'91 Honda Accord, all stock (to my knowledge)

So here's the issue:
When i'm driving, my dash light dims as though my car's alternator is not functioning and is running only off the battery. When i let off the accelerator, the dash backlight brightens back up as normal. The problem seemed related to the voltage regulator, so we replaced the alternator. I've had 2 batteries die due to this issue. After replacing the alternator the problem continued to persist, although previously it would do it seemingly randomly with regards to the stepping on the gas or letting off. However, if i hit 41 mph, the problem also goes away, which is extra wierd.

So to recap, the power on my car is dipping, and its not the alternator. We've also replaced the main fuel relay and the starter during the all of this, while i don't think they're related, it would make sense. Previously, before we replaced the alternator, we would be able to move the plug that hooks into the voltage regulator and the car's idle would change depending on which way we moved the plug.

If i had to take a guess, i'd say that this is likely an issue with either the plug (which i can't find anywhere to buy a replacement) or an issue with the eletrical harness, or wiring through out the car. Either way, giant pain in the ***. Any suggestions or thoughts would be appreciated.
So my question, what's wrong with my car? should i beat it with a stick? I want to buy stock in batteries at this rate - pushing my car is no fun.

 

<Reserved for problem solution>

 

Cheerios,

You know that it could be one of a million things? And as you stated E-gremlins are a PIA.

That being said, let me take a few wild swings. '91 Accord... Older cars can have issues with corroded, broken, or missing grounding straps/wires. Bad ground will play hell with the electrical system and drain batteries. Engine/block straps are usually low on in the compartment and exposed to moisture. It would be great is someone had a few photos of grounding wire locations, like the infamous T-stat wire that seems to go missing on older hondas.

Every thing you described indicates bad regulator or pending alternator failure (when regulator is internal.) It is unlikely that a new alternator could be faulty, but not unheard of. Before beating the car with a stick, I would at least pull the new alt and have it benched tested at the local parts house. Don't rule it out until you know you know.

How old are the belts? Could they be slipping under load until you hit 41 mph? When was the last time it was replaced? Time for a new one? A little belt dressing will give it some grab for a short time period to check it.

My best guess is you will find a lose ground wire that is moving when the engine is under load and not making good contact. But before you drive yourself nuts chasing electron paths, yank the alternator and give it a bench test. It will only take a few minutes and save a lot of frustration that uncertainty breeds.

 

ELD - Electric Load Detector