Please help: lighter socket weirdness

[ICB]

Have done plenty of searching but I think this is probably too unusual to have been covered before, hope Pippin or one of you other electrically sussed people can help steer me right because I'm a bit at a loss.

Background:

Our front and rear outlets are both running off the LB via the common option of a 4-fuse standard-blade fuse box. I know for sure that the rear socket was re-wired with 40a cable, I'm guessing the front was as well.

Both sockets are live without the ignition (as you'd expect) and a cheap £4 fan from Tesco powers up no problem. Unfortunately I don't have a multimeter to test further.

The problem:

Today I've blown several 20 and 25a fuses in the 4 fuse box trying to work this one out. Basically the only thing that seems to run off either socket is the aforementioned fan (useless for everything else but glad I've got it now). A USB adaptor with LED doesn't power up or supply any charge to my phone, an adaptor for a PDA and Bluetooth GPS doesn't get the LED on or supply any charge. Rather odd.

What's even weirder is that the LED on a 3-way socket adaptor's plug won't come on, nor will the LED by its power switch, BUT it will power the fan when the fan is plugged into one of its sockets and the switch is on.

What seems to blow the fuse is when I plug in a 300W inverter to either socket (purchased today from Maplin). It cheerfully takes the 25A fuse under the bonnet out without being powered up or having any load in the AC socket but it's own 15A fuse remains intact. The plug came prewired to the cables with 2 spade connectors and the spade connectors are in the correct terminals so unless there's something messed up with its internals I can't see the problem.

I'm guessing that there is an earthing problem somewhere (seems most likely explanation, especially as LEDs don't light but the little 6W blower spins up) but I can't think where as both batteries are very well earthed and everything else, including a 200W/20A amplifier and 15A head unit running off the LB, is working fine. The original LB conversion from the winter pack and the rewiring were all done professionally and I did the stereo work myself, very carefully.

So where should I start looking? If I can get a meter what readings should I be looking for from the sockets?

I guess I could hook the inverter directly up to the LB with croc clips and see if it blows its 35A fuse (or, more likely, powers up fine) just to rule it out of the equation?

Sorry for the epic, hope someone can help as I'm mystified.

We're off to Ireland in a week so I really need to get this nailed, preferably without a trip back to the dealer to hang around for half a day.

Cheers,

Paul

[pippin]

It is 01:00, I'm tired, just got home from playing a gig - so this is just a very quick thought.

I wonder if the negative and positive connections to the lighter sockets have been reversed.

That would account for fuse blowings and LEDs not lighting.

You need to get a multimeter to check!

[harpo42]

With pippin on this. Which way is the fan spining? Should be blowing not sucking.

[moonshine]

My immediate thoughts too. Reversed polarity at the sockets.

[ICB]

Thanks everyone.

Funny the way these things work, I woke up this morning with "reversed polarity" on the tip of my tongue. Fan is obviously going the wrong way and LEDs are polarity sensitive so it's just obvious really. I think I must have been tired last night as well Pippin.

I need to pop to halfrauds for levelling ramps later so I'll ask one of their chimps to put a meter on it to confirm.

[corblimey]

Forget the monkeys, get yourself a multimeter. Must be one of my most used tools, that and my hammer

[Colin Lambert]

Almost without doubt, reversed polarity.

[ICB]

Yep, some muppet had wired both sockets up the wrong way when the LB conversion was done

Now sorted and the inverter fired up fine with a rechargeable torch attached, likewise both LEDs and all sockets on the 3-way adaptor.