Hi All,

I've got my 240D running really well.. only this is it has a few nagging electrical issues.

There was a pretty intensive battery drain when i got it and I figured it might be the fan blower so I actually took all the fuses out and put some of the necessary ones back in to see if the battery would stop draining. It appeared to have stopped draining.

Replaced my glow plugs for easier starting and put a brand new bosch battery in two days ago.

Today driving home from work I noticed that the headlights were suddenly dim and that the lights inside the car were also basically off. Then the radio stopped working as well as the turn signals and windshield washers.

I pulled over to check the fuses and replaced the fuses for the low beams with some used but fair fuses I had in the glove box. Started driving again and noticed the lights were dim all over agin, basically barely illuminated. Then I noticed I had no windshield washer function or turn signal function once again.

Parked the car, ate dinner and got some new fuses. Replaced the light fuses with fresh ones and tested them and they worked fine. I think I may have put the wrong voltage fuse in the wrong holder by accident (it was dark because something under the hood made a rapid bussing noise). Checked all the fuses and made sure they were in the correct places.

Went to start the car and the battery drained instantly, like as soon as the glow plug light came on.

Jumped it and then the headlights were dim again. Any idea as to what this could possibly be? Everything was functioning fine the past three days and then all of a sudden nothing electrical works? I believe the car has a fairly new alternate installed but he PO.

Probably has nothing to do with your fuses and sounds like your alternator or its voltage regulator is failing.

Your going to have to do some additional investigation in order to determine what's going on.

I have experienced similar symptoms with a bad voltage regulator.


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1985 Euro 300TD Turbo

You probably have a stuck on glow plug relay

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

Sounds like worn alternator brushes, which gives no alternator light in these cars.

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Gone to the dark side

- Jeff

Use a voltage meter to test the battery with the engine off then with it running. If the battery is too low to start the engine, have it recharged and perform the same tests. Post the results here.

Update,

I charged the battery for about almost two hours today and re-installed it in the car.

Like a dummy I forgot to test it with the car off.

I tested the battery with a volt meter in the parking lot after work today. Got Approximately 11.75 while running. When I revved the motor up the numbers appeared to go down? I would think they would go up?

I checked the belt tension and I wouldn't call it loose per say. There is some deflection in the belt when i press on it with my finger and I can turn the alternate pully if I push on the fan without the belt turning.

Perhaps the belt needs to be tightened?

forgive me for my ignorance, I'm new to testing electrical systems in older cars!

You should be seeing about 13.75-14.4 volts across the battery with the engine running, 12.5 volts or so with the engine off battery fully charged.

First thing to check is - Does the instrument cluster charging indicator light illuminate when you turn the key to position #1 and #2? The alternators exciter voltage flows through that bulb to the alternator, if the bulb is dead no exciting voltage makes it to the alternator. You can check that easily and eliminate that as a potential problem.

Alternator belt is too loose when you can turn the alternator fan with your fingers.

The only light that comes on when I put the key in the ignition and turn it is the glow plug light. I would presume this means that the bulb you mention is out.
So this bulb can be responsible for the car not charging properly?

ALSO... I've spend a good amount of time searching and am having trouble finding an explanation of how to adjust the tightness of the belt. Car is an 82 and apparently the tensioning device changed over the years?

thank you all for you help!

I searched under "alternator indicator" and found this thread

http://www.peachparts.com/shopforum/diesel-discussion/362615-w126-no-charging-light-no-charging.html

Even with a belt as loose as you describe, engine running voltage will be higher than 11.75 . Testing the battery before cranking was just to see if the battery was fully charged and to give you a point of reference, given your low running voltage we really don't need the non running voltage at this point.

A fully charged lead acid battery that has been sitting for 8 hours will read 12.66, below is the full chart. Do not use the chart when the battery is being charged or discharged it won't help.

Also , this does not measure capacity of the battery, only state of charge. Load testing measures battery capacity. State of charge is how full the gas tank is, battery capacity is how large the gas tank is.

12.66 full charge
12.42 3/4 charge
12.18 1/2 charge
11.94 1/4 charge ( this is typically as low as you want to regularly go )
11.70 completely discharged, going lower than this can damage the battery.

The next step is to find a thread on diagnosing your specific alternator. If your system has an external voltage regulator, there are ways to jumper it out and make it charge at full strength. If the voltage goes up when this is done, the regulator is bad, if it does not, the alternator has a problem ( brushes , open rotor winding , many open / shorted diodes in the bridge )

And to get terminology correct.

A wire that is broken is called an "Open "

A wire that is inadvertently connected across a power source is called a " Short " ( The term short is frequently misapplied to open wires. )

Further terminology: Voltage is electrical pressure; Amperage is the amount of current; OHMS is the amount of resistance found in a particular size wire; wire size is by the gauge- the higher the number the smaller diameter the wire; a diode is a one way electrical check valve that only allows current to flow in one direction;AC- -alternating current;DC- direct current ; "excite the field" - to allow electrical power to flow to the alternator to cause the rotor to become magnetic; an alternator produces AC power that is changed through the use of diodes to DC power to charge the battery; every electrical device on a vehicle must be grounded (ground wires are brown in a W123 car).

More later.

BTW, I can concur with the diagnosis - and assertions - here.

Having just replaced my alternator and battery, I happened to have my instrument cluster out for other (maintenance) reasons. without that 'charge' bulb in circuit, the alternator diode circuit was not getting excited; IE, no charging of battery!

With the cluster re-installed, all is AOK again.

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'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

This sounds like the usual culprit : the brushes have worn out so it's time to order up TWO BOSCH brand (! only !) voltage regulators and install one to your alternator and keep t'other one in the glove box for the future when it fails again some years down the road when you're far from home .

FWIW, I have experimented with every different brand of regulator/brush holder I could find and -only- the Bosch branded ones will charge 14 VDC consistently .

This affects your cold starting heavily so pay the extra $3 or so and insist on only BOSCH brand voltage regulators .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better