Alternator or Battery? - PeachParts Mercedes-Benz Forum

GGR · #1 09-17-2009, 03:02 PM

This morning I started my W111 3.5 Coupe and I noted the battery was weak. Car started anyway and the red light on dashboard went off as normal. I drove for 40mns and when I came back I stopped the engine and tried to restart it but the battery was too weak. I shut off the contact, disconnected the ground at the battery and then put the cable on and off on the minus of the battery to see if there were any sparks which would have indicated some drainage in the circuit. But there were no sparks, so I assume there is no drainage. I think the battery would have had enough time to recharge enough during the 40 mn drive, so I think the culprit may be the alternator. but then, why would the red light come off when the engine is running? I wonder if that light is also connected to the oil pressure, and not to the alternator? I will try to borrow a battery charger and recharge my battery. Once the engine is running, how can I determine if the alternator is charging the battery? I can go buy a multimeter to the ACE store if needed.

Thanks in advance for the help!

dhjenkins · #2 09-17-2009, 03:10 PM

Quote:

Originally Posted by GGR

This morning I started my W111 3.5 Coupe and I noted the battery was weak. Car started anyway and the red light on dashboard went off as normal. I drove for 40mns and when I came back I stopped the engine and tried to restart it but the battery was too weak. I shut off the contact, disconnected the ground at the battery and then put the cable on and off on the minus of the battery to see if there were any sparks which would have indicated some drainage in the circuit. But there were no sparks, so I assume there is no drainage. I think the battery would have had enough time to recharge enough during the 40 mn drive, so I think the culprit may be the alternator. but then, why would the red light come off when the engine is running? I wonder if that light is also connected to the oil pressure, and not to the alternator? I will try to borrow a battery charger and recharge my battery. Once the engine is running, how can I determine if the alternator is charging the battery? I can go buy a multimeter to the ACE store if needed.

Thanks in advance for the help!

If the alternator is working, you should be getting around 14 volts at the battery.

If the battery is good, it should never drop below 10V, no matter what you do (accessories on, lights on, cranking engine).

If those are both fine, test voltage drop on your ground & postive wires.

Mike D · #3 09-17-2009, 03:13 PM

If the battery is bad internally then no matter how long you charge, it won't store the charge.

Voltmeter on the battery cables while running can give you a false positive reading but it's a good place to start. You're looking for a voltage of apx 13.0 volts at idle increasing to about 14.0 at a medium engine speed.

If you turn the engine off and the voltage immediately drops to below 11.5 then it's the battery since you have already check for drains.

If the battery slowly drops to about 12+ volts, turn the headlights on and see if the battery drops voltage quickly.

Cables and ground connections are the #1 cause of mis-diagnosed battery "failures". Double check these before replacement.

dhjenkins · #4 09-17-2009, 03:26 PM

Quote:

Originally Posted by Mike D

Cables and ground connections are the #1 cause of mis-diagnosed battery "failures". Double check these before replacement.

This cannot be stressed enough.

I had a nissan xterra do the same thing last week. The owners took it to a national chain shop, and were quoted $480 for a new alternator.

They brought it to me, and what I found was a good battery, a strong alternator and a loose positive battery terminal. 5 minutes with a battery terminal brush, a 10mm socket & some terminal protectant and they were back on the road.

Graham · #5 09-17-2009, 04:23 PM

I picked up a battery load tester at Princess Auto (our Harbour Greight). It puts a load on battery while testing. This will show if internals are failing. Any battery shop should test your battery free of charge if you ask them.

I had similar problem with my SL's battery. Kept charging it, even overnight, but it would run down again despite it seeming good on DMV after charging.

New battery did the trick - Bought one at Walmart that was not too expensive and yet a brand name (forget what they sell - maybe Exide ?)

GGR · #6 09-17-2009, 07:09 PM

I have checked my battery connection and everything looks clean and tight. I have bought a multimeter, and here are the results:

engine non running, measure at the battery gives 12V.

I then succeeded in starting the car, even with the engine cranking very slowly (note that a few hours ago and after a 40mn drive the engine would barely crank at all, so having a bit of rest did some good to the battery). Engine at idle I read 13V. revving the engine does not seem to make much of a difference, the needle stays on 13V.

Engine running, I put headlights, ventilation fan and whipers on, and the measure was 12V. Note the idle went slightly lower, I do not know if this is due to a weaker ignition due to the consumption of all this, or if it is because the alternator demanded more energy to respond to the increased demand in power (if the latter then this would mean it is charging).

I then stopped the engine and shut off all the things, and the measure was 12V.

I then switched on everything again without starting the engine (note the fuel pump was also running) and the reading was 9,5V.

I switched everything off and the reading was 12V again.

I tried to start the engine again but by then it was not cranking fast enough anymore...

What is it doctor?

Graham · #7 09-17-2009, 07:47 PM

Sounds like you need new battery!

If you need proof, try putting that battery into a different car.

GGR · #8 09-17-2009, 08:03 PM

Well, the battery is definitly low, but is it because it is worn out or because it is not getting enough charge? I guess if the voltage goes up from 12V engine non running to 13V at idle it is getting some charge... But what would explain such a sudden failure? I use that car everyday and the battery has always been strong. When a battery is on it's way out it weakens gradually...

