Strange Alternator Problem

How about the simple solution...a small short on the wire to the alternator?

Drew K

Thanks Drew. Alt wire is good.

Check the ground too. I have a funny memory of some older mercedes or jaguars being rigged with a positive ground.

Drew K

The last cars with a + ground were built in the '60s. CANBUS is ultra modern in comparison.

"Never did the twain meet" so to say....

Bear in mind that the alternator feeds power to a regulator that keeps the voltage from being too high for everything. The regulator sends the correct voltage to the battery and burns off the excess voltage as heat.

If there is a problem in the regulator, that would explain the limited amperage draw. Generally diodes fail in the open or shorted condition. If they are open, you get no current. If they are shorted, you should be blowing fuses.

The regulator, on the other hand, has the feature of shedding excess voltage through a resistor, thereby generating a little bit of heat. If the diode like switch in a transistor of the regulator shorts, it may be connecting that resistor to the line voltage, thereby pulling only 1.4 amps...

Hi Joe,

Thanks for your tips. I have a question for you about excess voltage. I know that some alternators adjust the voltage via changing the field current (alternator puts out more or less current). Do you know if the Mercedes voltage regulator burns off the excess voltage as heat via a resistor? For the C240 (W203) I think the regulator is part of the ECM, but I'm not sure.

German engineering would never ever use such an awful method of voltage control.....

By the way, is the CANBUS actually connected to the alternator on this car or not?

I found out that this year (2005) of the C240 does not use the ECM for voltage control (for newer models it does). I am not sure if the CANBUS is connected to the alternator, but I don't think it is.

Then a properly selected scrap yard unit, more powerful amps wise is usually better, the problem will be the fittings and the lining up of the pulleys correctly....you should find one for under US$100.

Just make sure that it has an internal voltage regulator and if possible, buy the connector with some cabling attached to allow proper connection. Though I thing there are only 3 connections if I remember correctly, chassis ground, + output and one other, but its a long time ago....(apologies if I have been misleading here!)

See here:-

Best of luck.

Andy, you're right, this is the correct set up. I'll have the mechanic check for a good used unit. There's a huge market out here for used parts, so it should be no problem to find one. I'll tell the mechanic to chance the brushes while the alternator is out.

"Bear in mind that the alternator feeds power to a regulator that keeps the voltage from being too high for everything. The regulator sends the correct voltage to the battery and burns off the excess voltage as heat."

Actualy the regulators for Automotive Alternators vary the excitation voltage of the field windings which in turn varies the output voltage.

There no need to "burn off" any excess voltage as you would do with a linear regulator in a PSU.

In Some of the more recent VW's the can buss controles the output current from the Altrenator to assist with braking and to minimise engine load when accelerating.

This idea might be complicating things a bit but it might be possible..

Perhaps the MB in this case uses a similar system and perhaps a faulty sensor is causing the EMS to tell the Alternator to produce more power ( but of course it cant because the engine is not running)

That is so far away from the truth to be ridiculous.

The voltage and therefore the amount of current/power produced is controlled by the battery voltage being monitored by the regulator and this controls the alternator output, by inputting current, via the slip rings to the rotor.

Low battery voltage, regulator increases rotor current, high battery voltage less current.

Here is a good YouTube video that tells you HOW it REALLY works in an easy to view format:-

How a Car Alternator works

You will laugh at yourself once you have seen it !! But at least nobody will laugh at your explanations anymore!!

Even the old dynamos were regulated and did not use resistors to burn off power as you appear to think and that is really going back a few years......'60s and earlier.

I think I see your point. If the diode is in some odd state (not fully open or closed), a voltage across the + and ground will go through the diode vs normally the diode blocks it. If this is the problem, I'm wondering why it's intermittent and also why it stopped after the alt cable was pulled and plugged back in? Can a diode fail intermittently?

If the regulator was bad, wouldn't the problem be consistent? And if the mechanical parts of the alternator had a short, wouldn't the bridge rectifier block current through the alternator and to ground?

If there is a capacitor that holds the circuit in conducting mode, wouldn't the problem come back as soon as voltage was applied (starting the car) and continue after the car was turned off vs being intermittent?

