1978 300 CD has a new battery, but will intermittently not charge. Had the battery tested, and they said the alternator wasn't putting out the full charge. Is this most likely the alternator, or the voltage regulator, and is there a way to be sure.

Thanks in advance.

 

If you have a multimeter or just a voltmeter you can check yourself if you are not sure to believe them. Get your car running, turn on some electrical loads like your lights, fan blower, and your radio, then check your battery with the leads from the multimeter/voltmeter it should read still some where between 14 to 14.6 volts or more. For the alternator I think the regulator is built in the alternator it is a whole unit. But wait around for a experienced Mercedes guy on the forum to answer this question also I could be wrong.

 

I have the same problem and I changed the alternator and put in a new battery on my 78 300d. Sometimes full charge sometimes not. Would love to know the answer.

 

A lot of good Mercedes/Bosch type regulators will only charge at 13.5-13.7 Volts. If you have a regulator that will allow you to put out 14+ volts consider youself very lucky.
Even when working the Alternator on my 84 300D is a 45 amp Alternator.

If you are in the US Harbor Freight Stores sell digital Volt/Ohm (Multimeters) for $3 (I have one in eac car). They can give you a really good indication as to what is going on.

The suspects for low charging rate:
Slipping drive Belts due to not enough tension or Oil on the belts.
The Socket that plugs into the Alternator can be cruddy and oxadized to the point where not enoug current is getting into the Alternator for it to charge well or just plan loose.
A weak Battery leads to weak charging because the Alternator still needs current from the Battery to work (the 2 fat Red wires going to the Alternator Socket/Connector have posative voltage all the time).
The Regulator on the Alternator controls the current by grounding. So if the negative connections from you Battery all the way to your Alternator are poor your charging will be Poor. (on my 123 there is a Ground Strap under the front Drivers side.)
Pretty much the same with the Posative; it needs good clean connections.

The Brushes could be worn on the Regulator. I think on your 78 the Regulator is only held in with 2 screws. Remove the Socket/connector and unscrew the Regulator and have a look. Also you cannot have Oil on the Brushes and have them work well.

Facing the inside one of the ends of the Regulator will have a metal faced hole for one of the screws. This is where the regulator grouds to the frame of the Alternator. Make sure the area on the Alternato where it touches is clean so it makes good contact.

 

Check your alternator's v-belt tension -- proper tension is critical on this car. Even with all components functioning 100% if the belt is a little loose you will not get sufficient charging and your battery will slowly drain over the course of a few weeks, even faster in colder weather.