Battery not charging - PeachParts Mercedes-Benz Forum

firemediceric · #1 07-20-2006, 07:27 PM

I just purchased an '81 300 SD with 208,000 miles. The battery is not receiving a charge. I pulled the alternator and had it tested, but was told that the voltage regulator and the alternator as a whole are working just fine. The belts on the alternator were tensioned.

Where do I look next?

Thank you in advance for your suggestions. I'm not much of a mechanic, this is my first MB, and I've only had it for 3 days. UGHHH:

Jeremy5848 · #2 07-20-2006, 07:44 PM

We need more information:
1. Is the idiot light on the dash lit up? All the time, some of the time, never?
2. How old is the battery? An old, damaged, out-of-electrolyte battery may not charge no matter how good the alternator and regulator are.
3. Who tested the alternator and what did they do in the way of a test?
4. Can you put a voltmeter on the battery and read it?
a) engine not running (you're looking for about 12.5 Volts or more)
b) engine idling in neutral (about the same)
c) engine running at 1500 RPM or more in neutral (should be 13.5 to 14 Volts, more or less).
If you can't do that, try this: with the engine idling, turn on the headlights. Now rev the engine up to 1500 or 200 RPM. Do the lights get brighter? If they do, the alternator is delivering current and the problem is more likely to be the battery. If the lights do not get brighter, then either the alternator/regulator is not working, or the current is not getting to where it needs to go.

I don't know the SD's electrical system well enough to know if there is a fuse or something in the alternator circuit. Nonetheless, check all the fuses in the fusebox anyway.

Jeremy

whunter · #3 07-20-2006, 08:21 PM

Inspect the wires from alternator to the screw junction block for damage or corrosion.
Inspect the battery to body and body to transmission housing GROUND cables for damage, corrosion and/or loose connection.

Engine Electrical:
http://www.peachparts.com/shopforum/showthread.php?t=142401

Have a great day.

firemediceric · #4 07-20-2006, 09:52 PM

The idiot light has not come on at all. I went to start the car and had a dead battery. I brought the battery to Wal-Mart, they tested it and said it was no good. I purchased a new battery and put it into the car. Car started and ran fine.

Today I did a little research and diagnosing. I started playing with the volt meter. With the car turned off the battery reads 11.5 volts. Once the engine is idling the battery continues to read 11 - 11,5 volts. No change when the engine is revved up. With the headlights, radio and AC on the battery reads 10,5 volts, even when the engine is revved.

The alternator was tested at Advanced Auto Parts. They hooked it up to their meter, attached a belt to the pully, turned on the machine to turn the belt which in turn caused the pulley on the alternator to spin. The alternator produced the voltage on their meter.

Thank you for your time in responding.

n0rt75o · #5 07-20-2006, 10:03 PM

wonder why mercedes never put a voltage meter on instrument cluster ? how good is the battery dummie light, what is the little black box that the cabel goes to from batt to starter ? so many questions,, hope i stuck to the topic ...n0rt

#6 07-20-2006, 10:08 PM

Quote:

Originally Posted by firemediceric

The alternator was tested at Advanced Auto Parts. They hooked it up to their meter, attached a belt to the pully, turned on the machine to turn the belt which in turn caused the pulley on the alternator to spin. The alternator produced the voltage on their meter.

Well, if your voltage measurements are correct, it's not working in the car. I would have the alternator tested in the car (free at some auto parts stores). Its either a bad alternator, voltage regulator, connection, or wire.

Jeremy5848 · #7 07-21-2006, 12:12 AM

The voltages you report are appropriate for a battery that is being loaded down by the car's electrical system while not receiving any charge from the alternator. The test that Advanced Auto Parts did sounds reasonable. That being the case, a failed connection between the alternator and the battery (and hence the rest of the car) is likely the culprit.

Follow whunter's suggestions in looking for bad connections. Keep the new battery on a trickle charger so you don't damage it by discharging it excessively. If you can follow the wire from the alternator to the junction block, you can check the voltage at each step along the way. Note that the alternator must also receive a small amount of current from the battery to get started generating electricity. All of the connections must be good for charging to occur.

pmckechnie · #8 07-21-2006, 07:08 AM

When you say "The idiot light has not come on at all.", does that also mean it doesn't come on when the key is on and the engine is not running? If so, that can be your problem. The light has to work. If it doesn't come on, find the small blue (I think it is blue, it is on my 500SEL) at the alternator, disconnect it, and jumper it to ground, turn the ign on, and make sure the light comes on. If it doesn't, fix that problem first and retest for charging. If it does, then the regulator is bad. Let me know.

firemediceric · #9 07-21-2006, 07:51 AM

The idiot light does come on when the key is first turned and the glow plugs are warming up. As soon as the engine is actually turned over and is running, the idiot light shuts off.

