Unexpected voltage drop

[Matthew johnson]

Earlier today I was working on car head unit that was DOA. No power at all, using 12v power supply I put power to the harness and found that my 12v has now dropped to .5v?! Just curious if anyone had run into anything similar??

Thanks in advance for any help on this!


Edit: it drops the voltage only when the constant 12v is given voltage, voltage can be applied to the acc wire without a drop.

 

[(*steve*)]

Your load is attempting to draw more current than the power supply can provide. The voltage drops as a consequence.

A further consequence is that if left this way you may damage your power supply.

 

[Kiwi]

As Steve advised the load is more than your power supply can provide.
What is the output rating of your power supply?

The Acc wire is usually just a "wake up" wire on newer car head units, so draws very little current.
The constant 12v wire is the main power wire.