I've had a few weird symptoms lately that have caused me to worry about my charging system in my 1989 300E. My windshield washer jets have lacked power intermittently and the horn seemed weak a few times. I've been doing lots of city driving lately and it's snowing here so lots of defrosting/rear defrost, etc.

I took the car to an independent garage today for an alternator check and they said it needs to be replaced - they said it was not putting out enough current. I was skeptical and I pulled the multimeter out when I got home from work and here's what I found...

Battery reads 12.96V at rest, engine off.

With engine running at idle, NO LOAD, RPM 600, battery reads 13.74V.

With a pretty FULL LOAD (lights, defrost on high, rear defrost, stereo), RPM 800-900, battery reading goes down to 12.46V.

Kicking the throttle up to about 2000 RPM, with the same FULL LOAD, the battery reading went up a notch to 12.57V.

I pulled the voltage regulator and the brushes are ok. They're worn but both surfaces are clean and are definitely making contact. The voltage regulator itself was very dirty though and I cleaned it before re-installing. I'm done for tonight, so I haven't driven the car yet. I checked the readings again and with the engine running at idle, 600 RPM and NO LOAD it was slightly higher - 13.96V.

With these readings can anyone tell me if my alternator is OK? Could these intermittent symptoms be related to the fact that it's a stock 70A alternator and at low RPMs it is just not putting out enough current (amps) to handle the load and hence the drain?

I guess the voltage regulator could be ok, but something inside the alternator could be worn?? Diodes?

Any help would be much, much appreciated before I drop big $$$ for a new alternator!

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

Zeus, what is the condition of your battery? How old is it, do the terminals look to be clean, or have you cleaned them as well?

Your latest voltage output
>600 RPM and NO LOAD it was slightly higher - 13.96V
sounds much better with no load. Have you cleaned all of your ground connections as well, like body grounds? I know the electrical system has weak points, as I am still making discoveries with my 300E.

A new regulator kit may be in order for you though ( I would, after your succes on cleaning). Good luck

As my auto shop experience reflects, the instructor said,
"Charging system at battery should read 13.2-14.1 volts."
This is under load or no load conditions. Funny, what applies 30 years ago still applies today.

Schucks or Kragens will check it for free and pull load test on battery.

I bought an alt for my 190E for 75.00 with core charge included
and free shipping this summer. It cost more to send core back than core charge so I kept it for parts.

After battery hassles, I will only buy Optimina Batteries. You can get them at Costco.

The charging system on the 1986-89 W124 chassis is under-powered. IN 1990 MB installed a much more powerful alternator. AT dealer & many indenpendant shops that early 65-70A alternator is replaced by the later 80+A alternator. The only part needed to install the larger alternator is the wiring loom MB part number 124-543-45-26. The alternator is 008-154-48-02-88 OR Bosch AL66X

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Concerning my battery - it is a new Interstate battery - maybe two months old. So I don't suspect the battery at this point. All contacts are nice and clean, no problems that I can see.

M.B.DOC - Yeah, I have an 80A Bosch alternator (66X) on hold at my mechanic's shop for me. It's expensive though, and I don't want to install it unless necessary....neither do I want to get stuck somewhere this winter with a dead battery either.

So I take it I cannot determine the output of my alternator with these readings alone?

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

check your grounds.you may be getting voltage drop across the engine to frame ground strap or negative battery cable.this will definately affect the current reaching the various loads in your car

You probably have some bad diodes in the alternator, causing low output. The alternator in my 220D did this, total output was 4 amps when I had it tested. Not the only thing wrong with it, either, but that was why I took it in.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Click on Fastlane right here & you will see that the AL66X($165) is less than the AL65X that you car calls for!!

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Yes, I saw that, thanks for the mention though! Unfortunately, FastLane doesn't ship to Canada. If they ever do, they'll have a good customer waiting here!!

With the exchange, shipping, core charge and pulley from PartsShop, it's the same as buying from my mechanic - roughly $350 CDN. So I'll just buy local this time.

I'm trying to avoid replacing it though...I wish there was a good way to test the alternator output on a DIY basis.

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

Hi Zeus,

IMO your voltage measure @2000 rpm with full load is an excellent test. This represents "normal operation", and the alternator output voltage should be where glmoy suggested - 13.2 to 14.1 volts.

The 12.57 volts you measured actually represents a discharging condition (if your battery was fully charged).

I'd feel comfortable with a decision to repair/replace the alternator.

HTH,
Dan

__________________
2001 e320 wagon
2004 F350
2006 Crown Vic

Zeus,

Naturally, I would like you to support Fastlane first, after all they give us this great venue. However, you might call these folks to see if they ship to Canada.

Here is the link for alternators:

http://autoexpress.safeshopper.com/157/cat157.htm?276

The problem has been solved! Thanks again to everyone who replied and once again, to this forum in general.

I replaced my ailing stock 70A alternator with a new Bosch 80A (66X) alternator. All symptoms are gone now. Cars runs perfectly.

I did some before and after battery multimeter readings right before I did the swap -

BEFORE (70A):
rest - 12.85V
idle - 13.86V
idle with full load = 11.87V
full load, 1500 RPM = 11.74V

AFTER (80A):
rest = 12.88V
idle = 13.97V
idle with full load = 13.56V
full load, 1500 RPM = 13.94V

A few other unexpected pleasant surprises awaited me as well - I had an annoying flicker of my instrument panel lights at idle - totally gone. Lights are all nice and bright. The monowiper now zips across the windshield crisply and the washer jets spray properly.

The reason I decided to finally abandon the old alternator and install the new 80A alternator was made for me by Santa, er, my wife. She bought me a new Alpine 4-channel V-power amplifier for Christmas to further upgrade my stereo.

It was easy to install the new alternator. 2.5 hours total. Old one came off easily and I took it off without removing the serpentine belt. I loosened (but not removed) the fan shroud and I just loosened the two mounting bolts (13mm) and the old alternator slipped out. I cut the old wiring harness near the base and there were three wires - two 10 gauge wires and one smaller (16 gauge?) wire. I soldered new eyelet connectors to each wire and sealed with heatshrink. The 80A (Bosch 66X)alternator doesn't have a plug in harness, it has a large bolt and a small bolt to attach the wiring (see picture here in the Fastlane catalog). The two large wires go to the large bolt and the small wire to the small bolt. That's it - thanks to M.B. DOC for that info in another post!

To install the new alternator, I slipped it roughly into place and got the belt on the pulley first. I then attached the upper mounting bolt. Then, from underneath the car, I pulled down hard to get the alternator into place for the lower mounting bolt. It took some strength, but I'm no Hercules (no pun intended) and it wasn't that hard. I had the lower bolt ready in place, so once I had leveraged the alternator into position, I slipped the bolt home and tightened it. That's it, no removal of the belt necessary - not sure if everyone is this lucky, I'd hate to mess around with the tensioner. Belt tension is fine (and the serpentine belt is in good condition in my car, btw).

So that's it, I've got a new alternator, a happy Benz and a great new project to look forward to - installing my new Alpine amp. That's a post for another section though. Life is good!

Thanks again to all!

Chris

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

Nice when things work that well!

My brother was having trouble with his 300D with flickering alternator light and poor charging. Replaced the regulator, but that didn't fix things, so we inspected the wiring. Sure enough, the wire to the battery was corroded, only a couple strands holding. Dissasembled the plug, soldered the wire to the spade lug, and problem is gone.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!