Hi, I have a 72 350 4bbl cutlass supreme convertible that was restored. It came with one of those classic auto sound radios that look like the original, but are modern-ish. I installed a 300w sub and a 300w amp for the front speakers. It actually worked fine for a bit but then the sub started cutting out while driving with the headlights on. I figured it was too much for the alternator so I upgraded the alt to 140a, but it still happens. When I step on the brakes, the radio goes off for a second and comes back on. The headlights, tail lights and gauge lights flicker when the radio is on, to the beat of the music. When I turn the headlights on, the radio goes off for a second and comes back on. Another really interesting thing is that when I push down on the back bumper, the tail lights and headlights get brighter. This does not happen when I push down on the front of the car. Any ideas? This is pretty crazy. This is my first time posting here. Thank you all.

 

Sounds like a loose ground somewhere. 600W of amps seems like a pretty heavy load to an otherwise stock electrical system, I assume you powered both directly off the battery (using an appropriate inline fuse close to the battery) and have a solid ground return to chassis somewhere close to the amps. The fact your headlights are dimming to the beat of the music tells me that the amps you added and the rest of the electrical system is sharing a current path somewhere where they should not be.

 

You said that the radio was in the car when you bought it. Did you do the install of the subwoofer and amp, or did you take it to a shop? Post some pics of the install when you can.

 

Hi. Yes, I ran eight gauge wire with two separate runs direct to the battery for the subwoofer amp as well as the amp for the front speakers. It has a 60 amp in-line fuse in each. It is grounded to a piece of metal inside the trunk about 8 inches away from the amp.

yes, I installed it myself. The only path that I am thinking it could share is the remote blue wire from the radio that runs to both amps to tell them it is getting signal and they should turn on. I can post pictures later when I get home from work. Thank you all.

 

Not sure please clarify. Does two separate runs include a separate new/additional ground?

 

Sorry. 2 separate power runs to the battery, then 2 separate ground wires with eyelets running to a single screw to a piece of metal inside the trunk.

 

Consider mirroring the positive side with an equivalent negative all the way to the battery. Relying on a 50 year old chassis for a high current ground is not good and it may damage other components that it tries to use as a path to ground.

 

I have solved this... It was 2 issues. The ground straps were weak so I replaced them. Motor to chassis, chassis to body. Next is the fun one. Whomever installed the radio, wired the power to the green/white BLINKER WIRE. Good lord.

thank you all for chiming in.