Hello ard_newbie.

I connected the ground from arduino to ground of HMC.

Here are the readings I get :

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00

X=-1, Y=-1, Z=-1

135.00