connecting plug or plugs under the dash, if it's misaligned in the wrong position will cause a short.  worth rechecking.

[Jack Jones]

The easiest way to check your alternator for overcharging is to simply measure the battery voltage while the vehicle his running. Normally the voltage will <15 volts. Not likely the issue but it is an easy check.

[ja17]

Hello Jordan,

Isolate the problem by un-plugging the items on that circuit one at a time. Eventually you will find one which causes the problem. For instance start b  un-plugging the dash cluster plug under the dash first and start the car and turn on the lights. Keep us up to date!

[Jack Jones]

If you want to prevent having to purchase a bag of fuses and you are handy, replace the fuse with a low wattage light. When the fuse tries to blow the light will just come on and continue unplugging devices until the light goes out.

[Jordan]

Many thanks everyone.  Jack that is a very neat trick and will try it out.  I'll let you know how I make out.

[w113dude]

If you want to prevent having to purchase a bag of fuses and you are handy, replace the fuse with a low wattage light. When the fuse tries to blow the light will just come on and continue unplugging devices until the light goes out.

Very cool trick Jack.