Help trouble shooting power seat issue

[Phil Gilsdorf]

My 1987 930 has one non-functioning power seat axis. I disassembled the switch as described elsewhere on the forum, no issues there. So I unbolted the seat to check the motor itself. I pulled the wires going to the motor in question and when the switch is activated the voltmeter reads 12v. Great... so I reconnect the wires to the motor and activate the switch and now it only reads .04v and no movement. The next logical step was to see if the motor in question would function with the wires from another motor, so I grab those (they also read 12v when the respective switch was activated) and hook them up to the motor in question. Voila! it works. So my question is...

How could two power sources both read 12v yet only one works when connected to the motor???

Thanks,
Phil