Electrics: Fuse keeps blowing

[Gearyjo]

I have a 2016 XC90 and tried to replace fuse #28 with a 5amp fuse, which was blown. (just trying to fix vanity mirror lights not working) When I replace the blown fuse with new 5amp, it blew immediately.
How to I figure out why it is blowing?
Thanks

[canis]

Sounds like a short circuit somewhere. Which means some insulation has rubbed off something. This usually occurs at places where wires physically move, in a hinge for example, or a pinch point due to normal vibration. Either way, you need to find out.

Leave the fuse out, use a multimeter to resistence check between the positive terminal in the lamp and the car chassis, or some other reliable negative. The meter will show a resistance value, the higher the better, preferably infinity. If it's zero, or nearly zero - that's a short circuit.

At some point you'll need to decide whether it's worth the hassle of inspecting the wire visually for broken insulation and fixing it, bearing in mind dashboards can get pretty complicated and the wire is probably inside protective tape which will need to be removed to perform a thorough inspection. Or simply run a new wire leaving the old one in place and dead. Might even be worth just living without a vanity light. I'd start with where the wires enter the visor, that strikes me as a place where abrasion might occur.

NB: I'm entirely new to these models, bought one only days ago, so I'm not entirely familiar with the wiring loom yet, and haven't looked at the XC90 interior lights at all. Therefore, I cannot speak with any authority on these particular cars, this is just general advice based on years of car maintainance. But that's what I'd do first.

[Moose Test]

Some generally good advice but it isn't quite that simple. The courtesy lighting is electronically switched, so any potential short could be in any of that circuitry and not the loom itself.

Then there's the fact that there is more than one lighting module connected to that supply, not just the driver's overhead. As this is a 2016 model you've also got rear courtesy, rear reading and foot well to consider (and also potentially door level and mood lighting, depending on the options fitted and how they are wired).

I'm not that familiar with the gen. 2, so I don't know whether or not it makes use of LED bulbs and/or whether or not they are user replaceable. If both are so, it would be worth checking all of them for signs of burning (sometimes this can be seen as tiny black specks, on the yellow/white chip background). LED chips have potential short-circuit failure modes which the old filament bulbs don't. Failing that;

TL;DR - you need to physically disconnect ALL lighting modules from the loom to be able to conclude a short in the loom itself, and you need to measure for that between the load side of the #28 fuseway and chassis ground. If the loom is ok, i.e. open circuit, then connect the modules again one at a time until you find the one that is shorting.

[TruckbusUK]

Also maybe post in the XC90 mk2 section of the forums .... https://www.volvoforums.org.uk/forumdisplay.php?f=201

You may get better more relevant answers to your mk2 XC90 rather in that forum rather than here (not that the advice here is bad,) just not as relevant to your model.