Thank you for the prompt reply. To help our techs diagnose, I was able to borrow a Fluke digital tester and did some voltage measurements again. Here are the readings:
1. Engine off before starting 12.45 VDC
2. Idle RPM, no accesories turned on 13.40 VDC
3. Idle RPM, All lights & stereo on 12.01 VDC
4. Idle RPM, All lights, stereo & A/C 11.80 VDC
5. same as #4 except 2000 RPM 12.13 VDC
After the engine had been shut-off for 3 hours, the ABS light went off when started and the speedo works. What stumps me is why I lose the speedo function when the ABS light turns on and why they return to normal after resting a while? All fuses in the fuseboxes are OK, also OVP fuse.