Alternator not charging no warning lights ? - PeachParts Mercedes-Benz Forum

rhodes2010 · #1 12-15-2009, 11:23 PM

On my 1980 300D that I have owned for 25 years and 200,000 miles
I have replaced alternators and voltage regulator/brushes and batteries.

So this week it died and I replaced the battery.
After several hours of headlight on driving the lights dim and battery dies.

I replaced the voltage reg/brushes, even though I get no dash lights for
battery or alternator(charging).

Found the alternator to spin a little to easy by hand so I tightened the belt too.

Still running off battery and car dies after 4.5 hours of headlights on.

I ran wires from the battery terminals into the cabin so I could monitor with a voltage meter.
Disappointed that I do not get a voltage boost or any change on volt meter.
Except for running basically down until it dies.

I am set to replace the alternator as I have no other place to go on this.

Is there a component other than the alternator and built in voltage-reg/brushes that could cause this situation ?

No light warning that battery or charging is in trouble.
Running on battery until it dies.

Any advice ?

Thank you all.

compu_85 · #2 12-15-2009, 11:29 PM

If the (small) field wire isn't working properly I don't believe the warning light will work, and the alt will never start charging.

-Jason

barry123400 · #3 12-15-2009, 11:32 PM

I would make sure the indicator bulb in the dash has not burnt out at least. With an open bulb filament the alternator recieves no excitement voltage to initialize and sustain charge.

#4 12-16-2009, 01:03 AM

A few months ago, I had a similar problem and it turned out to be the connectors on the alternator. The wires inside the plastic connector on the back of the alternator were corroded where the connected to the actual connector. I finally figured it out when I noticed that wiggling the cable with the engine running caused the voltage at the battery to change.

layback40 · #5 12-16-2009, 02:55 AM

I have had the blown red idiot light stop my alt from working. Other than that, I ended up replacing the main wire from the alt to the battery. The idiot light is the most likely problem.

rhodes2010 · #6 12-16-2009, 01:00 PM

Thanks for the replies. Several have alluded to the dash lights...
working or not working is the question.

When I turn the key to wait for glow plugs to light the dash lights
all light up, Brake, Battery, Charge, Seatbelt, Glow Plug.

So this makes me think the bulbs themselves are okay.

I will remove the connector from the back of the alternator and investigate
for corrosion.

Theorectically, should I be able to connect my voltmeter directly to
the alternator connectors and to ground to get a reading and see if it is charging ?

compu_85 · #7 12-16-2009, 01:50 PM

Yes, you can use a meter on the + output.. you should see the voltage go form ~12 to ~14 volts when the alt starts charging.

-Jason

oldiesel · #8 12-16-2009, 09:42 PM

check the small wire at the alternator for voltage with the key on before you start the engine,if the bulb is good you should have aprox batt voltage there with it disconnected from the terminal on the alternator. Don

rhodes2010 · #9 12-17-2009, 12:07 AM

Olddiesel thanks I'll try that next.

But for now I did this:

1.
disconnected the + battery terminal and
the plug at back of alternator.

2.
I used the Ohm/Resistance setting to check for continuity
from the alternator connector to the positive battery wire.
My thinking here was that if there is continuity (zero ohms /
no resistance) then the alternator will charge the battery if
it is working.

There are 3 wires on alternator connector, two larger, one smaller.
Both larger wires showed a connection to the positive battery wire.
So I am thinking that if it is working, it should charge.

3.
Next I clipped leads onto each large alternator connector.
Separate leads to make it easier to check each one for 12 volts or more
when engine is running.

4. Reconnected Battery wire to battery. Alternator connector off.
Leads on alternator for testing.

5. Started engine. Checked for voltage coming from either lead.
Got zero. I put the volt meter on the 15 volt DC setting and the positive lead
to the alternator connector, the ground to the negative battery terminal.
I assume the alternator grounds through the case and only send positive
through the connector and wires.

So now I am concluding that this alternator is not producing any output
and will replace tomorrow.

I suppose I could run the same test on the 300SD and check for different
results. But it is getting too cold out.

Hope my test makes sense.
Never had this much trouble before. Usually the charging light has always told
me the system was ill.

rhodes2010 · #10 12-17-2009, 12:30 AM

So.. I thought this alternator was a 65 amp already but the various
websites offer 55amp as the starting point and a replacement.

Then a 65 and even a 120 amp.

I like the thought of 120amp but can anyone say it is really needed ?

Diesel911 · #11 12-17-2009, 01:46 AM

The 120 amp Alternator may require that you modify the wiring.

patbob · #12 12-17-2009, 11:45 AM

Check that the alt is grounded.

That was what caused the exact same problem and symptoms on my old Horizon 20 years ago yesterday.

Graplr · #13 12-17-2009, 12:37 PM

Recheck the belt. And watch the pulley spinning with the car running.

I too have had plug problems like Craig.

rhodes2010 · #14 12-18-2009, 01:43 PM

So I acquired a rebuilt/reman alternator and installed.

Attached test leads to the two larger terminals so I could test for
voltage with engine running, like I did on the old unit.

I still do not get 12 volts from either one.
Not knowing what to expect I can only say that's the way it is.
If someone knows how to test a alternator for voltage on
car , running, with cable disconnected, please tell me.

Anyway I connected the harness and will find out in a few days if it works or not.

I'll drive it with lights on, if they die - t didn't work.

Graplr · #15 12-18-2009, 03:08 PM

Quote:

Originally Posted by rhodes2010

So I acquired a rebuilt/reman alternator and installed.

I'll drive it with lights on, if they die - t didn't work.

Where did you get the alternator from?

If your car loses charge it still may be the battery. Replacing a working alternator with another working alternator won't cure the problem if the battery is to blame. (I'm not sure it is the battery but, you most likely will be able to tell soon.)

You don't need to disconnect anything to test the alternator. Just hook everything up as normal and test the voltage across the battery. Should be at least 13.5V if not more. You should check the battery voltage first. It should be around 12.5V.

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