Alternator/Charging System problems - PeachParts Mercedes-Benz Forum

deerefanatic · #1 03-03-2007, 10:07 PM

Ok, so I spent the greater part of today troubleshooting my car's electrical system to see why I have constantly low charge voltage... So here are my observations.

This car has an AL129x 115amp alternator upgrade, BTW.

With the Headlights, Driving Lights, Rear Defogger, and heater on HI setting the car will have a dropping voltage that finally settles around 11.9volts at idle.. Engine has to be revved up to about 1200 rpms, at which the voltage will settle at 14.5v and show about 80amps of consumption by the car. (I had an ammeter that monitored current traveling to the fuse box.) Headlights draw 10amps, as do the driving lights.... Just about right I figure for 60w bulbs... The rear defogger draws 19amps, and the CCU set on ECO with blower latched on hi draws 24amps.

Anyhow, It appears that at 800rpm idle the alternator can only put out about 35amps or so...

So, I decided to look into this... I measured the diameter of the alternator pulley and it is 3.25 inches in diameter. The drive pulley on the engine is about 7 inches. This equates to about a 2:1 ratio, meaning at 800 rpm engine speed, the alt will be turning about 1600 rpm.. Now, the alt bearings can withstand 15,000 rpm easily I'm told, so at a 2:1 ratio, they will never max out as 7,500 rpm is alot higher than our engine's 5,500 rpm limit... So, here is my hypothesis: The 55amp alt that was originally on the car appears to not be original (no MB stickers or symbols) but actually a replacement. I'm wondering if the pulley I have is two large? I would think that downsizing to a 2 inch pulley would give me 50% more power at idle, as now the alt shaft will be turning 2400rpm at 800rpm engine speed, which, right now 2400rpm is attained at about 1100 rpm engine speed, which is exactly where the alternator can keep up with my loads........ Plus, the 15,000 rpm alt shaft speed won't be achieved until 5,000rpm engine speed, which I don't see very often.

So, what do folks think of this? Does anyone know what the alternator pulley diameter is on their car? Anyone feel giving and want to measure their's to compare to mine?

deerefanatic · #2 03-04-2007, 07:03 PM

Bump.....

Anyone have any ideas on this?

Chas H · #3 03-04-2007, 07:12 PM

Hell, try it. But a smaller diameter pulley may not have enough "wrap" to drive the alternator at full output.
Or set the idle higher.

240Joe · #4 03-04-2007, 07:13 PM

I think it's fine like it is. If you're driving the car, you'll get enough charging when the RPMs are up to offset that small amount of battery current drain when you're idling. And you don't always have all that load on.

You've done sort of a worse case analysis that most cars probably wouldn't pass.

240Joe

deerefanatic · #5 03-04-2007, 07:21 PM

Except, I do spend quite some time in the "worse case" scenario alot of the time. I drive in town......... Sitting at a stop light with heater on, headlights and driving lights, and rear window defogger...... The little bit of running wont make up for the stop light drain........ So therefore, battery actually discharges over the course of the drive.....

Plus, I want to be able to start the car and turn on the heater and rear defogger without worrying about discharging the battery....

Raising the idle isn't a hot idea as it just cooks your tranny fluid quicker.... Plus, 800 rpm is mercedes recommended....... I kicked it up to 900, but I really don't want to leave it there......

240Joe · #6 03-04-2007, 07:32 PM

That driving scenario seems rather rare to me but OK. You could try the smaller pulley but I'm not sure that is a great idea. Remember that a smaller pulley has less contact area with the belt so it is more likely to slip.

I think you're going to have to get a special alternator, like one out of the police cars where idling is common and they have that large radio transmitter load to deal with, and the light bar.

240Joe

Chas H · #7 03-04-2007, 07:50 PM

I guess you're screwed. But maybe you could turn off the driving lights in town.

deerefanatic · #8 03-04-2007, 08:13 PM

Here's a question for you though......

Brandon314159 (no longer a member here, he's at Schuman's website now) posted on here that he had an AL129x that would run all the stuff I mentioned and maintain 14.2v...... That's why I upgraded this stuff, I dont need 115 amps of output, but I want to be able to run all my stuff and not have to worry about battery, EVER...........

Does anyone know how to positively identify the difference between a 115 amp and the 80 amp unit? The place I got this from has an 80 amp for sale here:

http://cgi.ebay.com/ebaymotors/MERCEDES-BENZ-190E-300-350-ALTERNATOR-W-WARRANTY_W0QQcmdZViewItemQQcategoryZ33573QQhashZitem7941870100QQihZ019QQitemZ7941870100QQtcZphoto that is the exact same price as I paid them for my "115 amp" unit. Actually, I paid them $100 even for the unit with free shipping and no pulley....

My alternator doesn't have the red band around it like the one in that listing does, but that's just paint too........

Chas H · #9 03-04-2007, 08:20 PM

An alternator with the suffix "X" is "rebuilt". I quit buying such years ago because the quality was not acceptable, so I take my starters and alternators/generators to a local electrical repair shop for rebuilding and testing. I suggest you do likewise. It's possible your alternator is not operating properly.

Jkid · #10 03-04-2007, 08:48 PM

Quote:

Originally Posted by deerefanatic

So, what do folks think of this? Does anyone know what the alternator pulley diameter is on their car? Anyone feel giving and want to measure their's to compare to mine?

I destroyed the pulley on mine and replaced it with a smaller diameter. I reasoned that if anything it would at least put out more juice. It works fine, I just wish I had measured the out put before and after the switch to see if there was any difference.

carnut · #11 03-04-2007, 09:09 PM

Locate a shop that can put a "load" on the alternator. Years ago we used a machine labeled a VAT 40. It had three leads, two to the posts on the battery and one inductive lead that clamped around the positive wire on the alternator. We would raise the idle to about 1500 rpm, turn a reostat knob to put a load on the alternator and verify it on a meter regarding AMP output. A 80 AMP would max out very close to 80 amps and likewise a 115 AMP would peg at just below 115 AMPS. Any alternator can be re wound at a rebuilder to produce more amperage and look physically the same.

deerefanatic · #12 03-04-2007, 10:07 PM

Cool.

There is a very reputable alternator repair guy not to far from me. I bet he can test it for me, and get me the smaller pulley to......

Like I think I mentioned elsewhere, I really don't mind if this thing only puts out 80 amps total, I just want alot of my total amps down at idle.

Come to think of it, I don't think it is an 80. It most surely is a 115 because, the other day when I was testing, I had an ammeter on the lead to the fuse box. I revved the motor up to 1500 rpm and block the throttle to hold it there, then turned on every device I could... Current draw on that ammeter was about 82 amps, and my voltage stayed at 14.2 volts, which means that the alternator can attain 80 amps with negligible voltage drop...... I would think an 80 amp alternator would have been seriously sagging in voltage (like 12.8 - 13.2 volts) when completely saturated like that......

Am I on track thinking this?