Battery light dimly lit up on idle...Alternator problems? - PeachParts Mercedes-Benz Forum

azitizz · #1 11-04-2014, 09:46 PM

ON my 85 300TD w123, I've been noticing for the last month and a half that the little battery light is dimly on when I start up most of the time, but not all the time. Sometimes nothing.

As soon as the rpm go up, the light goes off. Its a very new and large battery put in less than a year ago. Also the alternator was replaced only about 4 years ago.

What are the typical reasons for this symptom. As its getting colder I want to make sure I'm going to be able to start-up when its time...

Thanks

uberwasser · #2 11-04-2014, 10:00 PM

Could be a loose belt. Corrosion on battery connections or loose connections. A failing voltage regulator. Or a problem with the alternator. The first two are easy to check. You can visually inspect the voltage regulator by pulling it out but not all failure modes are visual. You should consider taking your car to an auto parts store and ask for their free charging system test.

azitizz · #3 11-04-2014, 10:04 PM

Quote:

Originally Posted by uberwasser

Could be a loose belt. Corrosion on battery connections or loose connections. A failing voltage regulator. Or a problem with the alternator. The first two are easy to check. You can visually inspect the voltage regulator by pulling it out but not all failure modes are visual. You should consider taking your car to an auto parts store and ask for their free charging system test.

Is the voltage regulator your talking about the one thats directly on the alternator? With the two carbon brushes that can wear down?

I know its not the belt or the terminals.

Whats a healthy voltage coming out of the alternator?

coolram62 · #4 11-04-2014, 10:04 PM

First make sure the battery posts are clean and there are good tight connections from the cable to the posts. After this o get a better idea if you have a multimeter (digital is easier to read) check the voltage at the battery. With the motor off (static) the voltage should be between 12.2vdc to 12.75vdc. With the motor running you want to see it between 13.7vdc to 14.2vdc. The voltage regulator brushes could be worn if the readings are below 13.7vdc.

ah-kay · #5 11-04-2014, 10:53 PM

I strongly suspect it is a worn out voltage regulator brushes. I have the same problem once with my 190D. Replaced the regulator and all is well.

uberwasser · #6 11-04-2014, 11:04 PM

That would be what I lean towards too.

To the original poster, if you want to understand the various components of the charging system to help with your approach to diagnosis, take a look at a synopsis a friend of mine put together a while back when dealing with charging issues. In his case, the voltage regulator looks OK, but ended up being the problem.

Quote:

1. The alternator has two outputs (called D+ and B+) B+ goes to the battery to charge it. That's the "main" output of the alternator, and we are able to measure it when we check voltage at the battery while the car is running. There's also D-, which is just a ground.

D+ shares the same voltage as B+. The only difference is that it is lower amperage. This is the famous "blue wire" and it is used by the voltage regulator. The regulator's only job is to sense the D+ voltage and variably increase resistance to the line called DF (Dynamo Field). This is the key to the whole thing.

DF is an input to the alternator. When the voltage regulator changes the resistance between D+ and DF, it is effectively raising or lowering the current that can go to the alternator's magnetic field. Altering the magnetic field strength causes the alternator to change output. More power to the magnetic field = more voltage from the alternator.

In other words, if the alternator is cranking out a lot of power, D+ is high and so the voltage regulator increases resistance, which lowers current to DF and causes the alternator to lower output. At about 14.5 volts, the voltage regulator will cut off the circuit entirely, which brings DF to zero and stops the alternator from producing any power.

Similarly, low D+ voltage means the voltage regulator will lower resistance between D+ and DF, to increase magnetic field strength. So to sum up, D+ is identical to B+ (battery charge) and is used simply as a feedback loop so the voltage regulator can know what to do.

This means that one possible failure mode of the regulator is for the integrated circuit to incorrectly cut off current to DF because it "thinks" that input voltage from D+ is too high. Note that this will cause the battery to fail to charge but not cause the charge light to illuminate while the engine is running until the battery is entirely dead, for reasons I'll explain below.

2. If the D+ output is at zero (as when the ignition is ON but the engine is not yet running), the voltage regulator lowers the resistance between D+ and DF to nearly a short. This means the alternator gets its DF power from the battery, via a long chain of components: BATT --> ignition switch --> charge light --> D+ --> regulator --> DF. This is why the charge light illuminates—there's a 12v differential between the battery and the D+ circuit. Power is flowing from the battery, through the charge light, to DF.

