Typically, it's the accumulator that goes bad on these units. It's entirely possible that you had a bad accumulator, and that you bought a used unit that also has a bad accumulator.

Does the ABS light illuminate on your dash at any time?

Does the pump run at all times or what's going on there? What color are the wires at those plugs?

Let's see about getting the ABS codes. Here is how to do that:

1. With ignition switch turned to the On position, allow pump/motor to charge the accumulator. If accumulator is discharged, the ``BRAKE'' and ``ANTI-LOCK'' lamps will remain on for up to 30 seconds.
2. If the ``ANTI-LOCK'' lamp does not go off within 30 seconds, note that lamp did not go off, then proceed to step 3.
3. With ignition switch turned to the Off position, place a jumper wire between Assembly Line Diagnostic Link (ALDL), pin H and A or between ALDL pin H and body ground. The ALDL is located on the driver's side of the vehicle near the parking brake pedal assembly.
4. Turn ignition switch to the RUN position, then count the lamp flashes to identify the first digit. If the lamp turns on for four seconds, then turns off (and remains off), no trouble codes are present. Proceed to ABS FUNCTIONAL CHECK then proceed with diagnosis as directed. If the EBCM has stored any trouble codes, the ``ANTI-LOCK'' lamp will turn on for approximately four seconds and will then start flashing. These flashes represent the first digit of the trouble code.
5. Count lamp flashes for the second digit. The lamp will pause (remain off), for approximately three seconds after flashing the first digit. It will then flash the second digit of the trouble code. When the ``ANTI-LOCK'' lamp flashes the second digit of the trouble code, be careful to count only the number of times the lamp turns on and goes out. When the EBCM is done flashing the second digit, the ``ANTI-LOCK'' lamp will remain on. The last pulse, when the lamp remains on, should not be counted as a flash, Fig. 50.
6. Record trouble codes obtained.
7. Check for additional trouble codes. Without turning the ignition switch to the Off position, disconnect the jumper wire from ALDL pin H. Connect the jumper again to ALDL pin. If an additional code is present, the ``ANTI-LOCK'' lamp will begin flashing the first digit as described previously. Count lamp flashes for the first and second digits as was done for the first code. Record the additional trouble code if present.
8. Repeat steps 6 and 7, until no additional trouble codes are displayed. Up to seven trouble codes can be displayed. The ``ANTI-LOCK'' lamp will remain on continuously when all trouble codes have been displayed. ABS trouble codes cannot be cleared unless all codes have been displayed.
9. After recording all ABS codes, remove jumper from the ALDL, install ALDL cover, then proceed with the ABS FUNCTIONAL CHECK.

Sure, let me know if you need anything else.