HU runs without ground connected

[412s]

Here's something weird... Installed new head unit and realized it would power on without ground wire connected, traced it down to the rca's shorting out somewhere? running 3 amps with one on a cap. getting the typical ground loop noise too yet it does it with the car off as well, and all connections are clean metal to metal.
Had the same problem off and on with previous HU... system is pro installed and had no problems, just started out of the blue.

Any suggestions would be great.

[turboMiata]

Maybe the HU is grounding itself through its chassis onto the car. Take an ohmmeter and connect it to both the ground wire and deck chassis (metal) to find out.

[412s]

I completely isolated the HU and it would still run without the ground, but ONLY when there was an RCA connected... never seen this before.

[Gonthro]

then your rca must be grounding it

through the amp or something

[412s]

yeah I pretty much figured that the problem is with the rca grounding through the amp, but it seems to be happening with all three amps.

I think I'll give up and just connect everything as it should be and hope it doesn't kill anything...

[stockEK]

Alot of devices will have the same ground plane for power, input and output grounds. So, power negative, RCA negative and speaker output negative will all be attached to the same point within the unit.

Your input device will be the same way, and it's power ground is connected to the RCA shields, and is providing your HU with a return path (ground)

It is not going to hurt anything, except if you are pulling alot of power which may melt your RCA wires, which are not desiged to handle alot of current.

[Zero102]

Yep, usually the outside ring shaped part of the RCA plugs is grounded in car audio. Won't hurt anything, as long as the current draw is kept really low, so connect the damn ground wire, but it might cause a bit of interference in the short term.