Morris Minor Owners Club

I have fitted a cigarette lighter wired to a permanently live fuse running through another in-line fuse. The lighter works perfectly well as does a test lamp put across the cigarette lighter terminals. Plug my Garmin Nuvi satnav into the cigarette lighter and the in-line fuse blows. I've upped the fuse as high as I dare and it still blows. Any ideas?

Blowing fuses often means a short circuit somewhere. A sat nav is not going to require so very much current - it likely runs on a supply of 5 Volts from a one amp supply. I suspect there may be a polarity problem, or similar, with your installation.

Tell us more about your electrics.

Does the Sat Nav work fine in another car?

It's not unknown for the cable to get damaged at the rear of the power adaptor .
They usually go open circuit, but they can short out.
As oliver90owner said, a serviceable one will take less than an amp.

So if it is OK, then the issue is with the car, so as he said, "tell us more", starting with if the car is Neg Earth.

Is your car positive or negative earth?

The car was converted to neg earth years ago.

The cig lighter works OK and doesn't blow the fuse

Just plugging in the satnav lead does not blow the fuse

The satnav works fine in other vehicles

I've taken out the lighter socket and everything looks fine and it's not shorting to earth.

An internet search on other forums (fora?) produces numerous other drivers with other makes and ages of cars with the same problem and no conclusive answer.

A test lamp only needs voltage and most aren’t polarity conscious same with most ciggy lighters so they should work no matter which way you have wired the socket,

It might be worth checking that you have the 12v positive coming through the centre pin and the earth is coming from the sides of the socket, wouldn’t be the first one I’ve seen wired backwards. It’s an easy error to make.

if not remove the socket and wire it directly to a battery (with an inline fuse in the positive) if that works it looks to be a car issue not a socket issue.

Steve

As it's defying all logic, my thoughts are that it's been wired up incorrectly.
I assume it was a new one and not a period piece salvaged from somewhere?

Any chance of some photos of the offending item showing the connections? And the general condition of it.

Hang on, only just seen that this is a Garmin Nuvi, have had lots of problems with my Garmin Nuvi as well. It would work no problem in the 12v socket of a boat (!), (not much use out at sea eh) and not in the socket of one classic car, (blew fuses too) but works fine in the Morris. There are known issues with this particular satnav and I think, as I remember ,that it is something to do whether the unit thinks it is in normal mode/data transfer mode or not. Was some time ago.
There is some info out there on the web that I consulted, will try and find it for you or you can have a look yourself.

Does it only blow the fuse when the engine is running or does it make no difference?

The voltage can be significantly higher than 12 volts when the battery is being charged. Plus if you're still running a dynamo, then the regulation tends to be not as smooth. Perhaps there are some voltage spikes the satnav doesn't like
Just a thought.

The OP is either coy about the polarity of his power supply (formerly referred to as a cigar lighter, but later as a fag lighter), or just doesn’t know how to check the polarity?

It is almost certainly a polarity problem, in my book. Concentrate on that. I would guess that the 5 volt supply to the sat nav runs a circuit that draws excess current due to the earth path being shorted to the live side of the car electrics. Why it only blows the fuse when the sat nav is connected leads me to believe the voltage reducer simply does not actually provide a 5 volt supply at all - it is likely protected against a reversed polarity connection. Likewise the sat nav is protected against a 12 volt supply - particularly if of reversed polarity.

Problem not answered but solved. Direct connection to the battery worked OK. I then moved the spade terminal over to the other fuse (which went through the ignition switch), plugged in the satnav and no blown fuse! Whether it will last long-term remains to be seen when I next do a long journey but in the mean time thank you to all who helped.

Problem not answered

I expect it was, even if you deny or don’t know.
I expect you have simply changed the polarity of the power socket.