Macbook Pro A1989
Will it power on with power cable, even if the battery daughter board cable is disconnected
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A1989 appears to be a MacBook Pro 13" 2018 or 2019 model.
What daughter board and what cable?
These USB-C Apple laptops may or may not boot when internal components are disconnected and the behavior can be different between two identical laptops. Unfortunately many times if any internal item is disconnected, it can lead to the laptop running the CPU at idle speeds which is 800MHz which is extremely slow. However, I have had some of these Apple laptops take hours to boot when an internal component is disconnected....sometimes I have left the laptop for half a day to boot and with some I just gave up...since you cannot really tell if the boot process has gotten stuck or whether it really is that slow. It is really hit or miss whether these laptops will boot when an internal items has been disconnected. These laptops are missing the sensors on that component so it puts the laptop into a lower power protective state usually with the fans running at high speed.
What issue are you trying to troubleshoot by disconnecting internal components?
Have you tried running the Apple Diagnostics?
Or tried an SMC Reset (make sure to read the instructions carefully since the process is really different from older models).
Or a PRAM Reset (hold for at least two chimes if possible....sometimes it barely will allow for one chime).
It is not easy to troubleshoot this model even with known good spare parts.