This is also what they mean when they say the alternator needs "exciting". E.g., the alternator needs a 12v power source to initially magnetize it. It won't produce any power otherwise. Once the alternator is running it produces its own power via the D+ line. (In the Pirate4x4 discussion, the initial 12v power source is referred to as the "field current supply," and it says that this is a separate input to the voltage regulator, but in the Bosch system it isn't.)

3. The charge light illuminates only when there is a voltage differential between the battery and the alternator, OR the alternator's D+ and B+ outputs. As I noted above, the light goes on when the car is turned on but not running, because the battery is supplying 12.6 volts and D+ has zero. Likewise, if the battery is dead or refusing to charge but the alternator is working, D+ will have up to 14.5 volts but the battery will have perhaps 11.6-12.0, and so the charge light might illuminate weakly. (It would be interesting to test if the 2-3 volt differential is enough to illuminate the light enough to notice in normal daytime use.)

Why would D+ or B+ not have the same voltage? Because they have separate diodes (3 each) that rectify the output of alternator (each one converts a "phase" of the alternator's AC output to DC). If a diode fails on either D+ or B+, output voltage drops by 1/3.

Actually, a single D+ diode failure can mean there would be much lower or even zero output at D+. According to the Peach Parts description, the drop in voltage at D+ would cause a cascading effect, since this would cause a commensurate drop at DF and hence a further drop in alternator output ... which of course means a further drop in D+. The overall drop in voltage would illuminate the charge light while the engine was running, which didn't happen on my car. So I can eliminate D+ diode failure from my troubleshooting process.

Similarly, B+ diode failure could mean the battery doesn't get enough voltage to charge. The battery will act as a "buffer" in this case, providing 12.6 volts for a while, but there would be a differential voltage between D+ and the battery. The voltage differential would be larger whenever the engine was revved because alternator output would go up a little with higher revs, even with a bad diode. So the symptom would be a voltage light that comes on weakly when running, and gets brighter as the battery runs down or the engine is revved up.

In my case, I didn't observe a charge light at any time. This doesn't prove all the B+ diodes are OK, since one could have failed and the rest could still be putting out some power, but I would have likely seen a dim charge light when I was driving home, especially toward the end of the drive when the sun was setting. The best test is to check voltage while the car is running, at the D+ terminal ("blue wire") and the B+ terminal (battery positive). They should be identical. (The trick is getting to the D+ terminal with a probe while the engine is running; it's down on the rear of the alternator.)

There's another set of "common" diodes in the alternator that affect both B+ and D+. However, if they failed, I think the alternator would have no output at all. Once again we'd have a large voltage differential between battery and D+, illuminating the charge light.

(By the way, as I'm sure you've read, if the charge light is burned out or incorrect wattage, it breaks the electrical circuit between the battery and D+, and thus the alternator has no magnetic field and will not put out any power. A burned out bulb would be obvious because the light wouldn't illuminate when the ignition switch is on. Mine does. In the case of incorrect bulb wattage, the charging problem would have manifested a year ago. So I can rule out the bulb as a cause as well.)

4. All of the above explains why the charge light is not illuminating on my car, either at idle or high RPMs. Even though the alternator's output voltage is too low to sustain battery charging, it is producing enough voltage to ensure that there's no significant differential between the battery and the D+, at least not enough to brightly illuminate the charge light. Most likely the alternator is putting out identical voltage at B+ and D+ (although I'll verify that shortly).

As we saw, the alternator did increase output to >13 volts when the car was revved. So if the alternator can produce >13 volts, why isn't it doing that all the time? Low output could be the result of mechanical damage to the alternator, but inspection shows that it is in relatively good physical condition (no oil dripping in to it, no rust, good belt connection, electrical connections appear OK). I'm still expecting the problem will be the regulator. My theory is that it's because the voltage regulator is misreading the D+ input OR it is no longer able to variably lower resistance between D+ and DF on the correct curve. I could not find any bench test procedure to verify this, and I don't have a variable DC power supply anyway.