We are baffled by this problem and I may be thinking wrong about the solution. If you can expand on your ideas, I would greatly appreciate it.

Thank you.

Are you sure the battery is being drained? ...because it sounds like a bad ground connection to me....

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....also update on 5 watt solar battery charger, I had a problem with my battery running low, do to vehicle sitting unused for weeks at a time....The solar panel sits on the dashboard and plugs right in to the cigarette lighter socket....works great....Car wouldn't even turn over, but after 3 sunny days it cranked right up....what a find!...will be installing charge controller today....sure beats pulling the battery out to charge it....

Solar, it's definitely a drain on the battery. The mechanic has been able to duplicate the problem and a draw of 1.4A shouldn't be there when the car is turned off. I don't think it's a ground problem, since once the alternator is disconnected and reconnected, the problem goes away.

Thanks for you input.

In that case it sounds like a bad diode....

http://www.youtube.com/watch?v=xgikeXt91vM

If the diode were bad, we would have an open circuit at the diode or a short. I recall from college that diodes do get weak, without a catastrophic failure. I just don't know if that could happen intermittently.

You seem to be overthinking this, just change the alternator and be done with it....imo

I'd tell her to do that, but the job is over $600 including labor. I recall being told to never use non MBZ alternators in later model cars.

If she cannot afford the prices of Mercedes parts, she needs to drive a simpler, less expensive to repair car.

Wait up till a back axle or gearbox needs replacement on the Merc, original replacements can cost up to $5,000 + labour or more each, depending upon the model......which is why I drive a Japanese car....BMW is no different to Mercedes.

Some years ago, if an ABS sensor went on the rear axle of a Merc, you had to have the whole rear axle replaced.

I am sure that it is better today, but how much better I cannot say.

Rich people drive new Mercs, poor people are the ones who drive ex rich people's Mercs.....repair costs are tremendous, $600 is nothing......which is why secondhand the cars are relatively cheap for their age.

Andy, what you say is true. Japanese cars are designed and built to run a long time with minor problems. She loves her C240 and it's getting to the point where it's going to start getting expensive to own. The C Class is the most basic Mercedes we can buy here - I have friends who swear by Mercedes and they aren't buying C Classes. They can afford the repair bills, or they lease them for a 3 year term.

Here in LA County, there are so many European cars. Most should not own them, because they can't afford the repair bills. But they sure look good in the driveway and they do impress the neighbors, relatives and co-workers.

I have to admit that I own some European cars, but I do so for the way they drive and the comfort. And when they get too expensive to repair, they are sold to someone who "thinks" they're getting a car that will run for 1/2 million miles.

Best advice so far reading from the top, assuming that the regulator is in the alternator.

I would bet on the regulator, which is usually a part of the alternator nowadays, so to repair it, assuming it is in the alternator, you will usually have to replace the whole unit.

If its not, only the regulator will need replacement.

With luck, you may find a junkyard unit that is still good that will have the same fittings, assuming only that the CANBUS is NOT connected to the alternator on this car. Easy to check.

With CANBUS cars, even simple work is made far more complicated, often you cannot even swap out the radio/CD on such cars without going to a CANBUS specialist to get it working fully again, I know, been there and got the T-Shirt and the ragged nerves.....

Andy, a friend of mine had a C 230 with a bad stereo (Becker) that drove him nuts. The CANBUS received an error signal from the stereo and caused all kinds of error messages to show up on the display - he thought the car was going to self destruct. They went through all the circuits and couldn't find a problem. It also caused a parasitic drain situation, but this time it wasn't intermittent. Finally, they pulled the stereo fuse and the drain was gone, but even more error messages. My stereo guy had a spare Becker unit and they swapped out stereos. Voila! Problem fixed!

I'm not sure if the Command system is a good idea and it makes it difficult for regular mechanics to work on cars.

You could not say a truer word when you said:-

"I'm not sure if the Command system is a good idea and it makes it difficult for regular mechanics to work on cars."

I won't touch it, but luckily I know a real expert who I guide all my misguided friends who are Mercedes owners to.....

It makes a few people rich......