If the voltage regulator is bad, wouldn't that have showed up when the alternator was bench tested?

Thank you all for your input. I will be checking back throughout the day as I take breaks in an effort to mitigate my frustration as I try to trace and test the circuit.

wenchris · #10 07-21-2006, 09:37 AM

Just went through the same thing on my 84 300d. Turns out after a new battery, voltage reg, it was corrision where the ground of the voltage reg connects to the alt. Take off the voltage reg and a little sand paper to clean the back of the voltage reg and alt and I was good to go. Charging at 14+ now. Good luck. Jimmy

P.E.Haiges · #11 07-21-2006, 10:08 AM

fire,

Have U checked the alternator brushes? They can wear out and the alternator will not work. Brushes are easy to replace. Just remove the V-regulator, the brushes are attached, and solder in new brushes. Or U can pay twice as much for a new regular but that won't solve your problem if the ALT is bad like mine was.

The idiot light for the alternator is named after the idiot that put a light in the car instead of a ammeter or voltmeter. It doesn't light when the brushes wear out or when the alternator doesn't work. Seems like the only time that the idiot light goes on is when the key is turned on before the engine starts and that is just a bulb check. So what good is the ALT light?

MB would have done better by putting in a light for low oil pressure. It would be noticed a lot quicker than an oil gage if oil pressure was lost. Or use a oil pressure gage in conjunction with a buzzer. That would be a lot better than having a buzzer sound when I leave my lights on. Worst that could do would be to discharge the battery.

P E H

mespe · #12 07-21-2006, 10:17 AM

I had a similar problem, wound up being the connection on the plug going into the alternator. It was green. I in troduced PB blaster to the connection, been fine since, but I'm still leary about it. Once corrosion sets in, it's like rust, it'll never go away, so periodic blasts to ensure good connection are in order.

Look for any green copper, that's probably your problem.

Marty

firemediceric · #13 07-21-2006, 02:38 PM

Thank you all so much for your suggestions. The problem has been remidied. I am not much of a mechanic and get easily frustrated, but working on the MB was not at all the aggravation I expected.

I'm not sure eaxactly what I did to fix the problem. This morning, while out on some errands, I brought the alternator by another store to have it bench tested. It once again came back fine. I reinstalled the alternator and cleaned the terminals on the plug that goes into it as well as the receptical for the plug on the alternator. I then followed the wire harness and removed and cleaned other terminals as I came across them, such as the ones in the little black box on the fron fender. I also removed and cleaned the negative battery cable where it sits on the fender/ fire wall.

I put the volt meter back on the battery with the car turned off. It was reading about 11.5 volts. I started the car and the volt meter went to 12.3. As I revved the engine to 2K RPM it stayed between 12.3 to 12.5, and remained constant even at idle as I turned on the headlights, radio, etc.

I think that the problem is fixed, for now. When it does become time to replace the alternator, I think I will opt for a heavier duty one that will put out more volts. I am also going to be ordering a votage regulator to have on hand, as the brushes on mine to appear to be getting down to the minimum.

How many miles are the brushes usually good for?
Where is the best place to purchase the votage regulator?[/B]

Thanks again to all, I couldn't have done it without you. Now on to the water leak into the trunk

whunter · #14 07-21-2006, 02:51 PM

Quote:

Originally Posted by firemediceric

How many miles are the brushes usually good for?
Where is the best place to purchase the voltage regulator?[/B]

Brushes; OEM new is generally 200K, aftermarket is unknown = 25 miles to four years average.

Where are you?

Please add your car, mileage and location = nearest major city to your user CP.

pmckechnie · #15 07-21-2006, 03:59 PM

Yes Tom, the test should have caugt a problem like that, but I never trust "TESTERS". The only real tester for an alternator is the car it belongs to and the tech (you) that is working on it.
What we have proved is that the light is working as it should, and that is very important and the most overlooked problem when diagnosing alternators. Now we need to check the output of the system. Connect your volt meter to the output of the alternator (the big wire) and the other lead to a good ground like the battery -. It should have battery voltage on it at all times. At idle, it should have atleast 13.5V but not more than 14.5V.
Get back with these results.