1978 Corvette Battery Fluctuations
Thread Starter
4th Gear
Joined: Aug 2023
Posts: 4
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1978 Corvette Battery Fluctuations
I am having a weird problem with my 1978 L48 C3. I have attached a video below that shows the fluctuations on my battery gauge. My battery gauge begins to fluctuate roughly to the point where the car wants to stall. The way you see the gauge fluctuate in the video is the way the car feels, like its losing power and super lumpy. My gauge reads fine when no lights are on at around 13.5-14. However when lights are turned on it wants to drop under 13 very often. When I bought the car, it had a high idle of 800-900. I turned it down to about 600-700 and also adjusted mixture screws to get the correct vacuum and was able to achieve about 20-21 on the vacuum reader. I have a hunch that the idle was high to make up for these fluctuations at the appropriate rpm. I recently installed a new battery and 150 amp alternator (as I wanted to add accessories eventually) but that didn't seem to be the problem. I checked the grounds and they all seem clean and tight unless I am possibly missing a ground somewhere. Any ideas on what it might be and what else to check? Thanks!
Last edited by HokeyPokey; Aug 20, 2023 at 04:43 PM.
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
When you installed this new bigger alternator, did you install a new bigger charge wire? Or are you running that much higher output alternator through the factory wiring?
If so. The factory charge wire goes from the alternator along the left inner fender to a plastic Junction block located just under the brake booster. This plastic connector is prone to corrosion on the inside connectors which will create a lot of heat when the amps go up. Like when you have your headlights on.
inspect said Junction block connector for signs of melting. Run a new much heaver charge wire from main post of alternator to solenoid or battery positive post. Fused of course.
If you have already done this.
Then check the ground strap from engine block to frame. Right hand side, just in front of the starter.
If so. The factory charge wire goes from the alternator along the left inner fender to a plastic Junction block located just under the brake booster. This plastic connector is prone to corrosion on the inside connectors which will create a lot of heat when the amps go up. Like when you have your headlights on.
inspect said Junction block connector for signs of melting. Run a new much heaver charge wire from main post of alternator to solenoid or battery positive post. Fused of course.
If you have already done this.
Then check the ground strap from engine block to frame. Right hand side, just in front of the starter.
Thread Starter
4th Gear
Joined: Aug 2023
Posts: 4
Likes: 1
I suspected that might have played a role. I did not upgrade the wiring I left everything as it was. I do have the previous alternator. I think I'll install that one again and check the junction block for any corrosion. I won't be adding any upgrades for a while so its probably best to go back and upgrade when I am ready. I'll keep you updated to see if the problem persists. Thanks!
Race Director
Joined: Nov 2006
Posts: 13,195
Likes: 7,795
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Make certain you check that engine block ground strap. Mine turned green on the inside years ago. Put on a new one and my charging system started working 80 percent better.
Thread Starter
4th Gear
Joined: Aug 2023
Posts: 4
Likes: 1
I was able to swap the alternator earlier today and it worked perfectly. Nice and steady at 13 with lights on now, little to no fluctuation at all. I will double check everything else this week just to rule out any other potential areas of improvement. Thanks so much.
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