Andy, I own three Mercedes cars and at times they are frustrating, but very rewarding. My E-Class has a valve problem and is in the shop. At 118K miles, it shouldn't need a valve job, but my mechanic told me it's pretty common. He also warned me about the transmission, window regulators and catalytic converter. It's a very nice car to drive, but I don't like these problems happening at the mileage it's at. I also own an old 300CE that I love to drive, but the transmission isn't shifting properly, so it's also in the shop. Those old Mercedes drive really nice and the straight six is a smoother engine than the V-6. I also like the look of the CE, but the interior sure is dated - materials were better back then and it's still in good shape for a 1990. Finally, we have a C230 Kompressor coupe that was our daughter's car. She had twins and couldn't keep it, so we took the car back. It's a 6 speed and I have to admit that it's a good driving car. The miles are a little over 90Kmi and the only problem is the keyless remote (not the remote, but the car's locking system), a couple pieces of broken/missing plastic on the inside and the panoramic moonroof (motor works, but gears are bad). Oh yeah, this car eats tires every 25K mi. Still, a very comfortable car to drive, great seats, good gas mileage, pretty reliable and handles well.

I believe your experiences are typical - sadly. If parts were more reasonably priced, not a problem really.....

They do drive well when everything is working correctly, nothing else is quite the same...

As a valid alternative, I do find that the Audi A6 Diesels with the 6 cylinder TDI engine (2.5 - 2.7 liter depending on the year) are fine cars, even with well over 200,000 miles on the clock, they still drive like new cars, its a valid alternative with far fewer problems.

They also tow heavy trailers well too.....

A friend of mine sells high mileage A6 leasing returns and they just run and run and run.....as long as the maintenance is carried out correctly at the right distances, over 500,000 miles should not be a problem.

They are also very economical when one regards the performance available.....they are nice in either manual or automatic versions, the 4 wheel drive can pull heavy boats on trailers up launching ramps with ease....

The A6 Diesel is probably my all out favorite today.....

BMW are bad news with their "Run-flat" technology suspension and tyres, when the tyres are half worn, they are very unstable at high speeds on autobahn joins on curves, almost dangerous I would say.

The best fix is normal (not Run-flat) tyres.....but then there is no spare tyre available which is a problem.....I like a spare......I do believe that even the "well" is missing in the trunk......but I have not looked, I only deliver the cars.

Here in the US, Audi's are more expensive to run than either BMW or Mercedes. A friend of mine has an A6 Twin turbo Quattro. The car is awesome, but at 70K miles, the dealer charged her $6,500 for regular maintenance and she didn't even get tires. The car is now at 119K and the dealer wants $12,500 to fix a couple oil leaks and routine maintenance. The car isn't worth that much!

The worse that I've had myself is my Porsche at 14K miles. It was in for some warranty repairs and I figured I'd do the first oil change. The snooty service writer turned up his nose and proclaimed that the don't do oil changes on Porsches. A Porsche needs a full service, which includes cleaning the air passages, lubricating the suspension, checking the car thoroughly and changing the oil - to the tune of $721! Needless to say, I took it to my mechanic and paid $125 for a Mobil 1 oil change.

Maybe I should start exporting 3 year old A6s to the USA, at under $20,000 each.

Exact I cannot be today, but maybe next week if anyone is interested.

Yes, the oil got changed, but you failed to get your "passages" cleaned.

Sure sounds like a faulty regulator to me. Very common problem.

Was any work done on the car prior to when the problem started?

How about a ground bond removed somewhere?

Something oscillating? Put a scope on the electrical system.

I wish I had some sort of diagram of the electrical system. We can only guess.

What about insulation breakdown within the altenator?

How does the environment enter in the equation? Temperature?

With only guessing, which is not good to do. The ECM or alternator could be to blame. But, what if it was the ignition switch? The ACC section, not turning completely off?

of course, you can't fix it if it isn't broke.

Or just put a battery disconnect on the car and disconnect the power when she leaves the car for more than a few minutes until you can replace the diode or alternator. saw something to day I have never seen before an alternator cooled with antifreeze in a BMW. They were amazed when they took the alternator out of the front motor bracket and the antifreeze went in to the drain pan by the grace of God and lucky mechanics it did not even make a mess on the floor for the floor kid to clean up since he hates antifreeze on his floors go figure. Duke

It could be a faulty Regulator in the Alternator

I'm thinking you could by-pass the alternators sense wire and fool it with different voltage ranges to see if its functioning properly.

