oil pressure guage

hi lads,, i have a vdo oil pressure guage out of an audi 80, the back of it has terminals saying G which means earth and + which is 12v live or ignition and T which i think means sender, i have made a loom and put the 12v on to battery and earthed it, when i put the T wire on the sender nothing happened,, then i swapped the live and T and it went all the way to 5 bar, think im doing something wrong? i have read something about a t piece n tappin in to some pipework,, but the guage doesnt have the facility to plumb a feed into it,, HELP please as its the only thing tht doesnt work n its been in for 9months, cheers

G=guage, the T is actually the ISO earth symbol (ie -ve)?

You need a t-piece if you need to keep the original pressure switch at the engine end.