Summary: If the charge light doesn't come on when the ignition is on and the engine is not running, replace the bulb and/or check all the electrical connections at the battery, bulb, ignition switch, and alternator. Note that instrument cluster grounds are not the problem because the battery and alternator serve as the grounds for each other here.

If the charge light stays on when the engine is running, the problem is likely in the alternator or battery. Check the battery first.

If the charge light isn't on when running, but the battery isn't getting enough power, it's either the voltage regulator or (less likely) a failed B+ diode in the alternator. Comparing the output at B+ (battery terminal) and D+ ("blue wire" on alternator) will isolate the issue. Replace whichever component is at fault, since neither are user-repairable.

psaboic · #7 11-05-2014, 08:47 AM

Well, I had the same issue a year or so ago. New regulator on the alternator fixed it.

toomany MBZ · #8 11-06-2014, 07:45 AM

My vote goes to a new voltage regulator.

TX76513 · #9 11-06-2014, 08:32 AM

Its a small diode - when it fails it allows power to reverse through it (that's whey the light is dim). It has a technical name that escapes me but referred to as a "backout" diode. Same type used in solar panels to keep the electricity from reversing when the sun goes down.

funola · #10 11-06-2014, 09:15 AM

Quote:

Originally Posted by azitizz

ON my 85 300TD w123, I've been noticing for the last month and a half that the little battery light is dimly on when I start up most of the time, but not all the time. Sometimes nothing.

As soon as the rpm go up, the light goes off. Its a very new and large battery put in less than a year ago. Also the alternator was replaced only about 4 years ago.

What are the typical reasons for this symptom. As its getting colder I want to make sure I'm going to be able to start-up when its time...

Thanks

I had a non Mercedes turbo diesel with a Bosch alt which exhibited the same symptoms. The previous owner never was able to find the problem and it took me a long time to find. After changing the alternator to a known good one and changing the bulb the problem remains. Turned out it was loose connections on the starter motor. Snugged them up and the problem was solved. Something to try if nothing else works.

azitizz · #11 11-23-2014, 03:03 PM

So testing at Idle wint no accessories running, my alternator reads 13.9 Volts after running for a few minutes. Ignition off the battery reads fine at 13.1 Volts. Ive recharged it a couple of days ago worried it may be dying and its a new battery. But I never tested it before I charged it. I could just tell the engine was harder turnign over...

Any Ideas?

SD Blue · #12 11-23-2014, 04:28 PM

Depending on if there were changes made, 13.9 Vdc may be a little high. What kind of Vac are you reading? You will get a dim charging light with high AC voltage from a rectifier diode starting to fail as well.

azitizz · #13 11-23-2014, 04:28 PM

I think this may be the problem. Took out the Voltage Regulator and see that one of the brushes are much sorter and are probably just skimming the alternator shaft at this point.

azitizz · #14 01-14-2015, 11:31 PM

Well, Ive exchanged the V.R. with a new one. No difference.

I get a voltage reading at idle with the car warm of 12.9 to 13.07 or something close to that. Never hitting 14 unless I rev it up.

Belt is relatively new, battery connections are spotless and tight.

I couldnt get the car started the other day because the battery was dead. So its obviously not charging enough. I have an Espar diesel pre-heater running on in to enable startups in the cold without electrical plug in. (had it for 6 years now)

It works great but I have to let it run a good 30-min to an hour for a fully warm engine. (one that I can switch-over to WVO immediately).

So next thought Im having is perhaps the Alternator itself...

leathermang · #15 01-15-2015, 12:59 PM

Quote:

Originally Posted by TX76513

Its a small diode - when it fails it allows power to reverse through it (that's whey the light is dim). It has a technical name that escapes me but referred to as a "backout" diode. Same type used in solar panels to keep the electricity from reversing when the sun goes down.

If normal tests are ok... really consider this one...
I think Lucas Electrical had something to do with this circuit when it came out decades ago.... and that ought to make some of you cringe right off the bat...
but the thing is... the circuit is designed to be balanced on both sides... when things are doing fine... but all electrical things have tolerances.... and some can deteriorate over time.... so many cars...not just MB... have this weak light on at idle without actually having problems....
You might be thinking about a Zener diode....

http://en.wikipedia.org/wiki/Zener_diode