Replaced stock radio with an amplifier and now turning on headlights turns it off

[dkraven]

Electrical amateur here, getting that out front. Hooked up old wires that went to the stock radio to an amplifier. Only change was there was a ground wire going to the old radio and no spot on the amplifier for the ground, so just let that hang free. Figured it was just a wire grounding to the chassis so it'd be fine on its own. The amplifier works as is, but as soon as I turn on the headlights the sound cuts out. This is a 1977 GMC Sierra Grande. Looked at the wiring diagrams in the Haynes book and can't see why the headlights would affect this particular circuit. What am I missing?

 

[Ricko1966]

My guess is you used the radio illumination wire as your amplifier ground. So with the radio off its pulling ground through every bulb in the lighting circuit. You turn on the headlights, now your amp has 2 hots, no ground

 

[WFO]

I would run a dedicated amp wire, with an inline fuse, from the battery to the amp.

 

[Grit dog]

Yup need to be a little more specific than “hooked up the old wires”.
And honestly “new wires” and redoing all the connections. Proper fuses, etc is worthwhile.
Also it’s best to keep new “accessories” from using any real power or amperage from the existing OE electrical system for a couple reasons.
1. It’s old
2. It’s old technology and less durable.
3. Less stress on all the original stuff which likely needs all the help and luck it can get to keep working properly.

 

[Matt69olds]

Look at the wires a little closer, I’m betting what you thought was a black ground wire is actually a brown wire for the taillight feed to the radio.

 

[Catbox]

I like pictures.

Do you have a photo of the equipment that you installed?
Even a pic of the wiring diagram it came with can help...

I am in the area as well.
Radios are pretty simple to get wired up.

 

[Ricko1966]

What I told you in post #2 real easy to diagnose. Take every light bulb out of the truck and see if the amp turns off and stays off. Or second option relocate the amplifier ground to the chassis,dash,cigarette lighter any good ground,and see if the amp stays on with the lights on.

 

[Scribbles]

Go to Harbor Freight and grab a cheap multimeter and test the wires you are using.

 

[WP29P4A]

If you skip the faith based wiring and switch to fact based wiring you will save massive amounts of time. Instead of hooking up random wires that you believe might be correct, why not use a meter and actually verify what the wires you have actually do?
Trust is a feature that will only slow you down when doing wiring, trust nothing, test everything, save time and frustration

Step one, set meter to continuity and find the ground, or run a new ground.
Step two set meter to DC voltage and find the always hot (+12 volts) wire.
Step three turn key to accessory and find the +12 volts that comes on with key switch.
Step four turn on dash lights and find the the +12 volts that comes on with the dash lights.

Then figure out the speakers, and you got it. You can use a 1.5 volt battery to test the speakers to figure out which is which and to figure out which wire is + and -. If the speaker is connected to the battery with + battery to + on the speaker, the speaker will move outward. It the speaker is connected with reversed polarity (backwards) the speaker will move inward.

You can NOT run an amp off the little power wire that goes to the stereo, and most amps have a low voltage cut-off that keeps them from killing your battery and damaging the amp from low voltage. The only shared wires from the head unit (stereo) will be the trigger/remote wire that turns on the amp and the RCA that sends signal from stereo to amp. 8 gauge is about the minimum wiring size for a small amp.