I have a 84' 300SD. The other day the charge indicator light failed to come on when the ignition was in the 2nd position before start-up but the car started anyways. After driving the car approx. 10 miles or so, I parked the car for several hours and then the battery was completely dead. I charged the battery and the car started right up but again no charge indicator light. After driving a couple miles back to the house, the car sat overnight and then the battery was completely dead again. My local parts and info guy says it is the alternator, that it is faulty and leaving the circuit open when the car is shut off. Before pulling the alternator what should I check?

P.S.- I cannot find R&R directions in the factory repair manual CD-ROM. Any tips if I need to pull the alternator?

P.P.S- I also posted this in the Tech Help section.

Before you pull the alternator:

1) Disconnect the negative battery cable
2) Make sure everything in the vehicle is completely shutoff and all doors and the trunk is closed.
3) Connect an ammeter between the negative battery cable and the negative terminal on the battery.

The reading should be not more than 80 milliamps. Probably less than 50.

If the reading pegs the ammeter, then look around, again, for something that might be on, such as a dome light or a courtesy light.

Make the test again. If the reading is above 80 milliamps, disconnect the wires to the alternator.

Make the test again. If the reading is now below 80 milliamps, you have confirmed that the alternator has the leak.

If the reading is still above 80 milliamps, then you have determined that the problem is NOT the alternator and you need to start pulling fuses to find the circuit that is causing the problem.

First off, THANK YOU BRIAN for being the only one with some input. I ran through the series of tests you suggested and all circuits were above 80 milliamps, so I disconnected the battery entirely and found that the battery itself has more than 80 milliamps across it. In fact every circuit I tested including the battery were at 93 milliamps. Any other suggestions?

I am NOT even close to any kind of an expert- huge disclaimer!!!

But I just replaced my voltage regulator. Not sure of what tests you can do to determine if it is or not. Fastlane has some off-brand for $18 that they have never had trouble with. I had been having trouble with my lights dimming when rpm's dropped, and it wasn't cranking as hard or starting as quick as before, and the battery light had been coming on.

It wasn't too hard of a job, did it with the alternator still in. Had to custom fabricate a supershort screwdriver by ductaping a slotted screwdriever bit into a 1/4" ratchet socket to get to the bottom screw, which I accessed from underneath. Overall a 2 hour job, for an advanced beginner. Running great now...

__________________
1983 300D- silver blue, tan leather, 205,000.
1975 Datsun 280Z. Stock, ready to restore.
1983 San Juan 23. Dry docked.

I'm a bit confused.

The first test, with the ammeter connected between the negative battery cable and the negative post on the battery produced 93 milliamps, correct?

Then, with the alternator disconnected completely (all cables removed from the back of the alternator), the ammeter now continues to read 93 milliamps?

If so, then the alternator is not causing the leak.

One of the circuits is leaking more than it should. With the ammeter still in place, remove one fuse at a time until the ammeter goes to zero. Then advise what is on that circuit. It might even be that the clock is pulling more than it should. I once had this happen when the clock became old and was about to quit.

I have the ammeter connected as described (negative post to disconnected cable).
I have 93 milliamps with alternator disconnected.
I have 93 milliamps with all fuses individually disconnected except one.
I have 88 milliamps with the #13 fuse(clock,hazards, radio, locking sys)

This might explain battery drainage (I left the fuse out to check) but I still have no charge indicator light with the ignition 'on' before start up.
Is this something where the "diagnostic plug" would come into play if I took it to the dealer?(I hate to do that, I never have)

Am I correct in reading that with all fuses removed and with the alternator disconnected you are reading 88 milliamps?

Please confirm that both the positive output cable and the ground cable are disconnected from the alternator during the aforementioned tests.

There seems to be something amiss. With no fuses in the panel, the leak starts looking more and more like the alternator. Please explain exactly how the alternator was disconnected.

The diagnostic plug is unlikely to provide any assistance for this issue and the dealer will only perform a walletectomy.

One other possibility, although remote, is that the starter is leaking current.

To test this, you would need to remove the positive cables from the starter, however, the cable from the battery needs to remain in contact with any other cables that are now disconnected from the positive terminal on the starter.

If the reading now drops to near zero, the starter is the culprit.

If neither the starter, nor the alternator, nor any of the circuits in the fuse panel are the culprit, I would need the electrical schematic for the SD to attempt to go further. I have a newer schematic for the SDL, but, have not found one for the SD.

Clarification of my tests.
While pulling each individual fuse one at a time, the #13 fuse (while pulled) showed 88 milliamps (at the battery neg. post to disconnected cable).
At no time did I have all the fuses pulled at the same time.
The alternator was disconnected during the fuse circuit checks.
Also, in disconnecting the alternator I simply pulled the connector from the back of the unit. This connector has 3 wires (2 large red wires, 1 small blue wire). I cannot see any other electrical connections present.

Lorenzo7,

This post is very similar to a post where someone took the instrument cluster out of the car and drove it, with similar results. One of the problems is that without the light lighting at the beginning of the starting sequence it is unlikely the alternator is connected to the charging circuit. This can be as simple as the bulb behind the charging light is burned out, or it can be a problem with the voltage regulator. I doubt it is the result of a short circuit in the system if any of the other lights on the dash light up when the charging light does not.

