Just picked up a 1981 280 SEL sedan ( grey market ) .
There is also a voltmeter wired on the car by one of the previous owners.
My guess is they installed it due to this issue and they couldn't figure out the problem and used it to keep an eye on the volts.
The car has a new good Mercedes Benz battery and a new Bosch Alternator.

Still when I drive even with no accessories on the volts read between 12 and 13 max. When I come to a stop it drops down to 12 or lower. If I use the AC it drops lower. I got stuck 2 days ago in the rain using the AC, wipres and headlights together. The volts dropped to 5 at a traffic light and after parking the battery was dead. After a AAA jump I was ok. he starter is good. The battery and alternator are new. Any ideas what can be draining volts on this car?

* Originally this was a well taken care of Benz but also was an up north car in Chicago and Buffalo NY so there is some rust from salted roads but not "rust bucket" condition. I know that road salt can effect electrical contacts under the car.

Again, any help is appreciated.

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."

Did the alternator come with a new regulator or did you reuse the regulator? Make sure there's a good cable and connections between the alternator and battery.

Sixto
83 300SD can be yours
98 E320 wagon

I was dealing with a similar issue in my MY98 R129. Long story short it turned out to be that the light in the trunk was staying on and draining the battery. Turn the cell phone video function on and place it in the trunk and close the lid to see if light is staying on.

__________________
82 380SL 41K miles sold, 98 SL500 44K miles, 13 C300 4Matic, 14 CLA-250 4Matic

I bought the car with the new battery and alternator. Taking a look I think the voltage regulator is built in or on the alternator. *** Edit to locate I found this link here from oldtimer "Diesel Giant" on voltage regulator -> http://dieselgiant.com/mercedesvoltageregulatorinstall.htm

Yes my voltage reg is new and so is the wiring to and from it. As are the battery cable and alt' wiring..

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."

Engine running voltage should be 13.7 to 14.3 ish. The alternator is barely charging, I'd have a look at the drive belt and make sure is isn't slipping then move on to wiring , voltage regulator , alternator warning light < this needs to work to light off the regulator / alternator.

Its a good suggestion to check it but my drain is not a gradual overnight type. Mine happens while driving and the drain is stronger.

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."

Back to voltage regulator, I do get some erratic instrument cluster readings on my oil pressure gauge while driving once in a while. The rest of the needles seem ok. My clock is in-op, and also the odomoter and tripmeter. Can there be some other older or corroded wiring going from VR to the cluster ? ( Or my engine strap if I can find the sucker. )

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."

So that was a very fun test. ( Not as much fun as throwing a neighbors kid in the trunk and driving through The Everglades..)
My lights do go out though so I am back to the drawing board.....

__________________
'87 924S
'81 280SEL

Sold ->

81 300SD -
93 300E w/ 3.2
85 300D-
79 300SD
82 300CD
83 300CD - CA
87 190E 5 spd
87 Porsche 924S

"..I'll take a simple "C" to "G" and feel brand new about it..."

I'd make sure the ground cable from the battery to engine is making good contact. If you battery is in the trunk, there will be a large ground strap running from the engine to body. Being bolted together isn't good enough, the connections need to be removed and cleaned while checking the crimp on the terminal for corrosion.

Make sure the wire from the alternator to battery is good ( might run to the starter terminal ) . Does this alternator mount with rubber bushings? I've seen a missing ground strap cause charging problems on other cars.

Past that, find a way to full field the

Might be a bum alternator. Pull it and have it tested off the car.

Sixto
83 300SD can be yours
98 E320 wagon

Since your alternator isn't charging at all, there are two possibilities:

1) The alternator is bad. This isn't as strange as it sounds, I went through three newly rebuilt alternators before finding a good one. Many of these cores are now beyond saving, and competent rebuilders are a dying breed. I don't care if it says Bosch on the box. Bosch rebuilds nothing, contracts out the work, and the name alone is not a guarantee of satisfaction.

2) The alternator isn't getting D+ voltage. Does the cluster light glow when you turn the key to the on position? If not, it may be as simple as the bulb being bad. It may not be intuitively obvious, but the indicator light is a key part of the circuit. If the bulb doesn't glow at all, then the alternator may never start charging. On some of these cars, there are alternative paths supplying D+ to the alternator, but on others, the bulb is critical and a burned out bulb will disable the alternator. If you have a test port on the right inner fender (see photos), it's easy to check D+. Just make sure you have voltage between the D+ post and ground when the switch is "on" but the engine not running.

http://www.peachparts.com/shopforum/attachments/diesel-discussion/141479d1490225317-87-300d-no-tach-no-charging-no-idle-control-img_3170.jpg

http://www.peachparts.com/shopforum/attachments/diesel-discussion/141480d1490225324-87-300d-no-tach-no-charging-no-idle-control-img_3169.jpg

Check and clean all ground connections. If necessary put a wire wheel on a drill to clean up the metal where the ground connections are at. Then at least you can rule that out.

If there was a bad ground connection, the car wouldn't start. Since it does, we can say with certainty that both the battery and the block are adequately grounded through the body. The alternator itself is bolted directly to the block with multiple bearing surfaces, there is no separate ground connection for it. So the problem is either a bad alternator, alternator harness, or an open in the D+ circuit.