Charging system

[bbboomer]

My 84 300SD doesn't appear to be charging. I put a DVOM across the battery and it shows just over 12 volts with the engine off or at any engine speed. I have had the battery and alternator checked and they checked okay. I ordered a new regulator and when I pulled the old one the brushes are fine. They have very little wear. I did not install the new one. Are there checks to the regulator/alternator that I can perform with a DVOM?
I checked the connections at the battery and also at the alternator and junction block. All the connections are fine.
The charge light goes out as soon as the engine is started but the ole girl left me at the market with a dead battery so something isn't working.
I did check for voltage drain at the battery and when connecting the meter between the neg cable and post it shows battery voltage. I removed each fuse and each relay one at a time and it made no difference. Is there another area I need to check?

[John Schroader]

I've heard that pulling the battery cable (with engine running) on more modern cars isn't a good practice, but my guess is that it should be OK for an '84. That will very quickly tell you if the alternator is charging. My regulator went bad last year, causing my good alternator not to charge. Replaced it and now have appx. 14 v to the battery. Hope this helps.

[bustedbenz]

Originally Posted by John Schroader

I've heard that pulling the battery cable (with engine running) on more modern cars isn't a good practice, but my guess is that it should be OK for an '84. That will very quickly tell you if the alternator is charging. My regulator went bad last year, causing my good alternator not to charge. Replaced it and now have appx. 14 v to the battery. Hope this helps.

What exactly will removing the cable prove in this case?

It's a diesel. It'll keep running with or without voltage once it's cranked.

[carnut]

How was the alternator tested. Was a specific amp load put on it to verify output under load? is the alternator belt tight enough? How old is the battery. How clean are the battery connections?

[bbboomer]

The alternator was checked at O'Rielly Auto Parts. Don't know what process the tester uses. The belts are fine and tight. The battery was checked at Wal Mart, again, I don't know what process the tester uses. Both places said they were the latest and greatest testers.
I am going to take the batt and alternator to Advance to be re-checked. If they say they are good I will have to assume they are.
I cannot find a source of drain on the battery but I have always believed if the VOM shows voltage between a cable and post when everything is supposed to be off there is a problem.
I found a switch that looks like it may be for an underhood light. It looks as though it is supposed to be tripped by closing the hood. I cannot find a light and after disconnecting the wire leads to the switch the VOM still shows voltage.
Any help/suggestions would be appreciated. I though I had a pretty good handle on electrical problems but I am running out of ideas.
Thanks, Darrell

[compress ignite]

(for questioning minds)
EVER,EVER detach any cables from any Alternator or The Battery cables with the engine running...
'Burns out the Diodes the Alternator in a Flatbush second.

[TOPGUN]

Reading voltage from an unattached cable back to the post would normal. You need to check for amp draw between the post and a unattached cable. The will always be a low amperage for the clock etc. An excessive amp reading would indicate a problem, but it seems like the alternator has failed.

[Diesel911]

Random thoughts:
How old do you think the battery is?

While the engine is not running disconnect the (+) cable and again put your Volt meter between them. Go to the fuse box and pull one fuse and see if it cuts off the voltage if not put that fuse back. Go down the line and do the same thing to each fuse. When you find the one that cuts off the voltage and it is not the clock that is the circuit that is draining the voltage.
While you should have voltage at these 2 points no current should be flowing through the GP Relay itself.
http://i242.photobucket.com/albums/ff279/Diesel911/ZZZ7.jpg
You should get battery voltage from both of the large connector slots with the key out of the car.
http://i242.photobucket.com/albums/ff279/Diesel911/ZX6.jpg
The Alternator needs to be grounded as the Regulator contols the field current through the ground.
Look at your new regulator and you will see where the regulator ground tag makes contact with the Alternator housing (around one of the screw holes). While the Alternator is on the car try using 1 Jumper cable going from the (-) battery terminal and ground it to the Alternator housing and see if your alternator charges.

[bbboomer]

I meant to put this in my last post but forgot to. The battery and alternator are about a year old. I just bought this car about a month ago, the PO told me he had replaced the battery about a year ago. The alternator has a data tag on it showing a build date of 11/07 so I'm thinking this isn't the first time for this problem.
I checked the fuse panel but I used the ground cable. Would the + cable make a difference? The alternator plug and the junction block check ok, that is 12v at both terminals. I connected a ground cable from the alt to bat but it didn't help.
Darrell

[yellit]

Always a good idea to add extra ground cables to major parts...include the block, head, transmission,alt...even extra battery to frame cable......this eliminates thin layer corrosion related voltage drops....quick,easy and cheap.....this fix has solved many tail chasing electrical issues...make everything going to the negative post... one big, fat,low resistance, happy conductor....no one can say (probably a bad ground).....