So, the question is, do the rest of the lights on the dash light up when you turn the key (typically the brake symbols light up as well as SRS if the car has that and some other items like windshield washer level fluid and oil level indicator lights)? If so, the problem is more likely that when you started the car and noticed the charge light did not light up the first time the battery was already nearly discharged and that start cycle - glow plugs and cranking the starter - further weakened the battery. If you drove it with the A/C or heat on for a while, and any other items on, it is likely the battery was further discharged so the next try the battery could not start the car (a glow plug cycle can be a strain on a weak battery). When you failed to start did the glow plug light cycle on and off at all? Also, when you say you charged the battery, how did you do this and for how long. Merely getting enough juice in there to survive a start cycle and then driving with lights, fans and other auxiliaries on for a while could put you back into the condition where the battery cannot support another start cycle.

So, when you jump start the car or charge the battery, does the charging light come on? If it never comes on I would check the voltage regulator (easy to replace, but limited access - do a search there is a load of data on the subject) and if the brushes are completely worn off, replace them or the regulator. If the regulator looks ok, I would pull the instrument cluster and check the bulb behind the charging indicator. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

You have done all the tests correctly.

The leak appears to be somewhere outside of the fuse box and outside of the alternator.

This is quite uncommon. As mentioned above, the starter is one possibility.
The circuit diagram is required to determine what other items are powered directly from the battery, and not through the fuse box.

You must determine the source of this leakage in order to solve the battery drain problem.

The alternator may additionally be an issue if it is not charging. Have you tested the alternator while the vehicle is running? Simply check battery voltage with the engine turning at above 1200 rpm. The battery voltage should be in the area of 13.5 or so. If it reads 12V, then either the alternator or the regulator is not functioning correctly.

However, the combination of the faulty light and the discharged battery does seem to indicate a problem with the alternator. However, it is impossible for the alternator to discharge the battery if it is disconnected.

I greatly appreciate all the help!
To answer Jim's questions:
I have been taking the battery to AutoZone (close to the house) and have had them charge and check the battery.
Yes, the brake wear light as well as the fuel low light are not coming on but the others are. (I will pull the cluster and check the bulbs to be sure)
With the charged battery the glow plug indicator lights up properly.
The charge indicator light does not come on with a fully charged battery.
The battery holds constant voltage (engine not running) when the alternator is disconnected but is slowly losing voltage since I reconnected the alternator (engine not running).
[loss of voltage while connected equals bad alternator or internals, correct?]
My local guy was describing the probability of the regulator being the issue but is suggesting replacing the whole alternator due to age (almost 5 years, 30,000miles) on a reman.'d Bosch. What do you think?

I will check the voltage while the engine is running in the meantime, and please let me know if you have any other thoughts!

Brian,

Some of the problem may lie in terminology.

A "dead battery" is usually one that will not accept a charge. A weak, or partially discharged battery is one that has less than a full charge, and can be detected by checking voltage across the terminals of the battery. If you suspect a high resistance short in the system (needed to avoid a current draw that would pop a fuse) you can try this measurement with the battery disconnected from the car's electrical system (disconnecting the positive terminals should do this). In any case, a weak battery is susceptible to being unable to start the car. A normal battery that is not being charged by the electrical system of the car will eventually become unable to start the car, just because the energy stored is depleted every time the car is started. Lorenzo7 doesn't say where he is, but if it is below 40 degrees F, the starting cycle will deplete the available energy significantly. Driving without recharging will further deplete the battery unless Lorenzo7 meticulously turns everything off and removes the fuses for items with no switch.

I concluded from the discussion that Lorenzo7 was able to stuff some energy back into the battery so the car would start, so the battery is not "dead" but "weak." I was confused by the testing results he got, but figure if he has had this condition for any length of time, it is normal. A short that is high resistance will eventually become low resistance and blow a fuse, at which point it becomes infinite resistance. A high resistance short without fuse protection will eventually fail spectacularly as it progresses to infinite resistance. And possibly dangerously. These things typically do not take weeks to progress from an annoyance discharging the battery to a blown fuse or worse. This progression is typically a matter of minutes as the battery and the charging system together can pump a lot of energy into a short, and they are predisposed to do so furiously if left without fuse protection. The only such long term failed parts that can sustain a long term current draw are typically items that are designed to draw the low current and the shut-off mechanism has failed. In these, relatively few, instances the short can be sustained for long periods, but these are not shorts in connectors to ground, or wires to ground or components to ground. These tend to be shorts in switches or other devices that enable a design condition to persist after the device design intended it to be shut off.

Anyway, without some answers to the questions about how the battery was recharged, we cannot determine if the battery went from a weak condition to a fully charged condition or from a weak condition to a slightly less weak condition and the car started. Just driving and trying to restart the car is enough to give a car with a weak battery that is not being charged by the engine/alternator combo a condition where the starter cannot start the car. We need more information, and more specific information. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

This clearly points to a problem with the alternator. However, the previous test procedure should have confirmed the leak.

Can you repeat the test measuring milliamps from the negative battery cable to the negative post on the battery with:

1) the alternator connected
2) the alternator disconnected

There must be a difference between these two readings if the ammeter is working properly.

I agree with all your analysis, Jim, however, the 80 ma leak is abnormal.
My suspicions lie with the alternator, as this is common when they get old. However, I fail to see why the test with the ammeter cannot confirm this leak. Any ideas?