Fault find blown cigarette lighter fuse

Question

I have a 2005 ford focus mk2, nothing custom, nothing modified, all stock.

The cigertte lighter has never worked so I thought it was about time I attempted to fix this.

I removed the fuse (20amp) and found it to be blown.

So I thought that i'd replace with a new 20amp fuse after checking the manual and online to ensure the fuse rating is 20amp for my model/make, it blew instantly.

I then thought that the new fuse may have been defect, went to put another new 20 amp fuse but it just starting burning the pin as I was putting it in. I realised something was very wrong and stopped.

My question is, is there a probable cause and where would I start fault finding?

Answer 1

You have a short somewhere.

The purpose of a fuse is to be the weak link in a circuit. If more current is running through a circuit than it was built to handle, a fuse will self destruct and break the circuit, preventing overheating and possible fire. The most likely causes of a circuit having too much current are too many devices drawing too much power on a wire, or a wire that is connected directly from power to ground with no components providing resistance.

Since you say there are no addons in the vehicle, the circuit should be in stock condition. I would assume somewhere on that circuit the 12v power wire is being grounded. It's safe to assume that it's happening after the fuse panel, somewhere before the circuit returns to ground. You're going to need to check the wiring along the circuit and try and find it. Most likely a wire has been damaged. A wiring diagram for your vehicle will help immensely, as you'll be able to see what other components are powered off that fuse, and where and how they branch off. Otherwise, check for anything else that doesn't work. Dome lights, glovebox lights, and trunk lights are commonly on that circuit, and they can all ground in different locations.

The 12v cigarette lighter is a common source for power when installing addons. I know you said your car doesn't have any now, but it's possible it did at some point, and then they were removed. I'd check the back of the cigarette lighter first and ensure it looks ok. You may even want to check the socket itself with the continuity setting on a multimeter to ensure it hasn't shorted somehow.

Answer 2

How to test that there is/is not (still) a short circuit without burning out lots of fuses: get hold of a resistor and bridge the fuse with a multi-meter set to read current in series with the resistor. The resistor should be any value that passes a sensibly measurable current at 12.6V. 120 ohms would be a sensible minimum (expect a bit over 100mA current), 1.2 kilo-ohms a sensible maximum (10mA). If there's no short there will be no current.

Don't forget the resistor! Burning out multi-meters is even more expensive than burning out fuses.

An old-school alternative is a 12V automotive tail-light bulb with wires soldered to it. If it lights, there is still a short-circuit.

Answer 3

In most (but not all) automotive applications the power socket is on its own circuit. Multiple sockets are generally on separate fuses, but this varies with make and model.

First thing I would do is a visual inspection of ALL the power sockets in your vehicle, starting with the one in the ash tray. Unplug the actual lighter itself if your car has one. If you don't see any foreign objects (coins, etc..) in the socket, remove the socket and inspect the connector at the back, and the socket itself. I have personally seen the sockets themselves short out; the outer part of the socket is ground, and the button in the center is the positive terminal. These are often held together at the rear with a nut, where the positive wire attaches to the socket itself, separated with a thin insulator, I have seen these shift around or overheat, distorting the socket and shorting it. There is also usually (At least on the Japanese vehicles I've worked on; Ford may be different) a fuse at the back of the socket that is part of the socket itself, if this blows the only recourse is usually to replace the socket itself.

You can buy aftermarket add-on 12V sockets that usually come with wiring to connect it, this may be a good option if nothing obvious is found in the above inspection. If you've got a test light or other method of limiting the current as outlined in the above comments, I'd unplug the socket and see if the short is still present, from there I'd get ahold of a wiring diagram and see if there's anything else in that circuit that can be disconnected to test - will tell you where in the circuit the short is. If the socket itself isn't shorted, and it's not otherwise obvious, you may want to just add an accessory fuse and run new wiring to the socket vs tearing into the harness to find the problem. My suspicion is that someone overloaded the socket enough to melt it and short it out, but not enough to pop the fuse. (They're typically rated for 10A/120W or so each, the fact your fuse is 20A leads me to believe that there MIGHT be more than one socket on that circuit)