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2006 W203 C280 Estate 187 000km
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306 Posts
Discussion Starter #1
Hi there,

Beginning of the year I had some flat battery issues. Turned out that the alternator bearings was shot, and the alternator coils shorted out.

They installed a rebuilt one (mistake, I think now, but then I didn't have another option).
I've had some problems with it during the last weeks. Sometimes it doesn't seems to charge.
Replaced the regulator, and I get about 15V on the battery terminal.

Sometimes, when I test it (pushing the close button on the windows when they are closed), the rpms drop, but at other times it doesn't.

Also, I have an unusual roughness that I can feel on the pedals, all of them.
Might it me the alternator bearings that are gone again?

How can I test it with my limited tools?

Regards,
Erich
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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16,844 Posts
Make sure that you are only using a alternator that was rebuilt by Bosch OR get a alternator through the MB dealer network. Alternators obtained through parts stores are usually nothing more than a cleaned up old alternator with a new set of carbon brushes on it, and these units have a very high failure rate.

15V is much too high for a standard 12V lead acid battery, and is indicative of a bad voltage regulator.

FWIW, the dealer supplied alternators come with a new regulator already installed.
 

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2006 W203 C280 Estate 187 000km
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306 Posts
Discussion Starter #3
I think the biggest problem is that the rebuilders put in the cheapest bearings they can find.
I will look for either an original Bosch if I can find it, or a good rebuilt from a good source.

The 15V was after the new Bosch Regulator...

Regards,

Erich.
 

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91 500SL 37k,90 300SE sold, 91 300E sold , 92 300E137k ,02 Kia Sedona ,I know..... its a Mini van.
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8,119 Posts
use a digital ohm/volt meter.test bat should be above 12vdv engine off.
start engine test bat. should be between around 13 to 14 volts ,turn on your headlights engine running. voltage will drop a little and then recover above 13vdc . if it drops below 12 volts alternator not keeping up. if you drove around like this it would drain your battery and you wouldn't be able to restart.
if voltage is good with test, alternator good.
if battery is below 12v Eng off after test.bad cell in battery.
check the grounds for the system,make sure there clean.
my 91 I got last month has really bad grounds,the wires look bad too.and the battery is dead also.
voltage reg probably having a hard time regulating,
if the alternator is noisy like bad bearings ,you should hear it
again check the main connections for battery both ends,if your rpm's drop and engine runs crappy when you use windows a/c or the accessories that have a high draw,possible bad grounds. if its only bad with using one window then I would suspect bad window motor and so on.
I'm sure there could be other causes .but those are the basic things I've found in cars with that prob.
does the car stall on you? sometimes? could be OVP relay behind battery overload protection relay. I'm not sure but 15vdc sounds kinda high
on my American cars is never above 14.7vdc
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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16,844 Posts
I have never seen anything over 14V on any MB I've ever owned.

All 3 of my W124s behave the same electrically. The diesel with the lowest amp output puts out no more than 13.5-13.7V (post pre-glow warm up period) as measured at the cigarette lighter AND that number is with the car at full temp @ idle with high beams on, fog lights front&rear on, rear defrost on and AC on max with both aux fans turning at normal (low) speed.

Static battery voltage (after engine off for 12+ hours) is usually just under 13V as measured directly off the battery terminals..

These cars are very sensitive to proper battery type to get sufficient voltage as the stock alternators tend to be a bit underpowered.

Any W124 should be using a group49 battery to have a large enough electric "pool" for all the ancillary devices to draw from. The sole duty of the alternator should be to keep the "pool" topped off, not to power all the vehicle devices directly.
 

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1992 300D 218K MILES 4TH OWNER
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15 Posts
From above, then, is it widely known not to test alternator while glow plugs still on (after glow period)? Both Advance and Autozone said "bad alternator" until after glow ended, then "good". This was on a '92 300D which has had 3 new alternators (one for each owner) during its life so far.
 

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E320/E250 Bluetec Ford F350 6.7l
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35,900 Posts
Actually GP would give you good load test if you can figure out when they work and when they stop.
Batteries and charging systems can be pretty tricky to troubleshoot.
I confirm above, that good charging would be in 13.5-13.7 margin. Good alternator will keep the voltage regardless the car load. Most of alternators on those cars are above 100 amps rated, while in normal conditions you can not put 100 amps load in the car. The GP and starter are the only items giving higher load, but they work for short time only.
So for battery testing you should get one of the load tester, while when your alternator (not counting GP cycle) is dropping the voltage at idle with headlights turned on -you might have bad diode. With bad 1 diode the alternator will still charge, just at one point might not have enough charging power to keep up with demand.
 

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1992 300D 218K MILES 4TH OWNER
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15 Posts
Ok, last post on subject w/o answer yet. Is it widely known on MB diesels not to test alternator / measure charging voltage in first 2 minutes of starting due to glow plug draw? I thought I might add something to forum but so far not sure. My 300D, and most other early MB diesels have 70-100 amp alternator which only makes 40 or so amps at idle and thus does not stay ahead of glow plugs. This indicates bad alternator, especially with quicky Autozone/Advance type test. Always wait 2 minutes or do alternator test first then shut off for cranking test! Oh, almost forgot, you can tell how many GP's are working by measuring across the fuse and dividing by 5 or 6 (when all are good). Should be about 0.1v for 5 GP's decreasing in 30 sec. If this goes up, fuse is bad. It goes down in 20% increments for each GP and shows how long afterglow lasts. Handy.

Dick
 
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