1984 mercedes 300 diesel has no electrical power when running

If your alternator is bad,replace your regulator,its mounted on the bad of the alternator,cost about 40 dollars,too many people replace good alternators.

Old Diesels do not require electricity once started so luckily the problem wont stop you once you're up and running, though if at night you wont have headlights, brake lights, hazzards or turnsignals.

Even if it was a dead battery, your alternator would still give you power once started.

I'll need more information to help diagnose better. First, once started, do you have electricity? Or is it dead all the time?

Alternators need the engine to spin them, so first make sure there is a belt attached to the alternator...open hood, look bottom left of engine for alternator...look to see if there is a black rubber belt attached to the pulley on front of alternator.

Then make sure wire cluster plug is plugged in securely to top of back of alternator. If so, follow the wires up to battery and see if they are torn, detached, worn or corroded.

You will need to a good new battery and to pick up a cheap voltage tester from Radio Shack or similar place. Check battery power, set voltage tester to 12V and with engine off voltage should be around 11-12V...once engine is started, it should be showing 13 to 14 volts.

There are lots of "bad" ways to check to see if alternator works, but they can damage the alternator's internal voltage regulator...so I warn you not to detach from battery while running.

If new battery is installed, connectors and all wires connected tightly, and still no power once engine is started...or you see voltage slowly dropping while engine is running...and turn on headlights...see if it still goes down lower...reach over top of engine with a friend holding tester leads to battery terminals, you can rev the engine...there is a metal bracket at top center of motor that will have a red sticker that says, "engine stop" pull it away from engine to rev, push to shut off engine...this a fuel control gadget. If you rev the engine and there is no change in voltage...and all wires are intact from alternator...then your alternator is not working properly and will need replaced.

I stress this, before you decide this is the prroblem (which is expensive and time consuming to fix, MAKE SURE ALL WIRES are SECURELY ATTACHED and in PLACE.

Please let me know if this helped you and if not, tell me what you learned and the other clues I need to better diagnose. Thanks and good luck!