Battery not charging anymore - PeachParts Mercedes-Benz Forum

kahlil88 · #1 03-23-2011, 11:47 AM

I've had battery problems since I got this car, mostly because I needed a new alternator adjustment bolt to stop the belt from slipping and my '78 has an oddball bolt that took forever to find. I don't hear it slip very much these days, but lately the battery has been dead almost every time I go to start the car. Went to O'Reilly's and had them test the alternator while idling -- the results are it's only putting out 12V instead of 14 and is most likely on its way out.
Are there any other causes of a dead battery I should look into, and if not is it possible to fix the alternator instead of forking out a bunch of money on a new one?

leathermang · #2 03-23-2011, 12:04 PM

Take off the wires to the posts and clean them and the battery post...
It seems simple... but I had this happen to me lately ...and the battery top and wires looked nice... but had corroded just enough to not charge the battery properly...
Also check the grounding wires to the engine and the body... it has to get all around the circuit to work properly....

lutzTD · #3 03-23-2011, 02:14 PM

Quote:

Originally Posted by kahlil88

I've had battery problems since I got this car, mostly because I needed a new alternator adjustment bolt to stop the belt from slipping and my '78 has an oddball bolt that took forever to find. I don't hear it slip very much these days, but lately the battery has been dead almost every time I go to start the car. Went to O'Reilly's and had them test the alternator while idling -- the results are it's only putting out 12V instead of 14 and is most likely on its way out.
Are there any other causes of a dead battery I should look into, and if not is it possible to fix the alternator instead of forking out a bunch of money on a new one?

might also be the voltage regulator and brushes. you can pull yours and look at the length of the brushes, they should not be nubs. I always carry a spare I pulled from the PNP, it is the white plastic peice on the back of the alternator with 2 bolts. The brushes inside get worn but it is a pretty easy job to replace it. Just be sure to get the brushes in right before you tighten the bolts

here it is

http://catalog.peachparts.com/ShopByVehicle.epc?q=1979-Mercedes--Benz-300d-Engine--Electrical&yearid=1979%40%401979&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6186%3AED%7C10000132%3AMBC%7C1503%40%40300D&catid=240798%40%40Engine+Electrical&subcatid=240870@@Voltage+Regulator&mode=PD

leathermang · #4 03-23-2011, 02:22 PM

How about him checking the diodes ... is this one of the easy alternators for doing that?

lutzTD · #5 03-23-2011, 02:43 PM

Quote:

Originally Posted by leathermang

How about him checking the diodes ... is this one of the easy alternators for doing that?

Im not sure how to change the diodes on these, they might even be in the regulator part, usually a diode problem will give other symptoms. OReillys should have found that.

patbob · #6 03-23-2011, 03:03 PM

Could the glowplugs be sticking on? I know the '80 diesels have relays that sometimes stick on and can cause what you are seeing, but I can remember if the '78s have relays. Either way, when the glow plugs are on, even when everything else is working perfectly, the alternator tries and tries its little heart out, but it just can't quite keep up and the voltage sags. 12V sounds about right from the threads I've read (hasn't happened to me personally.. yet).

Orv · #7 03-23-2011, 03:12 PM

Does the alternator light come on when you turn on the key, before you start the car? If not, the bulb is burned out or you have a loose wire in the bulb circuit. The bulb provides the initial field current for the alternator; without it you will see either no charging or poor charging.

lutzTD · #8 03-23-2011, 05:05 PM

Quote:

Originally Posted by Orv

Does the alternator light come on when you turn on the key, before you start the car? If not, the bulb is burned out or you have a loose wire in the bulb circuit. The bulb provides the initial field current for the alternator; without it you will see either no charging or poor charging.

yep, forgot that.should always be the first question.....

kahlil88 · #9 04-03-2011, 01:32 AM

Thanks for the feedback everyone. I haven't investigated the diodes or voltage regulator and brushes, but to answer some questions:

Quote:

Originally Posted by Orv

Does the alternator light come on when you turn on the key, before you start the car? If not, the bulb is burned out or you have a loose wire in the bulb circuit. The bulb provides the initial field current for the alternator; without it you will see either no charging or poor charging.

The light with the battery symbol comes on, if that's what you mean.

Quote:

Originally Posted by leathermang

Take off the wires to the posts and clean them and the battery post...
It seems simple... but I had this happen to me lately ...and the battery top and wires looked nice... but had corroded just enough to not charge the battery properly...
Also check the grounding wires to the engine and the body... it has to get all around the circuit to work properly....

Good call. The positive clamp was a bit rusty and corroded on the inside so I cleaned that, as well as the battery terminals. The cable seems a bit corroded where it meets the clamp, and I suspect there might be more corrosion in hard-to-clean areas.

Quote:

Originally Posted by patbob

Could the glowplugs be sticking on? I know the '80 diesels have relays that sometimes stick on and can cause what you are seeing, but I can remember if the '78s have relays.

Interesting idea...the vacuum shutoff doesn't work, so is it possible there is something electrical tied to the shutoff system that prevents the glowplugs from turning off?