I cleaned contacts in the alternator triangular socket before taking any measure. Assuming something was wrong there and now fixed, I would need to run the car long enough so that the battery gets enough charge, and then try to restart it...

barry123400 · #9 09-17-2009, 08:05 PM

I assume you have checked the belt tension. If you can turn the alternator fan blades by hand the belt is too loose. Common problem and the alternator light will function but for all practicle purposes the alternator output is too low currentwise.

GGR · #10 09-17-2009, 08:18 PM

Hum, that's a good idea. So you confirm that light is indicating the alternator charge, not the oil pressure.

I know the belt is not that tight and I'm already at the maximum (because I'm using a 4.5 belt on a 3.5 engine!) But I drove 10.000 miles like that all across the States this summer with no problem. May be it is now too worn out. I can't check it righ now because mind you, I made it to restart the car again ans is now idling in an attempt to recharge the battery, assuming cleaning these contacts did something...

But frankly, that battery behaves like a good battery that didn't get enough charging. Even if the engine cranks slowly, it remains constant, while a dead battery would not be able the crank the engine at all after a few seconds...

GGR · #11 09-17-2009, 08:34 PM

Ok, while the car was idling, I took the multimeter and it read 10-11V at the battery. I stopped the engine after it idled for about 20mn, and the measure still read 11V. I tried to start the car again but it would bearly crank once or twice. I don't understand why the readings are now different than before. It now looks like the alternator is not charging, but then I don't understand why the red light goes off. I have checked the belt tension and it is OK. I can't turn the fan blades. Any idea?

Graham · #12 09-17-2009, 09:28 PM

If you charge the battery over night, then install in car, if the battery is good, it should spin the car over fast like a new battery. If it doesn't you know the problem is the battery. Check the voltage across the terminals when the charger is on. It should be about 14+v. You could also check the battery voltage while someone spins the starter - Not sure of exact number, but it should not drop below about 10v.

Car batteries don't fail slowly like a flashlight battery - they can go from being good one minute to bad the next. Don't ask how I know that! Basically a cell shorts out internally. If your battery is not sealed, you could also try measuring the acid density with a hydrometer after charging - all cells should be same.

Do you have another car? Try swapping batteries - if you see same thing with different battery, then you know you have a charging problem or a current leak. May be better to put your bad battery in the other car and see if it can recover there! Better than ending up with 2 bad batteries if the problem is in your car. Sometimes diodes in alternator leak current and run batteries down.

GGR · #13 09-18-2009, 10:04 AM

I got a battery charger from my neighbor and started charging the battery with the charger set on 10A. At the beginning the needle was well on the right, indicating the battery needed charging. After less than one hour the needle was all on the left, indicating a fully charged battery. The battery was not hot though, and no sign of bubbling as I usually see when I charge a battery. I put it back in the car, mulltimeter showed 12V. Started the car with difficulty, as if the battery was low again. When it finally started, multimeter read 13V at idle. Stopped the car, took the battery out and back to charging. The needle again showed a low battery, but went to fully charged in a few minutes. Battery still cold, no bubbling. multimeter reads 14V when I use the battery charger.

It could be my starter motor shorting, but I now really suspect my battery. I have to find another good battery and see if the engine cranks well. If yes, then it's the battery. Strangely enough, in my experience dead batteries would not take the charge and the needle would stay on the right, indicating a low battery. Why would this one first indicate a low battery and then a fully charged battery within a few minutes?

meltedpanda · #14 09-18-2009, 11:46 AM

I would rule out starter as a possibilty as well.

cgoodwin · #15 09-18-2009, 12:09 PM

Fully charge the battery and then tsake it to a shop and have it load tested. If good ut it in the vehicle and check all connections, they should be clean and coated in dialectric grease. Disconnect the - terminal and put a test light between the battery terminal and the - cable with everything off, if it lights at all you have a drain (Might want to pull the fuse for the clock). Put a miltimeter on the battery and have someone crank the engine, if the voltage drops below 10 then your starter is loading up. When running above idle you should see 13v to 13.9v. The simplest thing to do is go to any decent shop and have them run a diag test, all they do is connect to the battery terminals and put an amp clamp on the + cable, the car is started, then run above 2000, then at idle. The machine (Sun VAT 65 or similar) measures the battreries response to the starter load, the amps of the starter under crank, then loads the charging system both at idle and at 2000 and gives you the results. Really hard to do without 4 people, load testers and multimeters.

The most common issues I see with Bosch charging systems in Mercedes are bad cable connections and worn brushes. One alwaqys wears faster than the other, usually the inner, it can cause the alt to "Surface charge" where it produces volts but no amps, the light goes out but the batt does not charge, or particular to mercedes, all the dash lights will start to do wierd things, the gauges will bounce around and still the charging light will not light. Replace the brush pack and all is well.

I junked the alt on my 220D and replaced it with a 85amp unit off a volvo, all I had to do was grind one of the pivot points on the alt about 3mm. On my 560SEL when the alt went I replaced it with a 130amp alternator from a Chevrolet Suburban, it was $60 and was an easy fit with a little grinding on the alt body.

There is really no shade tree substitute for a good charging system tester, I use a Bear Sabre I got for $100 when a local parts place closed, it has paid for itself a hundred times.