Charging System woes.... - PeachParts Mercedes-Benz Forum

phinsup · #1 12-23-2009, 06:20 PM

About 5,000 miles ago I but a new alternator in, it was the "premium" from napa when my battery light came on. Now here I am 5,000 miles later and the same issue popped up, turns out my battery was bad so I replaced that.

Now the issue I'm having is that at idle the light is on VERY dimly lit, around 1500 rpm is goes away. So I'm guessing I'm back to having an alternator issue. on to my question.

Does the battery light just measure voltage? and if so at what point does it come on?

Thanks guys.

layback40 · #2 12-23-2009, 07:25 PM

I had a problem like that a while back. Turned out the 2 big heavy wires from the alt were not connecting properly. I ended up installing a new wire from the plug on the back of the alt direct to the battery, no prob now. It could also be a loose fan belt causing the problem. Did you replace it when you did the alt & have you retentioned it since?

Diesel911 · #3 12-23-2009, 07:27 PM

I have a feeling that each Alternator is going to have a slightly different Voltage that will turn the low charging light on.

This is where one of those $3 Volt/Ohm Meters (Multimeters) from Harbor Freight would come in handy. It would tell you what voltage you light is coming on at.

Then there is the usual suspects:
Belts not tight or oily

Oxidized or Corroded Battery and/or Ground connections

And, the Connector that plugs into the Alternator

The Regulator Brushes worn (easy to check)

phinsup · #4 12-23-2009, 07:31 PM

I replaced both belts when I replaced the alternator, checked tension and it's fine, replaced the negative lead and the positive looks brand new. I'll have to check the connection on the back of the alternator, this is something I have not looked at.

The regulator you are referring to isn't that built into the alternator, or rather would have been replaced when I replaced the alternator or is it elsewhere?

and i have a multimeter, so i guess the more appropriate question would be at what voltage is the light supposed to come on, ie i can test it and see if it should even be on in the first place in relation to the voltage being provided by the alt.

Diesel911 · #5 12-23-2009, 07:50 PM

With you Key in the off position you should get Battery Voltage at the 2 large slots on the Alternator Connector.
The 2 big slots connect to the Fat Red Wires.
The Blue Wire is the one that goes to the Low Charging Light. With the Key off out you will have no voltage there.

DrewGerhan · #6 12-23-2009, 07:55 PM

Go to AutoZone and purchase part number 14939.

phinsup · #7 12-23-2009, 08:04 PM

Quote:

Originally Posted by YoungBenz

Go to AutoZone and purchase part number 14939.

well this alternator is under warranty so I'm not going to go buy a brand new one, the thing only has 5,000 miles on it.

Hit Man X · #8 12-23-2009, 08:59 PM

Quote:

Originally Posted by phinsup

well this alternator is under warranty so I'm not going to go buy a brand new one, the thing only has 5,000 miles on it.

Simply have them refund your money.

Junkman · #9 12-23-2009, 09:22 PM

Quote:

Originally Posted by phinsup

and i have a multimeter, so i guess the more appropriate question would be at what voltage is the light supposed to come on,

Actually, the most appropriate question is "what voltage is your alternator putting out". Idiot lights give very little useful information. Since you have a volt meter, measure battery voltage without the engine running. 12.7 is fully charged. Next measure voltage across the battery terminals with the engine running. There was even a post within the last few days that quoted the FSM specs. Running voltage should be 13.5-14 volts at 3000 rpm with the engine warmed (meaning glow plugs aren't pulling voltage down). It is very common for a crappy reman alternator or voltage regulator to be bad out of the box or fail within a few miles. Also check for clean tight connections. These can be tested by measuring voltage drop. A generic google will turn up plenty of detailed information. Alternator rebuild procedure is well documented & used to be more common before cheap off shore remans became so readily available. A perusal of that information will turn up possibilities of what specifically may be wrong.

Diesel911 · #10 12-23-2009, 11:50 PM

If it is still under warranty drive it to AZ and let them test it. However, you need to have a fully Charged Battery for them to Test the Altanator while it is still on the Car. If they bring out their own Battery to do the test that is OK.

The reason a gave the answer I did concerning the Low Charging Light is that the little bit of current draw that the Low Chargeing Light uses; cause that Alternator to start Charging. Ounce it starts charging something inside of the Alternator itself causes the Low Charging Light to shut off.
Since a Rebuilt Alternator might use components from any where in the World during the rebuild process (Example: some aftermarket Voltage Regulators put out more Voltage than others) I would expect some varation in what Voltage would cause your Low Voltage Light to Light. Maybe even the Wattage of the little Light Bulb might have an effect.

layback40 · #11 12-24-2009, 12:30 AM

The reason the light goes out is because once the alt starts charging, the voltage in the alt where the light is connected becomes the same as the voltage at the battery/ignition system so there is no voltage diff across the light & so it goes out. If you have poor connections to the 2 thick wires, then the voltage in the alt will go higher than the battery & so the ignition light may start to glow some times. I would make sure all your connections are good especially where the thick wires join back in the elec system. I gave up when I had a problem & joined a new wire to one of the thick wires at the back of the alt & connected it straight to the battery. Have had not prob with the charging system since (80k miles)

phinsup · #12 12-27-2009, 05:40 PM

I tested the voltage at the 2 wires on the alternator and it's 12v, i also tested the voltage at idle and its the battery voltage 11.98 right now, it's got to be the alternator or the regulator. It doesn't overcome the battery voltage until about 1500 rpm, at that point it's putting out a whopping 12.52 volts.

patbob · #13 12-27-2009, 06:34 PM

It sounds like the alternator isn't putting anything out. Those wires on the alt are directly connected to the battery, so all you're measuring is the battery voltage on two different ends of a wire. The readings sure better be within millivolts of each other.

Despite it being a new alt, it sounds like maybe the main recifying diodes are fried or the voltage regulator is bad. You could try swapping out the regulator to see if maybe it has failed early.

Oops.. I take that back.. I got the voltage difference reveresed. The alt is putting something out, we know that because the voltage is higher at the alt terminals than at the battery. Assuming the 2mv isn't just fixturing issues, and making other assumptions, the alt is putting out at least 20 Amps, probably a lot more (like perhaps its full load). Are you sure the glowplugs are not stuck on? Your super loud woofer isn't sucking tons of juice? You are not trying to run everything electrical in the car all at once?

phinsup · #14 12-27-2009, 06:38 PM

Well i dug out the paperwork on this one, it's a year old. Reman'd by BBB Industries. I'm thinking i may take this one back and spring for a Bosch reman. Even though they charged me $130 for this one I don't think it's a $130 alt LOL

kerry · #15 12-27-2009, 07:11 PM

Could also be the regulator I think.

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