After my battery dying i checked my voltage to make sure my alternator was running. I'm getting the same voltage when the car is off as when its on (even after driving it around). For a while the light would turn on before i started the car and then go out.

I checked my brushes and they're good

I've got two questions
How can i test to make sure my regulator is working? I've got a cheapo multimeter that uses the DC circuit + a diode for the ac measurement so I cannot test the AC voltage. I read on a different thread that you should get 100mV ac. My meter reads 12.5 AC and DC :-P

I've read that the alternator won't do anything if somethings wrong in the blue wire. If the blue wire was working correctly would it end up at the - battery terminal? in other words can i test the alternator by running the blue wire directly to the battery -?

Thanks in advance.

If the idiot light no longer comes on when you turn the ignition on & goes out after it starts the alt will not charge. Sounds like that needs fixing first.
Can you please post some actual voltage values from your tests? was the 12.5 v with or without the motor running? Once you fix your idiot light, what values do you get?

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

To the best of my knowlege; unlike the old Delco SI Alterntors you could stick the end of Paper Clip in and if it charged you could tell it was the Voltage Regulaor gone bad. On yours you can only look at the output Voltage.

The Blue Wire comes from the No Charging Idiot Light and need to have a working Light Bulb to supply just a little bit if resistance to start charging (one article said it needs to be at least a 3 watt bulb).
When you first turn your Key on all of the warning lights including the No Charging Lignt should light up. This is to check the Bulbs.
If the No Charging Light lights up along with the Warning Lights the Bulb is good.

The only time the Blue Wire Gets Voltage is when the Key is on and it is the small Slot on the Alternator Connector.
The 2 larger Slots on the Alternator Connector (2 fat Red Wires) always have Battery Voltage.

It might be a good Idea to go to the nearest Harbor Freignt and update your Volt/Ohm Multimeter.Mine has an ACV settings of 200 and another with 750 on it.
The below is from my notes (it does not say what AC voltage to use to check for the Diodes).
ALTERNATOR DIODE check with volt meter
With your meter across the battery, on Volts DC, you should read 12.5 - 14VDC

If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.

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84 300D, 82 Volvo 244Gl Diesel

My voltage is 12.5 both with the car off and running (even after a trip before i shut it down). It stays 12.5 cause I'm keeping my battery topped off with a wall charger until i sort this out .

I don't think the bulb on my idiot light burnt out. Sometimes before this it would stay on after the car started. PO said he had replaced the battery and done the brushes and system was good, but maybe it has an intermittent problem. I'll check the ground strap tomorrow but i think its good cause accessories work well with engine off.

How do i go about getting at the idiot light bulb?

Thanks for all the help

911 has some good info for you in his post. Was the 12.5v with the motor running checked at a range of RPM's ? I think there are places you can go to & have your alt checked for free, that would be my next suggestion. You could test the blue wire at the alt with the ignition on. You should get a reading of anything more than about 3v ~ 12v with it disconnected from the alt If not, you have a problem in your wiring.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Sorry to dig up an old thread, but my problem follows along close to this. About 1.5 years ago my battery light started coming in and out while driving. After about 4 hours of driving over the course of a week it went to on while the engine was running.

W140 S350D 603 engine.

I bought a Bosch voltage regulator/brush assembly and had a local auto electric shop install it. We never could get the battery to charge so I ended up parking it.

Two weeks ago I started playing again. New battery (old one was shot). I bought a plug in volt meter that goes in the cigar lighter.
Test today went like this:
Charged overnight to 100% by a battery charger.
Plugged in volt meter and turned key to first position -> 12.8 Volts
Turned key to turn on the glow plugs -> voltage drops to 11.2 Volts
All idiot lights light up and stay on, including the battery symbol.
Light goes out at 12 seconds or so -> voltage stays at 11.2 Volts (Normal, the glow plug light only indicates it is ok to try starting. The glow plug relay stays energized for 30ish seconds.)
At 20 seconds I started the car, fired right up -> voltage went back up to around 12.4, battery light stays on and does not go off until I turn the car off.
Went to the battery and hooked up my Fluke multimeter -> 12.4 Volts DC.
Switched multimeter to AC ->started at .008 Volts AC, went down to .002 Volts AC. That is 2 mVac.

Why the heck is it not charging!!! What do I check next? Voltage on the blue wire right at the alternator?

(Other notes, yesterday I used the cigar lighter volt meter to verify that voltage did not change with engine speed. I ran a two mile straight away at speeds up to 65 MPH and it stayed between 12.1 and 12.3 Vdc.)

