Electrical System advice needed... - PeachParts Mercedes-Benz Forum

DrLou · #1 06-17-2012, 11:15 PM

Benz Friends,

Wanted to run by all of some recent symptoms - someone will surely have some thoughts on this:

First: I definitely need to replace my alternator - got a burning smell a few days ago, and now no charging.

Meanwhile, I've been trying to charge 'er up with an external battery charger, and am seeing weird stuff:
The red dashboard 'battery indicator' light remains on, all the time, while charging. It alone is clearly draining some power. Why would it even be on with the charger connected?

Through all this, I cannot seem to get a decent charge into the battery.

So, assuming I have a short somewhere which is draining all charge, I've removed all fuses one by one to try to isolate an offending circuit. No Joy! There doesn't appear to be any fused circuit at fault here! Huh?

Of course, I'm now wondering if some short is putting huge load on the alternator, and may be the root cause of my problems.

Hmmm... How to diagnose? Just happen to have read a thread about the auxiliary fan relay sapping juice. Any such thoughts? Where best to start?

Many thanks in advance. Lou

LoosBenz · #2 06-17-2012, 11:42 PM

One of the components of the alternator may actually be the culprit/short. Defective diodes in the rectifier can cause all kinds of weird symptoms. Have you disconnected the alternator to isolate it and tried to charge the battery?

BTW, what year/model?

Jeremy5848 · #3 06-18-2012, 01:01 AM

If the "charge" lamp remains on it when charging the battery (with the key off?) then there has to be a short-circuit or failed component somewhere. As Louis has indicated, the type of car can be significant. You should put that information in your signature.

Jeremy

DrLou · #4 06-18-2012, 08:02 AM

Louis, Jeremy, Tks for feedback...

I haven't yet unplugged the (dead?) alternator from the system - will try this.

But am wondering what in the system - un-fused - could be drawing so much amperage? IE, starter works; is not 'jammed'. Are there big pre-fuse relays which could do this?

On signature: Corrected! (Didn't realize 'Vehicle list' wasn't public-visible)

Lou

vstech · #5 06-18-2012, 08:29 AM

yeah, on the SD, it's simple to pop the spade connector off the alternator to verify the short is in there!

jay_bob · #6 06-18-2012, 08:48 AM

I'm thinking blown diode in the alternator.

There is definitely a path for the charge light to be always on if you have a blown diode in the alternator.

Disconnect the negative battery cable, then detach the electrical connections from the alternator.

Now let's check your diodes. If your meter has an option for diode test, select it. Put the red probe (which should be plugged into the V/ohms/amps socket) on the B+ terminal of the alternator and the black probe (Which should be in the COM socket) on the frame. You should read no continuity. With the leads in the other direction you will probably get a reading, because you are now reading through the diode in the normal direction (forward bias for you EEs out there).

Now try the D+ terminal, to the casing, there are a set of diodes there too for the field ecxitation.

If your meter does not have a diode check it is not guaranteed which lead from the meter is positive when in ohms mode. Therefore you will have to check both ways. You should have no continuity one way and something the other way.

DrLou · #7 06-18-2012, 10:33 AM

Tks, jay_bob, will certainly do some testing

However, given that this alternator has seen less than 4 hrs of use, and it's certainly burned up, I'm just getting it replaced. 65A, Rebuilt by Beck/Arnley, if we're keeping score!

My larger concern is that there isn't some huge current sink - apart from the alternator - which will overload, and burn, the replacement. Are there any other potential culprits?

Quote:

Originally Posted by jay_bob

I'm thinking blown diode in the alternator.

There is definitely a path for the charge light to be always on if you have a blown diode in the alternator.

...

Now let's check your diodes. ...

jay_bob · #8 06-18-2012, 10:59 AM

You can check for this by using a test light with a bulb in series with the negative battery post and cable. Large current demand will equal bright glow from bulb. If you get no or dim glow put your meter on highest current range it has, and put that in series with your battery. You should have less than 1A of current at rest.

Or when you hook the battery back up are you getting fireworks? Small crackles are normal, from things like the clock capacitors charging up, if you left the door open, your stereo, etc. but if you get big snap bang you still have a short.

Be really careful, the battery does produce hydrogen gas and it can explode from sparks. Don't be in the line of fire over the top of the battery, and wear safety glasses for sure.

DrLou · #9 06-18-2012, 08:50 PM

jay_bob,

First, yes: With the alternator disconnected, the the dashboard 'charge' indicator light now goes off. As I already know the alternator is bad, this comes as no surprise.

