NUMBNUT

Hi all, firstly i'm no electrician hence why i need you help. i have a psu out of a dell and want to use it as a power supply in my camper van, mainly to run a 12v fridge at max 3.6amps/42w now sadly the psu will only seems to want to kick out 11.6v not enough for my fridge (that works perfectly off 12v leisure battery as they push about 13v). the fridge is 12/24v now it has been suggested that i combine the -12 and +12 lines to get 24vdc but when i do this i simply get 11.6v and the cooling fan does not run. is there a way i can get either 12or24v out of this unit pref 12 volt as i believe (perhaps incorrectly) that the -12 being only 1amp that would be the max ampage i could run at 24v. i know this might sound odd as you can buy zig type units to convert mains to 12v but the are quite big and i have ltd space in the camper also i would like to use the psu as i can use the 5v to add charging usb ports in the van for phone/kindle etc. excuse my "ignorance" on these things. thanks in advance