...I go to start it up and everything is fine. But while it's running, my tach and radio will go out after running for several minutes. If I hit the gas pedal, the tach and radio come back on for a while and goes through the same cycle...
Ok, after re-reading the first post, I have NO IDEA how we got on to the subject of alternators. In no way whatsoever does this sound like a charging issue. If the engine starts and runs, the battery is good. If the battery is good every time, then the charging system is doing it's job.
If it varies from that norm in any way, then we can look at things differently. But since you're saying it's only the tach and radio acting up, the first suspect is the wiring under the dash.
-aftermarket CD player which was installed by a car audio shop over four years ago.
Does it work when the key is off, or only when on?
-I added a tach a few years ago - no issues since install.
Tachs are notorious for failing and killing the engine and keeping it from restarting, but they don't normally do any other malfeasance. But from reading further, I'd say you DO have an issue.
-neutral safety switch was replaced due to short in the original.
Do you know what was shorted? And what did it do to let you know? Or did it just stop working?
Either way, the NSS does not seem to be an issue here (with tach and radio)
-new battery (Optima red top)
Here again, ANY battery can be defective. Not just Optimas, but they did have an issue a few years ago where they did not last but a couple of years. All of mine are 10 years old or older at this point, so I can't say I've ever had a bad experience.
-I've had the battery on an Optima tender to keep the battery charged. If I disconnect the tender, the battery will be drained in a matter of a couple hours or so.
Ok, so now we're getting somewhere! This by itself could be an alternator issue. But did you mention this before?
Only two things usually can do this. One is the battery is defective. The other is the alternator's diodes sucking power. Or whatever it is diodes do when they die.
Are you talking about with it hooked up to the vehicle, or disconnected? If disconnected then it's only one thing. The battery itself.
-Something strange that I noticed tonight was that if I step on the brake pedal with the engine off, the tach light comes on and is very dim. And if I turn the key to accessory power only, the tach lights come on at the same intensity as when I step on the brakes. If I connect the tender to the battery and then step on the brake pedal, the tach does not light up. And when I turn the key to accessory power the tach light comes on with full intensity.
Ok, as said this is not right. And since the tach is not a stock factory item, nor is the radio, and the brake light switch has been replaced, something is rotten in Denmark under the dash. And you're just going to have to dive under there and dig through the wiring until you find it.
Something is either connected wrong, working lose, corroding, or wearing a hole in the wire's insulation so that it's touching some metal, or another bare wire.
Time to dig out the flashlight, flex your joints, exercise your neck muscles, and turn yourself into a half-pretzel to see what's going on up under there.
Follow the tachometer wires first, then the radio, then the brake switch.
The brake light switch gets it's power from the headlight switch. The tach (if it's lighted) has a wire connected at least indirectly to the headlight switch, and so might the radio.
Does your tach light up? If so, does it light up only with the headlight knob pulled, or as soon as you turn the key?
What about the radio? These days I guess most radios light up with the key, but does yours dim when you turn the headlights on?
Lots going on here. Mostly effecting non-stock items. So get to diggin'!!!
Keep us informed as you go and we might be able to make more suggestions as you find more clues. Even to the point of posting up pics of the wiring under your dash.
Good luck.
Paul