Engin try’s to die when lights are turned on

[Martin1313]

I have a 1973 c20 runs great until I turn the lights on. The rpm drops down and it will die unless I gas it and hold the gas enough to keep it running. Anyone know what the problem could be?

 

[dsteelejr]

What does your ammeter read before and after you turn on the lights?

 

[SirRobyn0]

What does your ammeter read before and after you turn on the lights?

That would be good to know. Also the RPMs the engine is idling at before and after the lights are turned on. This could be as simple as the idle speed being set to low.

 

[dusterdude]

Clean your battery terminals

 

[75gmck25]

I had my ‘75 worked on once at a shop and they must have left the door open and run down the battery. It started okay, but as soon as I revved it up to about 2k and also turned on the lights, the alternator kicked in full charge rate and tried to “fix” the low charge on the battery. It immediately burned and then broke the v belt.
Just turn your idle screw in about 1/2 turn and see if yours works better.

There is also a fusible link in the power wire that runs directly down from the firewall junction next to the brake booster. If you have ever overloaded the fusible link it might have partially fried inside, and it may not pass full current. It’s hard to see the condition of the fusible link, but a bad OEM link wire will usually look like the insulation is bulging out on the wire, and it may be discolored.

 

[SirRobyn0]

I had my ‘75 worked on once at a shop and they must have left the door open and run down the battery. It started okay, but as soon as I revved it up to about 2k and also turned on the lights, the alternator kicked in full charge rate and tried to “fix” the low charge on the battery. It immediately burned and then broke the v belt.
Just turn your idle screw in about 1/2 turn and see if yours works better.

There is also a fusible link in the power wire that runs directly down from the firewall junction next to the brake booster. If you have ever overloaded the fusible link it might have partially fried inside, and it may not pass full current. It’s hard to see the condition of the fusible link, but a bad OEM link wire will usually look like the insulation is bulging out on the wire, and it may be discolored.

I don't know what you are using for alternator, but having an oversized (higher amp output) alternator can increase the risk of this happening on a very low battery. One of the reasons I try to tell people not to jump into a bigger alternator than you actually need.

 

[Raider L]

Yeah, it sounds like your battery is about to die and can't keep up with the load, why, probably due to voltage regulator is failing to.

You know when you charge the battery and the charger shows it's fully charged, that doesn't mean it's fully charged. If you have the kind that has caps, take them off and get a "hydrometer". If you don't have one go get one! It's important because a hydrometer tests the specific gravity of the battery acid. All four colored balls floating, battery full, only three balls floating 75% full, two colored balls, 50% and so on. If there is one or two or three cells that don't have all four colored balls floating, keep the charger on the battery until all six cells a full up! This can take up to fourteen hours to arrive at. Then the battery is fully charged. As for the alternator, the voltage regulator may be weak, change it out. And folks, rebuild your own alternator's! There's no reason to go buy a new one when it might only be a diode trio that's bad or the voltage regulator. They aren't hard to rebuild and the internet is full of all kinds of tutorials, maybe even right here on this forum. I've not looked so I don't know. I've been rebuilding mine since I don't know when, decades.

 

[SirRobyn0]

Yeah, it sounds like your battery is about to die and can't keep up with the load, why, probably due to voltage regulator is failing to.

You know when you charge the battery and the charger shows it's fully charged, that doesn't mean it's fully charged. If you have the kind that has caps, take them off and get a "hydrometer". If you don't have one go get one! It's important because a hydrometer tests the specific gravity of the battery acid. All four colored balls floating, battery full, only three balls floating 75% full, two colored balls, 50% and so on. If there is one or two or three cells that don't have all four colored balls floating, keep the charger on the battery until all six cells a full up! This can take up to fourteen hours to arrive at. Then the battery is fully charged. As for the alternator, the voltage regulator may be weak, change it out. And folks, rebuild your own alternator's! There's no reason to go buy a new one when it might only be a diode trio that's bad or the voltage regulator. They aren't hard to rebuild and the internet is full of all kinds of tutorials, maybe even right here on this forum. I've not looked so I don't know. I've been rebuilding mine since I don't know when, decades.

You not wrong, but I'm also going to add that charging for "up to 14 hours" must be done at a low amp rate or you'll cook the battery at higher charge rates.