1981 240D Not Charging!
 

Ladies and Gentlemen for your expert diagnostic entertainment:


The 81 240D 4-spd will not hold charge and the battery croaks while driving after a few starts of daily driving. (I charge it the night before and it will be good for a few starts.)

Short version: Not the battery, probably not the alternator. Trying to figure out what it could be in between.

Long version:
-I have swapped batteries with my 300SD and the battery still dies on the 240 but works fine on the 300.
-Replaced the alternator, no change
-Battery terminals are clean, ground straps are tight.
-Plug for the back of the alternator slightly oxidized but probably not enough
to cause this issue and I did clean it and put some Dow-Corning on the plug
to help the conduction.
-Had the system tested at autozone with low volts reading, no output from alternator.
-And the voltage regulator is new with the alternator.
-(Put an alternator on the car 2 months ago for same symptoms but it fixed.)
-Belt is tight and not slipping.

---I can't really think of anything else and the car is so simple its really either the battery, the alternator, or in-between.

Thanks!
Pat

Is the indicator light in your dashpod coming on? Believe it or not, the alternator can't charge if the bulb is bad.

Wow, first time I have read that ! I don't doubt you, but good to know !

Maybe your radio antennae motor is odd. Try unplugging the antennae motor.

mostly true... with a bad bulb, the motor has to exceed 2000RPM to charge, but some vehicles do not have the alternate path to the exciter circuit, so the bulb needs to be good.

another thing to check is if the glow plugs are shutting off.
the alternator cannot overcome the draw of the glow plugs.

Years ago made an Alternator Test Cord that plugs into the Alternator. I go by 300Dman over on Benzworld.
As far as I know Myself and one Benzworld Member are the only ones who have made them.

If your Battery Charges OK with the Test Cord it at least tells you the Alternator is OK.
After you know that you have either a wiring problem or something is shorted an draining the Battery (or the Battery is shot).

Alternator Test Cord for W123s - Mercedes-Benz Forum

This is the other guy that built an alternator test cord Post #8
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1627361-300d-w123-electrical-problem-charging-system.html

Thanks for the input all:

1. Battery indicator light functions properly on standby (not indicating discharge during driving)
2. Not the antenna motor---thats been busted for a long time!
3. I can hear the glow plug relay clicking off as it times out. I guess that it would have to be off and not draining but I could try to crank then drive around with the 2 cannon plugs pulled from the glow plug relay.

Revised plan: Going to take the alternator that I had just replaced (but is only 2 months since installing) to the parts store and bench test it. If it tests good, then I'll know its not the alternator itself.

Any additional comments keep em coming!

Thanks

Pat

Does your alternator have a multi pin plug, or is there a separate connection for every terminal?

Since the dash light works, there's nothing wrong with the D+ circuit. There are only three other possibilities:

1) Bad alternator or VR
2) Wiring problem, including: miswiring the alternator if it has separate connections, heavy corrosion, or a broken B+ wire.
3) Bad battery.

That's it. The charging system isn't rocket science (which is especially lucky, since we're not rocket scientists). If the alternator is good, and there's current going to D+, then it will produce a charge current at B+. It's that simple. I wouldn't bother with that test cord thing, it's a great tool if your goal is smoke release.

I would pop open the connector going into the alternator...clean all the corrosion out of it, and re tension the plugs...

you need this. plug it into the ciggy to while the car is running. this has saved my butt many times. this is how I diagnosed the bad voltage regulator in my 190 after the big flood.

Equus Innova 3721 Battery Charging System Monitor - Walmart.com

obviously, if the voltage while running is less than 12.6v then you know the alternator (or the regulator) is shot

A $5 Harbor Freight multimeter will do the same thing.

I have experienced this same problem with Bosch 2 wire alternators that had a rough or pitted contact spring for the voltage regulator or bad voltage regulator too.

Try to get one off any other Bosch alternator if you can.

Thanks again everybody for all the info!

-This alternator has the plug that connects not individual wires
-MxFrank very true! haha Glad you can confirm my thought process

Will keep you all posted

Voltage readings across the battery, both Vdc and Vac, will give you a lot of information.
Check pre-start and post-start.

An area that caused my car not to charge, was corrosion on the surface of the alternator at the mounting location for the voltage regulator. The VR picks up its ground from the alternator case. A little cleaning and conductive grease keeps that problem at bay. (you only want a slight film of conductive grease, too much and it melts, causing shorts).