I'm not a wizard, but I went through the same ordeal. With the engine off, obviously, can you turn the alternator pulley by hand?

Alternator
 

What (amperage) did the alt put out on the test benches....?....It is possible for an alternator to show a somewhat normal (voltage) output 13-14 VDC but fail to deliver the (amps) needed to charge a system....I once worked as an alternator rebuilder and this was the most important test.....A voltage reading of 13.5 to 14 is a (general rule of thumb field test) but the actual amperage output needs to be tested with a suitable load...very possible for an alt to show normal charging volts and still not have the amps to deliver....

From what I've read, it seems that the alternator "appears" to be good until you put a fair load on it. Here is something I found on mine that had me scratching my head....check the grounding of the voltage regulator to the alternator. Believe it or not, mine had a little corrosion and, same as yours, wouldn't maintain a proper charge, especially at night. Use a little emery cloth to make sure both surfaces are shiny and use a small dab of white conductive grease (Radio Shack) to keep the corrosion out. Also, one of the screws, holding the regulator, was non-metric and had a small area of threads slightly stripped.

( Something I ran into previously......Ensure, physically, the cleanliness and tightness of the engine-chassis ground strap. The exhaust bracket mine was attached to was loose. Everything with a meter appeared good one time and not the next.)

Charging System
 

Thanks guys for all the help. I have already checked at least once, every suggestion all have made. I always thought I had a pretty good handle on electrical checks etc but this is a new one on me.
What I really don't understand is the delay before the alt kicks out voltage. I understand there is a big draw until the gp relay cycles off but my alternator still waits about 1 to 2 minutes after that to begin full output. Add to that the output is not stellar even then and I guess that could explain my dead battery problems.
I guess I will go for a 120 amp Bosch from an E-Bay seller. Anyone had any dealings good or bad with Eagle Auto Electric?

Alt problems
 

I think you are correct with the delay problem symptom.....the delay points to a regulator sense problem....If the bench test shows full output...this alt may have low level regulation problems....but may perform max output when called for...just no fine regulation so to speak...either full on or off....

There is a test known as full field test where you fully energize the rotor for maximum magnetic field to test the max amps and volts output....since you may get 16+ volts out of it this is best done on a bench .....
Probably a new alt will solve your troubles.....simply because it will have proper regulation....a higher output alt will still only deliver what is demanded from it....just will have more reserve output if needed.....
Please post back when you whip this thing...let us know what gets it going.....

Charging System
 

Yellit,
You may be right on the mark with the regulator idea. I do have a new regulator. The old one appeared fine and the brushes were near new length. The alt has a build date of 11/07 on a data plate so neither is very old.
I think I will put the new regulator in and see if that stops the delay problem. That may fix the whole thing!
Thanks again, and I will post whatever finally solves the mystery.

I'm noticing similar symtoms with my 83 sd. New regulator. Everything off, immediately after starting, 12v at idle, or 12.5 or so above 1500 rpm. Turn on lights and blower motor, goes below 12v. After it is running 4-5 minutes, voltage climes to what I would consider the low side of normal... 13.2 or so at idle, 13.7 above 1500rpm. With a load, maybe 12.9 at idle, but it will maintain about 13.7 over 1500.

Update... If I unplug the glow plug relay, it goes immediatelty to the 'normal' range 13-13.8 v with load. Plug it back in, and it immediately drops to 12-12.5. You may want to try this and see if you get the same result.

Charging system
 

Mayday Mike,
When you run your test can you see when the gp relay cycles off?
Mine will usually cycle off after about 30 seconds or so, this is clearly seen on the DVM. My alternator doesn't begin full output for another minute or two after the gp relay cycles off.
Is this what yours is doing too?

Alt problems
 

Got to be regulation issue....If the parts houses said the alt was ok.... they probably just check max output with a heavy load....this says the alt can produce the current needed... just probably did not test low level charging.....The delay is the regulator not sensing a need for increased field current in the rotor or either it is sensing the need and not kicking in as needed....this is a common semiconductor failure pattern.....several internal components in the semiconductor regulator could be marginal...I think the regulator is a basic current pass transistor setup....Big transistors can fail in strange ways.....

Charging System
 

I'll put the new regulator on this afternoon and post the results.

Alt problems
 

Will be very curious to see if this alt works fine on the test bench ...but won't work in the car.....this happens very often...sometimes it is a combination of factors in the vehicle ....not just one problem......always a challenge....let us know how this turns out.....

R+R Regulator
 

bbboomer,

What was the result of the regulator replacement?

Good News?

Charging system
 

I have to appologize fellas, I have not replaced the regulator yet.
I have been fooling around with making bio diesel and haven't gotten to the merc yet. I will probably change it in the morning and will post whatever I find.
Stay tuned...
Darrell

Alt problems
 

Understand the biodiesel fun...just got back from our farm based refinery...made about 1000 gals so far....accidentally cleaned the floor with about a 30 gallon overflow from new wash tank.....man is that floor clean...!

We are all waiting for the alternator diagnosis........