Hello,

I am having trouble diagnosing a charging system issue. The car is a 1984 Mercedes 300D.

My battery kept dying on me a month ago. I replaced the battery. The new one died too. Charged it back up and began to look at the alternator. The voltage at the battery was not rising when the car was on.

Took the alternator out of the car, got a new voltage regulator for it, put it back in car. Still no rise in voltage. Took the alternator to the autozone to bench test (it was fine), put it back in the car, no rise in voltage.

Checked continuity between the alternator plug (both B+ terminals) and the battery + (checked out fine), and between battery ground and chassis and engine (all checked out fine). Even took the ground terminal off the chassis and battery, cleaned it, put it back on. Took the alternator off the car, and back to the store to have retested. This time it failed.

Bought a rebuilt bosch 55amp alternator, put it in the car. Battery reading 11.8V before turning the car on. Did not rise with new alternator when car started. Took it back out and back to the store. Bench tested it again (it was fine).

Read about the battery light on the instrument cluster. Took the cluster apart. Checked the bulb with a bench power supply (it lit up, works). Put it back in the cluster, and tested continuity from the plug (hole 6) to the fuse (#12), and from the plug (hole 7) to the D+ third terminal on the alternator. This is essentially both ends of the dash bulb. Both tested okay.

Put the dash back together, and plugged the alternator back into the car (just wires, not belts). I turned the key and no battery light comes on.

What am I doing wrong here? What is left to check? I've been on this on and off for a month now, and now have a new alternator and battery and a car that will not charge the battery. I can now install or remove an alternator from this car in five minutes flat. Any help here would be awesome.

Thanks,
Blake

Upon further continuity tests, I discovered that actually there was a discontinuity between the dash pin 7 (the battery light) and the alternator D+ terminal. This is the blue wire that goes from the dash light to the alternator. I cut it near the fender and spliced a new wire to the alternator connector D+ terminal. Now it reads continuous.

With this I now get the battery light when I turn the key and when I start the car it goes out. But I am still reading only 11.7 volts at the battery with the car on and the new alternator installed. Revving the engine does not produce any significant change.

Unplugging the glow plug relay makes the voltage jump less than .5 volts.

Any ideas?

Thanks.

Introduce yourself to this group with some background about yourself. You will receive a warm welcome. Next, indicate the, make, model, and year of your car. If it is a Mercedes diesel from 1983-1995, you will probably find the answer by searching the site. Welcome.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

Some of this is a repeat of what you did but it is fast to check it again.

Do this with the Voltmeter.
If you Pull out the Electrical connector from the Alternator the 2 large slots on the Connector should have Battery Voltage all of the Time. On mine those are the ones the Fat Red Wires go to.

On the Smaller Single Slot you should only get Voltage to it when the Key is on and you will get slightly less than Battery Voltage due to the little Charging Light Bulb. That slot is the one that has a Blue Wire on Mine and goes to the Charging indicator/Light.
If your Bulb had been no good you with the Key on you would have had no Voltage at the small single slot.

If you know the Bulb is good but you are getting no Voltage when the Key is on it could be the Ignition Switch.

Also some sources say that the Bulb needs to be 2 watts and others say 3 in order for it to have enough resistance to excite the Alternator to charge.

As you noted the Alternator needs to have a good ground in order to function properly. Cleaning the Chassis to Engine Ground Cable and the Battery to Chassis Ground cable contacts normally takes care of that; except the if your Alternator Bracket is line mine where it bolts up to the Engine it is extremely rusty.
Rust does not conduct Electricity well.
To see if that is the problem People take one Jumper Cable and connect it to the Negative terminal of the Battery and then find a bare metal spot on the Alternator Housing and apply or clip the Jumper Cable to the Housing. If you start charging you know your problem is a bad ground.

I made the Test Cord in the below Thread to separate an Alternator issue from a Vehicle Wiring, Bulb, or defective Ignition Switch issue. Only one other person has made one that I know of.
However, the Test Cord does not eliminate the possibility of a bad ground issue. That can be taken care of with the Jumper Cable use I described.

Alternator Test Cord for W123s - Mercedes-Benz Forum

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84 300D, 82 Volvo 244Gl Diesel

Also you did not mention if whey you found that the Alternator was not charging your Charging light was glowing Red or not.
When you first turn the Key on all the warning lights should light up and go away when you turn the Key further; and when you turn the Key off the Charging Light will again light up briefly.
Off course if your Battery is entirely dead and not enough juice the Charging Light has nothing to light it.

If your Glow Plug Relay stays on a good Battery will be killed in a few hours or so.

