Charging System problem with 300D
 

Welcome to the forum.

Put the vehicle in Park, run it at 1500 rpm or higher, apply the full load, and read the battery voltage. You'll probably need a second person to do this........and holding it steady at 1500 rpm is almost impossible..........so just keep it above 1500 rpm.

If the voltage remains above 13.5V, the system is functioning normally.

Measuring voltage is not the way to verify alternator output. Amps output is the measurement. A fully charged battery will not need much voltage hence a low voltage reading. Have it tested for amp output above idle with a load tester like a VAT 40 or something similar at a shop. If, as you increase the load, the amps rise, then its working correctly. Its VERY important that the drive belt is tight. With the engine off, try to rotate the alternator pulley the opposite direction of normal rotation with your hand. If you can move it, the belt is loose! Also make sure the ground cable to the inner fender is clean by removing it there and cleaning the surface.

The stock alternators on these cars are relatively weak, and will not support a full electrical load at idle without draining some charge from the battery. However, that is not a normal operating condition. Under normal operating conditions (car movement) the stock alternator will keep the battery charged.

Sounds to me like you have a normal charge situation.

Ken300D

I recently had a slew of charging problems in my '79 240D. After replacing the voltage regulator (which you took care of with new alternator) I put the battery on a charger overnight, and make sure the belts are tight and not worn out. A slightly loose belt was causing me lots of headaches, I believe. Tighten that alternator up real good, and put some of that belt dressing junk on the belt if you don't have a new one handy. Working for me, so far. *knock on wood*

Not too tight. 1/2" of deflection. You don't want to kill the alternator bearings.

You're absolutely right on this. It's amazing how little tension is required. I think the alternator would be fine with 3/4" of deflection. After installing the new belts on the SD, after one 50 mile trip, I noticed the idiot light starting to glow a bit with the lights on at night. Found the belts had stretched so that the deflection was over 1". Still charged fine most of the time.

Most folks overtighten those belts.........to the detriment of the alternator and water pump.

The alternator will NOT support such a heavy current load at idle.

Tighten the alternator belt just tight enough that you cannot turn the alternator fan with your hands. That's as tight as it needs to be.

Dead Bat wild ones
 

I recently had problems dead batteries. The number one odd one was the glo plug timer staying on. That put a drain on the car running for an hour or two during the day. Three good alternators and only 12.4 volts will not keep it charged. The other was the power antenna. The mast broke and the antenna motor wanted to go down and would not shut off while parked.

Just two wierd problems you would not really think of checking otherwise.