Have also tested amperage, both the poor man's way - via the 'spark test' - and with a meter in series from battery terminal to cable. The battery is not yet fully charged, but the results are helpful; the car draws:

- 4+ amps with the alternator connected (to clarify, 2A of this is the electric vacuum pump in trunk, which shuts off after 20 secs or so), so
- 2-ish amps even with it disconnected (this seems quite high to me; wouldn't a couple hundred mA be more like it?)
- (so the above 2A was just the electric vacuum pump, running while I couldn't hear it!)

Quote:

If i haven't made the above confusing enough: The current drain dropped to 0, once I'd disconnected the cooked alternator, and after waiting a few seconds for the vacuum pump to shut off. Thanks to you guys for help with this.

jay_bob, have you (anyone) tested the amperage load in this car, at 'rest'? Wonder if it's available in specs somewhere?

Other (perhaps) useful data - I had this battery load tested at AutoZone at 850 CA about two weeks ago - AOK.

Quote:

Originally Posted by jay_bob

You can check for this by using a test light with a bulb in series ... with the negative battery post and cable

jay_bob · #10 06-18-2012, 09:36 PM

Sorry I'm on the road now so don't have ready access to my 240D to pull a measurement.

However I agree with you that 2 A resting load is too much. The draw going up with alternator connected is definitely no good and like you said the alternator appears to be bad. 4 A will drain a battery in a hurry, especially if it is not getting recharged when you drive.

First check the obvious things like dome/trunk lights on, light switch cocked slightly to left, which activates the standing light (lights on buzzer does not sound in this mode so it's easy to miss in daylight).
Aftermarket stereo with amp in trunk drawing power, seen that before.

If not then start popping out fuses 1 by 1 while monitioring the amp draw to see where it's going. When you have all the fuses out you should be at zero. If not look at your GP relay. Sooner or later you will find it, luckily there are not that many places for it to go in a 123.

Don't forget to keep an eye on the electrolyte level in the battery, car batteries don't like deep discharges and the water boils off. Hate to see a good battery get toasted during the ordeal of fixing the charging system.

DrLou · #11 06-18-2012, 10:15 PM

Yeah, I've recently pulled out all of the fuses one by one - but the alternator was still connected at that point; will have to do this testing again.

The Blaupunkt Radio - yes, with CD changer in the trunk (anybody remember CDs?) - has been unresponsive for about a year. It's occurred to me this might be implicated.

Have also recently replaced the Glow Plug Relay. It's virtually brandy-ass new!

(Now waiting for an RMA on my alternator)

DrLou · #12 06-26-2012, 03:10 PM

Ok, Diesel Friends,

More developments on the alternator replacement saga; in short, it still isn't replaced.

After the initial replacement alternator (we'll call this 'A') burned up, and I verified that it was itself drawing down the battery, I think I've got all the major mysteries out of the Why is the battery discharging so quickly? question... I may even have had the trunk light shorted on. In any case the total draw was only about 2A, enough to drain the battery, but not enough to cause SAD: (Sudden Alternator Death!)

So, now onto the Next Step, that of replacing the (replacement!) alternator. As I mentioned, 'A' was a '65A' rebuild from Beck/Arnley - this is the one which lived about 4 hours and burned up. So, I RMA'd it for yet another one - problem is, the output ratings are drastically different between the two! (Beck/Arnley includes a 'Computerized Test Report' with its rebuilds)

In brief: Alternator 'A' (burned) was putting out 64 A at 2500 (shaft) RPM
Alternator 'B' (a replacement/identical part #) was rated only 50 A at 2500 RPM.
Again, both ratings above are from Beck/Arnley's own test reports. 'A' and 'B' are precisely the same Part #, sold as a 65A alternator.

Without measuring both belt wheels, I can't say how engine RPMs translates to alternator RPMs, but can guess 2500 at the alternator might be around 1000 engine RPMs. Anybody know for sure?

My subsequent conversations with their 'Customer Support' people have done nothing to bolster my confidence, lemme tell you. They are trying to convince me of all kinds of crazy things! Before getting into all that, though:

Does anyone have specs - ideally, a tabular output curve - for a 65A alternator? I need a reality check here.

(vstech - I'm lead to believe, via other threads, that you may be the authority on this...)

Any help appreciated.

vstech · #13 06-27-2012, 12:01 AM

sorry, I'm hardly an authority on any electrical specs!
I'm a good parts changer though!
the only thing I have high knowledge on is the a/c system in the car. (refrigeration in general is my thing)

DrLou · #14 06-28-2012, 09:00 AM

Wonder if anyone else here may have data on this?
Does anyone have specs - ideally, a tabular output curve - for a 65A alternator? I need a reality check here.