Darkveldrin

1996 Honda civic ex
b16 engine swap( was in the car when i bought it)
didn't have this problem until about 2 years after ive had the car

I've had this problem for some time now and its just getting worse.
At first it started where as im driving and had my stereo on, if there was a part that made the sub kick it would kill the radio and you could see a massive voltage drop. So i checked the battery and the alt, they both checked out fine but since the battery had died a few times to the point of having to charge it i replaced it since it was under warrenty.
After awhile the speedometer was acting funny when by occasionally jumping from the speed i was at to 120 and back again.
When i moved i parked it for an extended period until i had the money to mess with it, i did a tune up on it, yet it was doing the same thing. Again tested the battery and alternator at 5 different parts stores and they all tested fine. Finally i just broke down and replaced the alternator just so i could rule it out, now that fixed my speedometer but now its like the battery isn't charging. I can charge the battery and drive it a few days and it will be fine, show 14v when driving and 13-14 when under heavy load ( heater, brights) i don't have my stereo hooked up at atm so i know its not that.
After a few days the voltage drops to 10-12 and when I have my foot on the gas i can see the voltage drop to 9 or less, my head lights dim to almost non existant and my dash lights all shut off.
But if i either push in the clutch or put it in neutral the volts go back up.
I've checked the #15 fuse tonight, and its not blown, doesn't look like the belts are slipping, the belt seems to have good tension on it, i'm just running out of idea to check.
Ive asked a few mechanic but they give me the same answer of, it sounds like the alternator or you need a new battery.
I have no idea what is wrong with it, and its getting very frustrating.

tony_2018

You might have to pull your engine harness back out and check those dead plugs that sit under your intake manifold.

Former User

Any CEL codes thrown?

When you turn the key from OFF to ON(II) [engine off], does the charging system warning light in the cluster turn on and stay on? Does this light ever turn on while driving?

Have you cleaned and tightened the G1 battery ground on the chassis, the battery posts and connectors, and the large white alternator cable attached to the fuse box?

Do you have a multimeter?