WAWAUS:

Old school, probably. What I hear now is the the ECM controls the alternaator on some vehicles.

A scope should show diode anomolies, but something liks bad insulation that shows up when the car is warm or it's hot out or raining etc. might be another variable.

I've has two wierd alternator problems:

1. the car died without warning. Rotor wire fell off on the rotor itself.

2. The harder one. I had a corroded bulkhead terminal to the idiot light system.

I can think of some system that say decides to oscillate and draws lots of current. Disconnecting and re-connecting the alternator fixes it.

That's why I'm suggesting a scope to look for AC ripple, both when charging normally and when the system fails.

Now if there is an oscillating anomoly, finding it could be fun. Disconnecting and re-connecting anything might cause it to stop and, of course, it won't re-start when re-connected.

I want to thank all of you for your replies. It got me thinking and her mechanic and I have an idea. Let me know if you guys agree.

It seems like it's the voltage regulator, which I found out is in the alternator for the 2005 C240. I'm pretty sure the bridge rectifier is fine - I got some good advice from another part of this forum on diodes going bad and I don't think that's the problem. So our hypothesis is a bad VR. A new alternator is over $600 from MBZ and we don't want one from a rebuilder - I heard they're trouble.

So, to prove its a bad VR, the mechanic is going to put a switch on the Pos terminal of the alternator. At night, she can flip the switch, which will remove the alternator from the circuit (two sense wires will still be connected, but they're not what we're worried about). In the morning, she can throw the switch and start the car. If it works for a week, then we know the problem is the VR. If it won't start one of the days, then it's something else.

What do you think?

If you can't afford to fix em get something that you can afford. Why play around with a simple issue over a few dollars. Just my thought's

No need to wait that long.

1. Put in a "keeper" battery

2. Lift the negative lead off the battery and install a switch in series, then reconnect.

3. Remove "keeper", then place ammeter leads across the switch.

4. Open switch then read current.

5. Now remove Positive lead from alternator, record current, replace.

6. Now remove alternator connector, record current, replace.

7. Remove in reverse order, note any changes, then remove both leads, note any changes.

There's only 4 outcomes

1. If the current drops only on first test then battery is backfeeding into alternator.

2. If current drops only on second test then battery is backfeeding through sensing line into alternator.

3. If current only drops with both removed the problem is in the internals of the alternator so it's time to spend the $600.

4. If current doesn't drop with both positive and connector removed then problem is elsewhere, leave both disconnected and start removing fuses/circuit breakers one at a time until the current drops.

Note that I haven't said what's wrong in the alternator, it's a "black box" that gets replaced as a unit, unless you want to rebuild it yourself.

There are other test combinations that you can try, but I doubt that the order of lead removal makes a difference. If it does then consider repeating the test with the key out, key in accessory position, key in run position, before a start, after a start, after everything has warmed up, after everything has cooled down, etc. Sooner or later you'll develop a "truth table" that will pinpoint the conditions under which this occurs, but I think the first test sequence will tell you whether you have to go to these lengths. Soon you'll be testing the damn wiring harness connector by connector, then lead by lead! Let us know your results.

RAM, great advice, except once the battery terminal is pulled or the Alt wire disconnected, the parasitic drain stops. She has to drive the car for a while and then the problem reappears. This is why the problem is so hard to diagnose. My thought is to remove the alternator from the electrical system via opening an in line switch at night. Then close the switch in the morning and the car should start. Do this for a week to see if the intermittent parasitic drain reappears - I think it won't, hence the alternator is bad.

You're describing the action that a relay, solid state switch, or SCR would go through. It stays "latched" for some minimal holding current even though it was commanded to turn off, then when the power is totally removed it "drops out" completely, returning to its de-energized state. Once it is turned on again it stays in the "up" state even if commanded to turn off because of all the "sneak" paths that are available, only a total removal of power allows it to reset. Unfortunately there are hundreds of solid state switches in most modern autos. They're cheaper than relays and interface easily with all the computer control systems.

