I have an '89 240DL, B23F. After improper diagnostics by the shop, I have replaced the alternator to no avail to solving my charging problem. There was nothing wrong with my alternator to begin with.

My battery is a bit over 2 years old. I have checked cell to cell voltage and they range around 2 volt each. With the state of the battery at that time, I feel that this value was acceptable.

With the electrical system fully intact, i.e., the battery connected, the car will charge the battery to a full 13.7 volt if no other load is running (lights, blower, etc.). Once activating any load with the car running, the battery will rapidly discharge (voltage reading from the alternator dropped to less than 10 volt over a 5 minute duration). Upon de-activating the load, the system will charge again to 13.7 volt within a reasonable amount of time. I let the car sit over night and the battery still has more than adequate power to start the car.

If I start the car and disconnect the battery, the alternator takes over and allows for the car to continue to run. Upon activating a load (or even several loads) the output of the alternator remains above 13 volt.

All grounds appear to be good and I don't suspect a short.

I realize the logical culprit would be the battery but I don't get it due to the tests I have ran on the battery. It charges fine and holds a charge.

Additional info:

Start the car fine with no loads on - battery VERY strong.
Check alternator voltage at 13.+ volts - battery connected
disconnect battery - Alternator reads 13.56 volts
Turn on lights - alternator reads 13.3+ volts
re-introduce the battery - alternator reads 12.5+ volts
disconnect battery - alternator reads 9,6 volts and will not recover.
turn off lights - alternator recovers to 13.56 volts

Also, Today I took the car back to the "pros" to test the battery mainly but also to get feedback on the entire charging system. These are the same folks that mis-diagnosed my trouble in the first place. They say my battery is good and their equipment showed "no voltage to the battery indicating the alternator is not functioning. I put my fluke DMM on their testing equipment terminals and read 13.86 volt while connected to my battery.

Any thoughts?

regards

Rev

Hello again

The differential in voltage from the case of the alternator to the charging terminal is a mere 63 millivolt. My situation remains the same. I suppose it is possible that even though the "new" alternator tested the same as the old, the new one may be bad out of the box. Lifetime warranty, so I'll get a replacement and give that one a go. I'll keep you posted.

Thanks to all for your feedback.

regards

rev

Alternator good or bad, that does not sound right. Even if the alternator has no output you should at least see battery voltage between the very large alternator output terminal and ANY ground point..... alternator case, car frame, battery negative.

I think you have an open somewhere. Alternator ground path to battery negative or alternator output to battery plus.
--
David Hunter

David

Sorry, I meant 63 millivolt from the case to the negative side of the battery and a differential between the charging point of the alternator to the positive side of the battery of the same amplitude indicating some losses somewhere. I suppose I'm suffering of the simple stuff while working on this issue. There has GOT to be a simple solution that I am overlooking.

regards

I'll get back to ya'll.

rev

Hey Ya'll

It goes from bad to worse to funny. I'm glad I have a sense of humor. I installed the third alternator now. Under normal installation, i.e., everything connected as it would be as if replacing it. The alternator did not produce an output voltage, NO CHARGING. Originally, I got strange voltages from the output of the alternator, 45.+ volts with no load, that is 45V, not .45V. I took the alternator out and "bench" tested it with 12 V excitation, no load on the B+ terminal and driving the alternator with a drill motor. I got about 12.5 V output on the B+ terminal. That told me that the alternator was good. I re-installed it in the car and connected it with with jumpers to the battery excluding any connections to the wiring harness, i.e., direct connection to the battery from the excitation tab and direct connection to the battery to the B+ terminal. The car started and ran fine, for many hours charging to only 12.6V. Pulling the jumper off the battery connected to the output feed of the alternator revealed 13.5V of charging potential. At this point I suspect the battery again as when I apply the headlights again as mentioned in a previous post, the voltage of the battery drops. So, on to the next test.

STOP GAP
In the midst of this testing, I noticed the acid level low in the battery (not so much as to expose the plates) and I replenished the level with distilled water. After two days, the level was as low as it was when I replenished it prior to this issue. I was instructed to replenish the battery fluid level with battery acid and in one day the level of the fluid is back down to the original level as noted 3-4 days before.

I disconnect the jumper wire from the battery to the excitation terminal to the alternator and re-connect the harness connection (factory) to the alternator excitation terminal and left the B+ terminal jumper connection to the alternator (floating). I start the car and I now see the 45+ volt at the output of the alternator (B+) terminal. Hmmm. Such a simple problem with a (so you would think) simple solution. Any thoughts?

Anyhow, engine running and battery hooked up, what is the output at the alt measured at its +ve post and -ve casing with the excitation wire connected? If correct voltage (>13V, <15V) then alt is okay and connections from alt to battery bad.

