A Stern-Gerlach atomic beam ion source (ABS) with ECR ionizer and cesium gas-target as charge exchange units is under construction. The goal is to achieve intense negative beams of polarized H− and D− of high brightness for injection into the 14 MV MP tandem accelerator with beam intensity of about 2 μA on target. At the entrance of the ECR ionizer the observed intensity of the neutral beam of polarized atoms is 6.4±0.4×1016 H/sec. Comparing beam transport calculations with beam intensity measurements under various conditions allows for a consistent description of the beam formation in the ABS with respect to degree of dissociation (≃80%), peaking (≃1.6), beam absorption (≃40%) and pumping speeds.