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

Sounds like the regulator is no longer working. Right after starting, you should be reading ~13Vdc. As it stands now, you are reading the basic voltage of the battery.

Before changing the regulator, double-check your grounds. Both the battery-chassis and especially the chassis-engine.

Another area that I had a problem with was corrosion between the regulator and the alternator housing. After cleaning the contacting surfaces and applying a light dab of conductive grease, it started charging and has worked great for 3 years since.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Brand new Bosch regulator was installed by an autoelectric shop just before I gave up and parked it 1.5 years ago.

When I replaced the battery two weeks ago I removed the battery to chassis ground cable, wire brushed the cable end and the frame point to shiney metal, covered with battery connector goop (white petrolium jelly), and reconnected.

Looks like I need to go after the engine ground. After that it will have to be the regulator/alternator connections.

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

12.50 Volts is the Battery Voltage; so you are not charging.

I am not familiar with the wiring on the back of your Alternator and could not find a pic of the Back of the Alternator.
On the later than your medels some have only 2 Wires. One thick Wire from the Battery and a thinner Wire.
The thinner Wire is the one that goes to the Charging Light and the Ignition Switch. If you probe that Wire you should get slightly less Voltage than Battery Voltage when the Key is on.
You need to have a real light bulb in that circuit. There should be a 2 or 3 watt (sources differ on the wattage) Bulb in the Charging Lignt or you will not charge. A LED light is not supposed to work.

The Battery Wire should have Battery Voltage all of the time. On some Cars the Alternator Battery Wire is attached to the Starter and uses the Voltabe fromt the Cable that goes from the Battery to the Starter for Voltage. Could it be loose?
To see if the Alternator is not grounded well find someplace to clip on one Jumper Cable to the Alternator Housing and one to the Negative Battery Terminal and Start the Car and see if it starts charging. You want to connect the Cable before starting so there is no Voltage spike.

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84 300D, 82 Volvo 244Gl Diesel

I'd remove the regulator that you installed before and be sure it is touching the springy connector inside the alternator case. It's easy to check with the alternator removed. I got one in the junkyard once that had the spring connector pushed out of line and it didnt make a connection.

It's working just fine. I keep it for a spare for my car.

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My first Mercedes Benz. 4/6/07.

1984 190E (201 024) 2.3L four.

I'm shocked at how much this sounds like my situation before changing/cleaning grounds and replacing the alternator and the battery in the w210- and I'm still having problems...

What Are the Causes of Alternator Failure? | eHow.com

"Losing Ground-The primary mount of the alternator serves as the negative ground and usually the mount will be wired to the vehicle's frame with a large cable. This cable is held onto the mount with one bolt, and it is known to become loose or disconnected. When the alternator loses its ground, it can become damaged as it surges and could cause catastrophic damage to the voltage regulator within it. Most alternators manufactured since the 1980s have internal voltage regulators, which can vary in design. If the electronics are of a cheap make, the voltage regulator can overcharge the battery or even destroy the electrical system of the car from extreme power surges. Tighten the ground wire or braided strap to the alternator mount, and check it often."

What is not mentioned above is that if the Alternator Bracket and where the Alternator bolt to the Block is is rusted it also results in a poor Ground.

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84 300D, 82 Volvo 244Gl Diesel

it's pretty easy to test if the alt is not charging due to ground issues, measure voltage at the alt case.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

OK, one at a time:
The bulb/idiot light is the same one that worked for years before I had any problems. It still works now.

Wiring: I need to check for battery voltage on the big wire (since it runs directly to the + terminal on the battery) and something less than battery voltage on the small wire with the key turned to the on position.

Loss of ground: I need to run a test wire from the battery ground terminal to the casing of the alternator, then start the car and see if it starts charging. (I like this one!) I also can/need to check ground from the alternator case to chassis. (Also easy.)

Cables in general: I need to make sure the alternator ground cable/connection is clean/solid/tight. I need to make sure the alternator Positive cable to the battery and/or starter is clean/solid/tight.

Alternator itself: I need to remove the regulator, clean the connections make sure everything lines up right and reinstall.

I'll go after the two loss of ground checks first since they are the easiest.

Thanks everyone!

Jim

__________________
1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

If your battery light does not come on with the ignition ON without the engine running, you should replace that bulb. I think it all has to be in the circuit before the alternator will charge.

__________________
My first Mercedes Benz. 4/6/07.

1984 190E (201 024) 2.3L four.