Engine running, check ground connection by running jumper from alt case to block, then alt case to body while checking voltage at battery. Likewise check +ve by running jumper from alt +ve to starter and alt +ve to battery.
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

Hey Ya'll

It goes from bad to worse to funny. I'm glad I have a sense of humor. I installed the third alternator now. Under normal installation, i.e., everything connected as it would be as if replacing it. The alternator did not produce an output voltage, NO CHARGING. Originally, I got strange voltages from the output of the alternator, 45.+ volts with no load, that is 45V, not .45V. I took the alternator out and "bench" tested it with 12 V excitation, no load on the B+ terminal and driving the alternator with a drill motor. I got about 12.5 V output on the B+ terminal. That told me that the alternator was good. I re-installed it in the car and connected it with with jumpers to the battery excluding any connections to the wiring harness, i.e., direct connection to the battery from the excitation tab and direct connection to the battery to the B+ terminal. The car started and ran fine, for many hours charging to only 12.6V. Pulling the jumper off the battery connected to the output feed of the alternator revealed 13.5V of charging potential. At this point I suspect the battery again as when I apply the headlights again as mentioned in a previous post, the voltage of the battery drops. So, on to the next test.

STOP GAP
In the midst of this testing, I noticed the acid level low in the battery (not so much as to expose the plates) and I replenished the level with distilled water. After two days, the level was as low as it was when I replenished it prior to this issue. I was instructed to replenish the battery fluid level with battery acid and in one day the level of the fluid is back down to the original level as noted 3-4 days before.

I disconnect the jumper wire from the battery to the excitation terminal to the alternator and re-connect the harness connection (factory) to the alternator excitation terminal and left the B+ terminal jumper connection to the alternator (floating). I start the car and I now see the 45+ volt at the output of the alternator (B+) terminal. Hmmm. Such a simple problem with a (so you would think) simple solution. Any thoughts?

That amount of voltage drop is probably normal due to the resitance of the wiring and conectors. Your alternator should be charging the battery at well over 14V then dropping some when loads are applied such as lights and fan.

Weak alternator or bad Voltage reg inside the alternator?
--
David Hunter

Poor remanufactured alternators and starters are the norm at the chain auto parts stores. The lifetime warranty easily substitutes for quality because most of their customers are folks like us who don't have an hourly rate for installation, test, and removal.
--
Art Benstein near Baltimore

Everyone seems normal until you get to know them.

I think your alternator is intermittent. I've found intermittent charging can result from glazed pulleys and belts silently slipping under load, and also from internal stator winding shorts that take 10 minutes of driving to incur. This can be a real challenge to pin down.

Be very wary of disconnecting the battery while the engine is running. Although this is excused as common practice by some pros, they rarely will accept (meaning pay for) that's the reason your ECU or AMM fails later from brief overvoltage spikes.
--
Art Benstein near Baltimore

Be careful about reading health books. You may die of a misprint. - Mark Twain

Art -

Thank you for your reply.

FYI -

This is a "new" after market alternator (may be a rebuild but it came in a new mfg box) and I had it tested prior to leaving the store. The "pros" stated that the unit was good. Also, regarding intermittent. I don't believe it is as I can reproduce these scenarios any time I like. Someone suggested a poor ground but my meter reads a good ground. Their suggestion was that it may read good with a meter (due to the low current) but as the current demand increases, the ground is not good enough to handle the additional load. I have yet to run a separate #10 ground directly to the alternator case to the ground post on the battery.

What are your thoughts on that?

Thanks in advance.

rev

Well, that's a great benefit to you, that you can reproduce the trouble at will.

Their suggestion was that it may read good with a meter (due to the low current) but as the current demand increases, the ground is not good enough to handle the additional load.

Absolutely true. The correct way to determine if a ground "reads good" is how David Hunter advises, and for the reasons he gives. Do it. If there's any voltage drop greater than 200mV, replace that blue wire behind the alternator. You know where it is if you swapped the units.

The exciter voltage is passed through the battery lamp. The other lamps mentioned are in the circuit (for lamp test purpose), but not needed to provide excitation, and actually don't as long as the battery lamp is OK.

--
Art Benstein near Baltimore

By all means, marry. If you get a good wife, you'll become happy; if you get a bad one, you'll become a philosopher. - Socrates

No need to run a #10 wire, just check voltage on the alternator case in relation to battery negative post, it should be 0 volts or maybe a few millivolts at most with engine running and load applied.

What is voltage at the large alternator output terminal as load is applied? Any resistance between here and the battery on red wire would give the symptoms you describe.

I agree that your battery is good.

Following dash warning bulbs provide exciter voltage to alternator, are they all good?
Park brake
Bulb failure
Brake failure

--
David Hunter

I agree with David
To expand a little on the idea, while the engine is running put one tester lead on the alternator body the other on the Negative of the battery. Ideally it should read 0 volts, anything more than a few millivolts .001 is cause for for further investigation.

I would also try this with the big lead on the alternator to the plus of the battery.

CHECK the BELT CONDITION and TENSION, are they glazed? It is possible that the belts are slipping while under the heavier electrical load.
This would cause the voltage to drop with the added benefit of the engine over heating. It would explain your symptoms - - Ask me how I know this or should I say Been there Had that!

Disconnecting the battery while the engine is running could cause your alternator to run wild, and I don't know how happy things are on 16 volts.