Get a hold of a U89 schematic (they are online- google it), remove the mic's head assembly, put the pattern switch to figure eight, and insert one end of an ohm meter to the pin on the mic's head assembly that carries polarization voltage to the rear backplate*. Put the other end of the probe to the wire terminal screwed into the rear backplate (both diaphragms on the U89 are at ground potential and carry no voltage) .
If you show zero ohms, your head assembly is fine, and you need to look for a fault either on the capsule itself or on the mic's amp board.
Amp board trouble shooting: you should be able to read a few volts DC on the amp board contact corresponding to the head pin for the rear polarization voltage. It will not be 60VDC, due to the high impedance you are measuring, and the load through your meter. But it should be roughly the same voltage for both backplate connections (front and rear) when measured against ground.
Capsule tests require a capacitance tester: with the wires removed from both backplates, measure the capacitance between the rear backplate (towards the center of capsule) and the rear diaphragm ring (aluminum), then measure between front backplate and front diaphragm ring: both measurements should read around 80pf.
* I just looked at the schematic: unlike all other Neumann mics, the head assembly pinouts are not identified, so you need to probe and follow wires from pins to their terminations at the capsule. By deduction, you will find the correct conductor between head pin and rear diaphragm connection.
