Was wanting to find out a good check to see if alternator is working or not on 80's model diesels. If you take a voltage reading on battery with the engine running, and it is only a little over 12 volts is that a good indication the alternator has quit on you? I was told it should be 13.50 to 14 something volts.

Yes in general it is a good quick test. If alternator has totally cased charging the alternator light should be glowing as well I think.

The MB owners manual advises all unnecessary electric loads be kept off if you are driving a lot of stop and go, and idling a lot, so I wouldn't expect much more from stock alt. Check the output at secondary road RPM, 2000 or so, and see what you get. The alternator is rated at 14 volts (anyone with better/ more exact figures is welcome to correct me any time, please) so at speed anything less than 13.8 is probably not good. Keep in mind that voltage and amperage tests require a fully charged battery to be accurate.

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1984 300 D runs, interior
1984 300 D light hit in front, RUST, mint/perfect dash, decent interior (see above), parts car
(hate to kill one)
1998 Dodge diesel

Look for green stuff on the wires by the alternator (and by the battery - but usually if Voltage is high at the alternator). If there is green stuff on the wires, clean off the green stuff (soaking in coca-cola works good). At idle I wouldn't expect more the a little over 12 volts, but if you rev up to 2000 the voltage should go up to 13.8 if wires are fine (higher if green stuff is present)

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If your Alternator is not charging and the Dash Board Charging light is not glowing the Light Bulb may be no good.
If the Light Bulb is burned out your Alternator will not charge as it needs the resistance of the Dash Light Bulb in the circuit to get the Alternator to charge.

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84 300D, 82 Volvo 244Gl Diesel

The light in the dash is on but not very bright. Is that how it should be if the alternator is bad? Also, the three idiot lights on the left side of the dash the battery, brake light & the can't remember the 3rd one right now. That's how it is when you get older??? They don't come on now when the ignition is on ready to start. I don't know if defective alternator would cause this condition or not. Or if I have a different problem.

Got Lucky! I had replaced the v-belts on the car this spring. Well!, I didn't have the alternator belt tight enough it seems. I tightened it up like a fiddle string, and the alternator is now working. I removed a couple of relays, and it made a clicking noise, and now all the idiot lights are working again, as well. It made my day, and I didn't like the taste of the pricey alternator. I was going to have to buy. Thanks, for all the input!!!

Be careful of over tightening the belts. I will cause premature failure of water pump and/or alternator bearings.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K