When the Alternator is not charging and the Battery starts of fully charged it takes longer to kill the Battery then when the Glow Plug Relay is stuck on; if all you do is start and drive the Car not using Headlight or other accessories like the Blower.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the link Diesel911. I had actually come across your thread on the other site in googling this issue. I will try to make a test cord of my own. This problem really has me stumped. I also like the jumper cable ground method. I will try that too.

To answer your question, the battery light comes on when i first turn the key (now with the new spliced D+ wire). When I start the car, the light goes out. Checking with the multimeter at the battery terminals with the car running, the new alternator is not charging the new battery, with out without the glow plug relay connector pulled.

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1981 300TD
1984 300D

Car running there should be no voltage between the negative battery terminal and the alternator shell. If no voltage then the new alternator is starting to sound bad. You would use a clip on wire to stay away from the running engine components on the front of the engine for a ground voltage test. Always practice safely. Usually if the starter will crank the alternator is grounded though. Still not always.

Where did the replacement alternator come from? Mexican rebuilt? Using a jumper cable as already mentioned is also a good ground test.

Reaching down with the engine off you should not be able to turn the alternator fan with your fingers either.

Just started up the car and test the voltage from the alternator shell to the battery. It read around 40mV (.04V), which is pretty close to zero. I'm not sure that my multimeter is precisely calibrated.

I have not had a chance to check the ground with jumpers or to make a test light but I did notice that my belt is somewhat loose, even with the alternator adjusted all the way out in its bracket (no more outward adjustment is possible). I can turn the fan and pulley by hand will the car is off.

The belt is a Napa Premium XL 25-7405 (cross-ref: Gates 7405, Dayco 15405). After googling this, I think this is a approximately-sized belt, although the Gates website and Rock Auto both list this belt for an 82-85 300D. Gate's website says this belt has a 40.5" effective length and a 41.08" outside circumference. Pelican Parts lists a Continental as 10x1035, which in inches comes to 40.75. I don't know if this is outside or pitch diameter though.

In any case, my belt doesn't seem tight enough and the pulley turns by hand. I can see the pulley turning when the car is running. Could this be causing the charging issue? Is there a more appropriate or shorter belt I could put on this?

Thank you for all the help.

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1981 300TD
1984 300D

There is a thread within the last 2 weeks where vstech commented on the Belts needing to be a matched set.

I have Continental Belts on mine the same as vstech suggested in the Thread.

I have also read that the Metric Belts have a different cross section where they go into the Pulley Groove.

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84 300D, 82 Volvo 244Gl Diesel

Yes the belts could be the issue. Idealy you want belts that are not out to the extreme of adjustment when new as they typically stretch during use. So you would have no further adjustment if they were even tensioned properly at the extreme of adjustment and yours are not.

Anyways the fault or not they have to be dealt with. You for certain should not be able to turn the alternator pulley with fingers on the fan portion of the alternator.

The 40mv drop was low enough to establish the ground to the alternator is good enough. So you are reduced pretty much to the loose belts that I suspect may be the issue. Or the alternator is no good as well basically.

An alternator asked to give a high charge initially especially draws a lot of mechanical power through the belt drive. The belts have to be set up to transfer that much energy. You did put new belts on I think. So take the old ones and read their original length in the code on the flat edges may give some help if still visable and it usually it is.

Barry12345, you were right. For some reason, with the prescribed belt, my alternator was at extreme travel in the bracket. At first I went and got a belt that was 1 inch shorter. I could not get it around the pulleys. But with a belt 1/2" shorter (a Gates 7400 instead of a 7405), I was able to properly tension the belt.

This new belt, along with the D+ wire fix, solved the charging issue and the car now seems to be charging properly (reading around 14 volts at the battery).

I'm grateful to everyone who replied for helping me figure this out. Thank you.

Blake

P.S.: I was reading on the forums here about broken or bent alternator tensioner- the bolt that has a threaded rod coming out of the head at 90 degrees, used to tension the belt. Mine bent years ago. As a fix, I used a regular bolt with a 1/4-20 x 5" long eyebolt around the shaft as the adjuster. I then used a locknut with washer to tension. I hope this helps someone with a similar problem. It's been working well for at least two years now.

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1981 300TD
1984 300D

From My Notes; also a similar thread over on Benzworld:
Using an Eye Bolt to repair a broken alternator belt tension adjustment bolt eyebolt on Alternator
http://www.peachparts.com/shopforum/showthread.php?p=2620258#post2620258

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84 300D, 82 Volvo 244Gl Diesel