Alternator size

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ram250098

ram250098

69 Barracuda
Joined
Nov 25, 2007
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Location
Cookstown Ontario Canada
I have been trying to figure out a problem with my electrical system and thanks to all that have given me tips.
I pulled my alt out last night to look at the brushes and found a stick on label that reads the voltage at a certain rpm.
I was unsure of what capacity I had so this post is to double check.
The label reads 6000 rpm....57 amps
2000 rpm....28 amps
I assume the rebuilder tagged it and that is is a 60 amp alternator. But the amps seem low at 2000 rpm and what would they be at idle..next to nothing?
Does this seem right? To run my lights and electric fan, no wonder it drops to 11.5 when both are on.
Thanks
AL
 
Al, Do you want to try the Alt off my Cuda? The Car is just sitting in the shop. Hopefully coming out for new accessories soon
 
I tell ya ......you guys are the best. I put off mowing the lawn yesterday to figure out this problem so I have to do it today. With any luck maybe I will do a half assed job of it at try one of them out.
Thanks
AL
 
CRAIG COX said:
That's a better idea! Have you got the Bee out?
Click to expand...
Craig,had her fired up yesterday,ready to go .Any plans for today,come down for a cold one, I have to plan my deck build as I received the lumber yesterday for next weekend. Need to take a fuel tank to Pat,s to weld a sump and the alternator is here if Al wants to try it.
 
beezee said:
Craig,had her fired up yesterday,ready to go .Any plans for today,come down for a cold one, I have to plan my deck build as I received the lumber yesterday for next weekend. Need to take a fuel tank to Pat,s to weld a sump and the alternator is here if Al wants to try it.
Click to expand...
Rick, Going to See My Mumma! Then stopping in at friends in Holland Landing. I would come next weekend but my honey do list is growing! We still haven't cut the grass up north yet either! Not enough time, dam Monday to Friday work gets in the way! lol..
 
What you have is what is left of a 60 amp alternator.

The rpm ranges indicate a bench test after rebuilding, and would indicate a decent rebuild. Those speeds are alternator RPM, not engine rpm. Calculate alternator speed from engine idle speed & drive ratios. This is why underdrive pulleys suck - alternator performance falls off fast.

That old mopar alterator was a good design for it's day. It was designed to replace a 30 amp Generator, so the original versions were rated at 37 amps. Over the years they stretched that capacity to 65 amps or so.

When I say "what is left" I am speaking of the degraded performance from being rebuilt several times. Many rebuilt units come from high volume operations that part cores into bins and use anything that still works. If you put a 37 amp rotor in a 60 stator you get a 45 amp unit. Maybe. Looks like your rebuilder at least took the time to test his work.

When a stator is rebuilt, they put them in an oven and burn the insulation off of the wire so they can remove it. You lose about 6% of the magnetic quality of the laminations from doing this, because of the heat. If this is done more than once you can lose 18% or more of the capacity of the stator laminations. Your rebuilt stator with a good rotor managed to make 57 amps now.

That old mopar design is maxed out, rewound for almost double it's design amps. It was one of the very first units out there. If you want to run big power accessories with constant current draw like fans & audio you will need a more modern design in the 130 amp range. A 3G/4G hybrid will fit in the brackets, and make 130 amps minimum, and usually around 100 at idle. They can be hot rodded to 200 amps if you need it.

B.
 
bohica is right about rebuilds, but what it "was" even coming out of a "decent" rebuilder has nothing to do with what it "is" "now."

AND he's right about the RPM label. Those figures are ALTERNATOR SHAFT RPM, not engine RPM. My old sixpack car was close to 4:1 pulley size, which means that when that ol 440 was wound up into 5-6K, the alternator was spinning 24 THOUSAND RPM. And no, they didn't last long. (Way back in 72 I had an integral Delco on my 340, long long before Al Gore invented the www, or anybody had ever heard of "Denzo" whoever he is.

It might have a brush/ slip ring problem, IE worn brushes, greasy, poor spring tension

It might have a damaged rotor, IE a turn shorted internally which LESSENS the magnetic output of the rotor

It might have a damaged stator, ......

or one or two "blown" diodes

These are 3 phase AC devices, which means that each of the 3 phases does 1/3 of the work. If you take away 1 phase, you actually cut the output MORE than 1/3 because of efficiency changes and the "awful" wave pattern this generates, but simply........................

An alternator with reduced output has nothing to do with "what the label says."


You really need to do a "full field" test as I sort of outlined in the other thread.

One end of the field is supplied with switched 12V (the blue at the field) from the "ignition run" line

So unhook the GREEN field wire, clip lead that field terminal to ground, and turn the key to "run"

Measure the voltage at the blue field wire (hooked up!!) to be sure you have "close to" battery voltage.

Make ABSOLUTELY sure the belt is OK, is tight, and not slipping

Then run the engine, and see if low RPM performance is improved. Be careful, because at this point you have no regulator, and if you rev the engine too much with light electrical load, you can raise the charging voltage too much, especially if the battery is charged and hot. (warm)

If the low RPM output seems better, you probably have a field circuit wiring problem, or a bad regulator. If not, there is probably something wrong with this alternator.
 
Even if that alternator & charging system is in perfect is in perfect shape it can't keep up with more accessories than OEM

I can't say what you had for drive ratios on your 340, but the drives from a stock 318 are 7.25 on the crank & 3.00 on the alternator. That is a 2.41:1 overdrive.

So with an 850 rpm idle you would be turning 2048 rpm - just enough to make 28 amps. With some electric fans drawing more than 28 amps by themselves, you are using more power that you are making. Better have a great battery, and drive for lots of miles between stops to recharge the battery.

B.
 
You guys have been great..thanks very much. I tried a klnown good alt and have the same problem. So my plan is to find a good shroud and put the fan back on and use the electric as a back up.
That should solve all my problems.
Thanks again
AL
 
The only thing I'll add is that if you do go to a modern alternator you may want to be careful with the amperage. 100-130 amps is about the top end of what you'll want to run through the stock wiring. Much more than that and the bulkhead connector has a nasty habit of melting down...
 
If I did go with an 100 amp alt upgrade I guess one from a mid 70s B body with air would be the one to get. If so do they use the same mounting brackets?
Al
 
No, the large frame 105 amp stuff used a different bracket. I DID however, modify the small frame brackets to work on my old SB in my Landcruiser (12V winch)

But those things are HUGE. Someone was referring to Ferd 3G units. I'd think something like that would be a better option than the huge frame Mopar units.

I don't keep up on the so called "Denso" stuff, I'm sure they have 80A stuff.
 
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