One thing I forgot to ask, have you checked the "Battery" light on the dashboard? In some cars it provides a path to both the ignition coil/spark control computer (SCC) and the alternator through one of those leads on the alternator connector, sometimes there's a resistor in the harness and/or the connection to SCC/coil/ballast resistor (if there is one). A bad connection or faulty ground might provide enough of a path or block one so that something stays in a limbo state. I found something like that in a friend's car.

RAM, A sticking relay was my one of my first thoughts, but I didn't think of an SCR. I'll pass this on to the mechanic. I"ll also ask him to check the SCC ballast resistor (not sure if there is one).

Great ideas. Thank you.

Maybe, maybe not. here is a schematic of a Typical Regulator IC

I think the ignition switch position is somewhat not right.

Pulling the alternator connector might actually be a better thing to do. This usually drives the idiot lamp and has (+12) from the ignition.

My guess is that the field is driven from the IGN terminal and not the battery terminal of the alternator.

The switch in the (+) lead would have to be rated for say 30 to 60 Amps.

Still having a hard time wrapping my hand around what to disconnect. Both have merit.

I think the schematic is correct. The field is directly connected to the battery, but is controlled by the VR. Pulling the alternator connector will remove the switched input and the lamp from the circuit. The 1.4A drain stops when we pull the Alt wire (from the battery). I don't know if he pulled the alternator connector when the drain was occurring? If it did stop the drain, then the ign or bulb circuit is causing the problem or the VR chip is bad. Hmmm.

You have a sticky relay (or something similar, not being familiar with your car ...) That usuually supplies the alternator field at start then goes oFf when the alternator starts to generate and the VR takes over. Switching off the key is supposed to swith that circuit off. Start checking the circuit from the ignition key ...

Good idea. I'll tell the mechanic to check this.

My wifes 2006 GMC Envoy had the same symptoms, after hours of testing such as disconnecting various "freds" (frustrating ridiculous electronic devices), and replacing the alternator, a friend recommended replacing the battery. Voila no more problem.

Bill

Thanks Bill. We already put a new battery in.

A long shot here. Isn't it possible that it's the position of the alternator shaft when the engine stops that determines "if" the intermittent drain occurs?

Could be a relay switching the alternator field current sticking(if it has this setup) or one part of ignition switch feeding field not opening.

How about the solenoid sticking? Worth checking.

I would try disconnecting the belt and very slowly rotate the alternator, with the key off, while checking the current draw off the battery.

Andy:

way back when, you said 3 connections. There is 4 in your picture.

Battery

Field

lamp

Ground

I actually wrote, to cover all errors:-

"Though I thing there are only 3 connections if I remember correctly, chassis ground, + output and one other, but its a long time ago....(apologies if I have been misleading here!)"

I did not check further.....is it a critical problem with the drawing? How can I do penance?

normally the power should flow to alternator when the ignition switch is on .please check the alternator relay (which supplies the power to alternator when switch on ) most probably relay have been stuck and passing the power to alternator coil (rotor) .

I thought about this, but if it were happening, wouldn't there be a larger draw than 1.4A?

The rotor turned on permanently makes some sense. See http://www.otherpower.com/images/scimages/1431/Alternator_Secrets.pdf In their example, they claim 12V at 3A is the max rotor current.

Do you have any idea what the rotor resistance is?

Does anyone you know have an oscilloscope?

The regulator does seem like a possibility, but so does the idea of a short depending on rotor position.

My alternator troubleshooting and repair have been starting from the late 60's. Sometimes, you just can't fix it. But it helps when you can take it apart and KNOW what's wrong.

Brushes can wear into the slip rings making it impossible to repair. Corrosion on the windings can break down the insulation.

Jumping the car can result in a blown diode. I said before, the two oddest problems was a spot weld on the rotor that "just broke" with no warning and the charging problem due to a corroded bulkhead connector. That last one, was the one that did not jump out at me.

There were two tests that are useful: Max output. This is nice to know, but at the same time if you make a test when things are working and put on all of the high loads, like AC on MAX, headlights on High beam, you can measure the voltage from the battery to the alternator. It will be in mV and the wire will act as a shunt, but it gives you something to check without calibrated equipment such as 30A.

