jtranger

My alternator ('65 365HP) does not charge the battery under 900 RPM or when the headlights are on unless the RPM'S are above 900 RPM. Battery, Alt, Regulator, belt have all been replaced. What would cause a discharge at low RPM only. I replace the regulator twice. Thanks Larry

AZDoug

Alternator pulley too large or crank pulley too small, or you are using a 1 wire alternator. Possible VR if it is an external regulator.

Doug

Avispa

Flashback to about 1987 or so, and my 65 roadster did exactly the same thing. Disassembled the alternator and found one of the three stator coil terminals had broken off. Soldered it back together and all was well. You may want to check all your alternator internals if the regulator has been replaced.

Things happen to these car parts after 20 years or 40 that the designers really couldn't anticipate. The stuff just didn't have that design life.

magicv8

that's probably how they designed the 365hp solid lifter engine, to keep everything intact at higher revs than hydraulic lifter engines could attain. It's not good when the belts come off the pulleys.

If your stock alternator is working as designed, there are several 100amp or more alternators available that will support the headlamps at low rpm. Changing pulleys is also an alternative.

jtranger

If I take the alternator to the Parts store they should be able to find a bad stator or any internal on their test machine right? I had the alternator (original) rebuilt, but you know how that goes.

AZDoug

You can do it your self with an ohm meter, one of your diodes on the rectifier bridge may be bad also.

Google alternator testing and diagnosis, or something similar.

Doug

jtranger

Thx

BADBIRDCAGE

Doug:

Out of curiosity, what would using a 1 wire alternator have to do with it?

Rich

AZDoug

1 wire alternators won't self excite until you get to about 2500 alternator RPM, below that, they won't put out any power, though they may once they have been excited, but not from a dead start. 3 wire alternators put out power at lower RPMs.

Doug

midyearvette

good post and oh sooo true!....

jtranger

Doug, I have the stock alternator. It has a two prong plug, a batt post and a ground post. That would be what you call the three wire right? It also has an external voltage regulator.

toddalin

Nope. I have one and it energizes down around 1,200-1,300 RPM, or the first time you even moderately "blip" the throttle when its cold and stays energized until you turn off the key.

magicv8

That's right

I replaced the factory 61amp alt with a 135amp from Alterstart which has no problem charging the car with a heavy load at idle. In general, the higher the high rpm rating, the more an alt will put out at idle.

AZDoug

Read my post.

ALTERNATOR RPM

Doug

BADBIRDCAGE

Thanks Doug. I gotta check mine. I have a 105 amp 1 wire on my car.

I can remember a while back I started the car and went to roll over the headlights. They were slower than I am used to. I haven't had that problem since but I haven't tried the rpm trick.

That was the only time they were slow to roll over. Not before and not since.

Rich

toddalin

Fine. And how many people do you think know their alternator RPM? I would bet that more than 9 out of 10 misinterpret your post.

AZDoug

Necessary to be precise.

If you have 3" alternator pulley and 6" crank pulley, your engine RPM will be way different to get 2500 alternator RPM than in you have an 8" crank pulley and 2.5" alternator pulley.

if you are going to reply to a post, read it and understand it.

Doug

BADBIRDCAGE

Doug: Started the car and let it idle. Then revved to 3000 rpm. Interior lights brightened and stayed that way when back to idle. I didn't measure my pulleys but the single wire alternator did not self excite and supply voltage until I had cleared at least 2,500 rpm.

Thanks for the info.

Rich