Fuse 5 keeps blowing - advise new air con pump needed?

[beckywise14]

Firstly apologies if this has been posted before, i have spent a good hour looking on the site(and others) and can't find anything.

Fuse 5 keeps blowing, pretty much as soon as the engine turns over but only with the climate control unit switched on. If you turn this unit off before starting the unit then fuse 5 stays intact. With the key turned so that the ignition is on without the engine running you can use all climate controls.

I have followed instrustions to lift up carpet in passages footwell to check for water ingress and checked visible wiring to ccm, all looks ok and bone dry. I have a look at the wiring near where foot would be in passenger footwell and again no obvious problems.

I was advised by a mechanic (friend) that the air con pump needs replacing but these are quite expensive.

Can anyone advise if they have any experience of this? Also a lot of posts seem to suggest that the most likely fault is a wiring issue, would this still be the case of everything runs ok before engine is started?

Many thanks in advance.