You used to be able to ground a test point and cause the alternator to put out the maximum current. Finding that is probably as bad as figuring out how to open an Oxygen sensor connector.

My fear is that the regulator is oscillating or there is a rotor position causing problems.

The two most important checks are:

1. Max output or the ersatz max ouput

2. Look at the scope waveform under various load conditions.

3. Look at the AC ripple under various load conditions.

Trying to correlate the event to some external event is also useful. Like, i just got the car painted and "they" didn't tighten the battery cable. Blew the regulator or the car acted up just after the battery died and I got a jump.

I'd even like to suggest putting a 1 uf metalized polyester cap on the alternator output to ground.

We all know that $600 is a lot to put out for an alternator and the wrong way to fix things is to throw money at it.

A lot of problems are fixed by inspection. Taking the alternator apart and looking around can't hurt. You might find a hot spot or some other problem. You can probe the diodes.

The reason why I kinda quit doing the internal repairs is that it got harder and the internals were not available. But, I always took things apart and looked.

The only TRICK, I can think of is that when you re-install the brushes, you have to use a keeper like a paper clip or something and some bozo probably says you now need a tamperproof torx wrench to open the alternator. You generally don't have to remove the pulley. Mark the housing (alignment mark) before you take it apart.

I'm familiar with early Chrysler (late 60's), Toyota (late 80's) and Chevrolets (mid 70's).

Thanks for your tips. The mechanic tested the alternator and the output is good. I think it has to be the voltage regulator.

There is no alternator relay in most modern vehicles.

Your unit has a built-in solid state voltage regulator which gets its "turn on" command from the ignition switch lead that goes into the connector, the other lead senses the voltage somewhere in the wiring harness. If the regulator is faulty and is always supplying current to the field then 1.4 amps is not unreasonable, there may also be a turn to turn or turn to ground short in the field that adds to it. It's time for you to remove the alternator and have it bench tested, any reputable alternator repair shop will know if it's bad in less than 5 minutes. You can do it yourself by following the diagrams elsewhere in this thread.

RAM, you're correct, there is no relay in the circuit. I am convinced that the voltage regulator is the problem My friend has not had a problem since the switch was put in (every night she opens the switch and in the morning she closes it). The switch is on the red battery cable on the alt.

Throw 75 bucks at it and find out, shouldn't take 10 minutes to swap out:

http://www.ebay.com/itm/2002-MERCEDES-MB-C240-W203-ALTERNATOR-VALEO-A-0111546402-OEM-/111030753387?pt=Motors_Car_Truck_Parts_Accessories&hash=item19d9f2d86b&vxp=mtr

Thanks for finding this alternator. I'll send this to the mechanic.

If the replacement is $600, I would find someone to take it apart and repair it. It should have replaceable parts inside.

"When he pulled the connector, the current draw dropped to .1A - normal. When he plugged the connector back in, the current remained at .1A, not 1.4A."

This clue reminded me of when I had the exact same problem with my 1967 Pontiac. I also had an intermittent excessive current draw. (Note that my car is relatively simple, with no CANBUS and few auxiliary loads.)

My problem turned out to be my upper radiator hose pressing down on the main terminal on the alternator, where the heavy charging current output wire connects. The pressure flexed the contact enough to cause a partial short (not enough to blow fuses). The pressure was caused by the hood being shut - it pressed on the radiator hose, which in turn pressed on the alternator terminal. It was hard to find because I troubleshot with the hood open - I only found it by accidently leaning on the radiator hose.

My solution was cheap. I'd replaced the upper radiator hose with one of those cheap flexible radiator hoses, which was larger in diameter than the original hose and also just a little too short. I bought a new correct molded hose and noticed that it cleared the alternator with ease.

Have you had anything replaced around the alternator that could be pressing on its wiring? Or has the charging output terminal been damaged? The latter will probably require a replacement alternator, unfortunately. When you said that the technician pulled the connector off, and then found no excessive current draw when he replaced it, it made me suspect terminal damage. He flexed it by unplugging and re-plugging.

please also check the charging line which have the reverse blocking circuit (uni directional or reverse blocking to prevent back feed )a little leakage in a rectifier can accomodate reverse flow .