I have an 81 mercedes 300d. I recently replaced the battery. Then found the alternator wasn't putting out enough voltage. I bought a new one from this site and installed it after charging the battery. The problem is I still only get about 13v. Shouldn't the voltage be around 14.4? Revving the engine doesn't make the voltage go up. I have tried grounding the alternator with a jumper cable and it didn't help either. I did notice that the little black box on the alter Did I get a bum alternator? Anyone know the return policy of mercedesshop? What to do next?

Around 13.5 volts would be OK. What are you using to measure the voltage? I have found that the Dash Board Gauges in cars are not often that accurate.
Is this the reading you are getting with a digital volt meter (electrical multimeter)?
Before I would condem the Alternator (only the Alternator not the whole car)I would take it to one of those parts places that will test it for free.
For the specific minimum voltage might want to check out this site:
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Main.html

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84 300D, 82 Volvo 244Gl Diesel

Just as Diesel911 says 13.5vdc is okay but not great.
Where did you measure the voltage ?

I have found that some alternators need a little running before they kick out 14.6vdc because the brushes need bedding in. Usually, if measured at the BATT+ terminal on the alternator, you should see above static battery voltage of 12.65vdc by at least 0.5vdc making it 13.15vdc at idle....rising to a maximum of 14.7vdc at 3000 rpm.

How is the battery, should be fully charged, as this plays an important roll in alternator output ?

Check also the ground strap from the BATT- to the body at the hood hinge.

There is also a braided ground strap between the bell housing and the chassis just behind the steering box.

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Thanks for the replies. I am using a digital voltmeter to get the readings, I am checking it at the battery. Ground strap seems fine although maybe I'll put a wrench on it just to be sure. I couldn't find anything on the alternator at that site? Any ideas what else to check?

What is the reference resistance between the case of the alternator and the negative ground at the battery...You said you used a jumper cable between the alternator and ground but if it is not exactly 0 ohms between the alternator and the neg batt cable measured with a digital ohmmeter it will put out less voltage and of course less amperage...

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http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html

Perform the test again with all accessories on...........heaglights, high beams, blower, rear defroster.

Run the engine at 2000 rpm (get a helper) when you perform the test.

Report back with results.

so how was the test??

Hope you get this figured out and report back. I'm in the same boat with my 300D.

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Kevin

1978 300D
1979 240D

I copied this from part of a Mitchell Manual
"On-Vehicle testing
Wiring Continuity Test
Disconnect the terminal plug from the rear of the Alternator. Connect a Voltmeters negative test lead to ground. Turn the ignition “ON” . Connect the positive lead of the Voltmeter to each Alternator plug terminal 1 at a time. The volt meter should read battery voltage at each terminal. If not, trace each wire to find the fault.
Voltage drop test-Ground side
Connect the Voltmeter between the negative terminal of the battery and the Alternator housing. Start the engine and run at approximately 3000 rpm. If the Voltmeter reading exceeds 0.25 volts, you have a have resistance in the negative side of the charging system is indicated.. If so , check for loose, dirty or corroded connections.
Regulator Control Voltage Test
All applicable regulators are designed to maintain from 13.7 to 14.5 at a load current of 5 to 7 amps. The Alternator should be driven at 4000 rpm and load current set at a load current rating. Read voltage within 1 minute. If not within specifications the regulator requires replacement.
If the Regulator fails to keep voltage within specified limits, it must be replaced.
Minimum brush length is 0.2” (5mm)."

If you have a terminal bolck you might want to disconnect the batter negative cable and remove one screw at at time and wire brush it. I believe this is were the Alternator gets the (+) current from.

I have also read that under the screw closest to the engine on the Voltage Regulator is th area where it grounds to the Alternator housing if it is not making good contact with the housing itself the ground will be poor. This type of Voltage regulator uses the ground to control to control the field current.

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84 300D, 82 Volvo 244Gl Diesel

and don't forget the battery terminals themselves... a new battery often has a thin oxidized layer on it and unless you cleaned it and the battery cable ends thoroughly, this can be the whole problem.

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