[yellowbenz]

Be sure the belt is good and properly tensioned. If you don't know how old the belt is just replace it. 3 months of similar problem and chasing down all of the good troubleshooting advice given here all boiled down to a problem with the belt in my case.

[compress ignite]

Your mystery underhood switch is most likely used as the sensor/trip for the
alarm system.

Man! You are working through all the correct steps to diagnose your problem,
with no joy.

'With the engine running place the VOM leads on the appropriate Battery terminals,
What voltage reading are you getting? (You're looking for a range
between 13.0 volts and 14.1 volts)[this is telling you what voltage the
Alternator is producing].
Now try the same test with the Headlights,Rear Window Defroster,
Air conditioning(With the Blower Fan on HIGH) all turned on
with the engine idling.The Voltage will drop ,but should maintain somewhere
around 12.1 to 12.5 Volts[If the Alternator cannot produce this minimum
the battery slowly drains down]

You may also have a Phantom Drain.The Multimeter in line test is valid
(Even using the Voltage scale...I've seen Amateur Wrench Turners,Who
are even cheaper than I,use a 12 Volt test light in line between the
Negative battery post and the disconnected Negative Cable to chase
down Phantoms,and THEY GOT LUCKY because there was only ONE
gross Phantom,Also because [Kinda like a Divining Rod] They claimed
the ability to judge the amount of current drain based on the Brightness
Factor of the Test Light.)
However,TOPGUN is correct, the definitive and most informative testing
is Amperage.It tells you the amount of power being drained(Milli-Amps or
whole Amps),[Small Current drain or Large].
The Voltage (or test light) in line only tells you that there is A drain.
(As others have mentioned,it might only be the Clock...OR it could be
the Clock and a Phantom...There's no way to tell)

Set your DMM/Multimeter to the DC Amperage scale(It will probably only
allow a 10 Amp Fused scale as the largest)and place it in line between
the Negative battery Post and the Negative Battery Cable.
[If you're spooky about your Phantom being larger than 10 amps and
damaging your DMM's fuse (Not Bloody Likely!) pick up a cheap DMM
at Sears (usually less than $10.00 USD) ]
Now run your "Pull the Fuse and watch the DMM scale" tests.If you're
showing less than 1 amp you can step down to the Milli-Amp scale.
The Clock,Etc will be pulling a low Milli-Amp level...Anything else is
your Phantom.

The very first thing I'd do,as soon as the DMM is hooked up in line,
is to pull the Trunk Light Bulb out of it's socket.Something about
that Circuit frequently hangs up in the closed position(usually,
it's the switch).

'Have you done Continuity testing on the Power Circuit cables?
(I.E. The Cable from Alternator Positive to the Battery Positive...OR
the Negative Battery cable [basically a Ground] )

[bbboomer]

I am taking the alt and bat down to be checked at another store. As soon as I get back I'll re-do the checks as all have suggested. Can't believe I am saying this but... I am hoping the alt or battery check bad.
I'll post whatever I find.
Thanks to all who have helped so far but don't go away I don't think this is over yet!

[bbboomer]

Battery checked good, 850 amps and good load recovery. The alt checked good, 13.8 volts, it could have been a little higher but well within the good range.
I installed the battery and checked the amp draw. This is interesting, it had a 3.5 amp draw when I first installed the battery which dropped to about .2 amps after a few minutes. I am going to put the alt back on and start looking for the culprit. Seems like it may be an intermittent problem.
I will post more as soon as I finish tests.

[bbboomer]

Okay everything is back on the ole girl.
When first started it was charging 12.1 at idle then it raised to 13.4 and would drop to 12.2 with a/c, lights and defog on.
It will recover to about 13 volts at 2000 rpm with everthing on.
What I saw happening is when first turned on the voltage drops about two volts till the gp relay cycles off, about 30 seconds.
There is a one to two minute delay before the alt begins charging, once it kicks on it seems to be able to maintain at least 12 volts depending on the rpm. (from 12.2 to about 13.0)
1. It appears the alt is marginal for the load requirement,
2. It doesn't begin charging immediately.
Any suggestions?