I have the same problem (erratic or no control on the touchpad) and found it disappeared as soon as I unplugged the replacement (and probably not from the laptop manufacturer) power supply from my HP laptop. I then found this post on another forum that seems pretty accurate:
My guess is that there might actually be something wrong with the
touchpad circuitry, or you have a power supply problem as suggested in
this clip from Wikipedia:
If the computer is powered by an external power supply (PSU), the
detailed construction of the PSU will influence the virtual ground
effect; a touchpad may work properly with one PSU but be jerky or
malfunction with another (this does not imply any electrical risk
whatsoever, a delicate capacitative ground, not a contact ground, is
at issue). This has been known to cause touchpad problems when a
manufacturer's PSU, which will have been designed to work with the
touchpad, is replaced by a different type. This effect can be checked
by touching a metallic part of the computer with the other hand and
seeing if operation is restored. In some cases touching the
(insulated) power supply with some part of the body, or using the
computer on the lap instead of on a desk, while working can restore
I can confirm that my issue disappears when I do the following:
- unplug the power supply from the laptop.
- touch a metallic part of the laptop (i.e. USB port)
- place the laptop on my lap (probably earthing it as above).
- touch the power supply with my hand.
All in all it seems to be down to the electronic circuitry of the replacement (non-manufacturers) power supply is not the same as the manufacturer's original unit.
So I either have to use it on my lap or plug a USM mouse in if using on a desk.