79 monte courtesy fuse keeps blowing

This is driving me nuts.

I have removed the radio and cig lighter. Everytime I put in that fuse it blows right away.

This circuit is hot all the time... any tips on what to do?

look into the lightter recepticle with a flashlight to make sure there isnt something in there you are missing ... or unplug it all together.

trunk light wire shorted?

hood light?

visor vanity mirror?

glove box?

clock power wire?

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yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.

cig lighter is unhooked.

no hood light, no visor mirror, glove box wire seems ok?

choke wire is unhooked

Its there a way to hook up a multimeter to test the wire to see where the short is?

Did you recover the headliner?

tony_ss wrote:

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Its there a way to hook up a multimeter to test the wire to see where the short is?

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Replace the fuse with a bulb. It should light.

Play around with wires (move them) until the bulb extinguishes - that's your short.


I presume the disconnected "tails" to the radio and cig lighter have been taped of and insulated properly?
(And no insulation pinched during reassembly etc?)

i am an idiot wrote:

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Did you recover the headliner?

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no, didn't do that

oldspark wrote:

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tony_ss wrote:

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Its there a way to hook up a multimeter to test the wire to see where the short is?

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Replace the fuse with a bulb. It should light.

Play around with wires (move them) until the bulb extinguishes - that's your short.


I presume the disconnected "tails" to the radio and cig lighter have been taped of and insulated properly?
(And no insulation pinched during reassembly etc?)

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Yes they've all been taped, no pinches..

If I put a light in that fuse, won't that just start to burn or melt where the short is?? Or melt down that whole circuit?

No - provided the bulb is rated less than the circuit eg, 10A fuse => 13.8V x 10A = 138W bulb.

But a 3W or 23W bulb will pass a maximum of 1/4A or 2A - neither of which is likely to melt a wire.


Thinks of the lamp as a current limiter.
The brightness depends on the circuit impedance.
If a dead short or maybe a starter motor solenoid, the lamp sits between +12V <whatever> and 0V (GND) and hence lights fully.
It will be dimmer for less than a short. EG if another 23W bulb, each will be the same ~half brightness. If a radio, it probably won't light.

I take an already blown fuse, and solder a few feet of wire to each of the ends of the fuse.  Solder the other ends of the wire to a lamp or a lamp socket.  Hang the lamp from the rear view mirror so you can see it from many points of the vehicle.  It will probably illuminate brightly, move wires and see if the intensity of the lamp changes.  If a particular movement makes it flicker, look further into that area. 

The reason I asked about removal of the headliner is because it is common to run a visor screw through the constant wire that powers the domelight.

Oh here we go again, why I WON'T get rid of my Snap-On test light.
Place the croc clip on your battery +, remove the fuse and look for wire joins coming away from the fuse, cut away all the joined wires then probe. The one that lights up is your short. Rewire it's quicker than the alternative which would be to pull that loom apart. Also look for grommets dropped out and where the loom bends round an obstruction, such as entering the trunk area. Also any retaining screws having gone through the loom

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I was gonna say an old blown fuse with wire extensions. ( <-- FULL STOP.)

Then plug in test lamps, or ammeters...
Those fuse-ammeter adaptors cost a fortune - though they do have a series fuse to replace the original fuse, or protect the ammeter (ie, usually 10A), but that is easy to add.
A shunt can be used for higher Amperages.

oldspark wrote:

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No - provided the bulb is rated less than the circuit eg, 10A fuse => 13.8V x 10A = 138W bulb.

But a 3W or 23W bulb will pass a maximum of 1/4A or 2A - neither of which is likely to melt a wire.


Thinks of the lamp as a current limiter.
The brightness depends on the circuit impedance.
If a dead short or maybe a starter motor solenoid, the lamp sits between +12V <whatever> and 0V (GND) and hence lights fully.
It will be dimmer for less than a short. EG if another 23W bulb, each will be the same ~half brightness. If a radio, it probably won't light.

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thanks I will try this.

Alot of the info is new to me, so I'm still researching and trying to find time to diagnose it further..

thanks again for the help everyone.

What about using a short circuit detector / finder? Like this?

https://www.amazon.com/Tool-Aid-SGT25100-Short-Finder/dp/B002XMUGZC/ref=pd_sim_sbs_auto_4

Or one up - the SGT25300 (for intermittents).

If the distance ranging works ok, it might be an advantage.

Otherwise it's no different to a test light. (I've used a stop/park lamp assembly.)

Expensive waste of money if this is a one off job.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.