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...

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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.

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

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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)

HA! You were refering to my post right? Well, I almost panicked when I started my car earlier on and realized the charging light wasn't coming on!! I almost got so angry that I was going to drive back to KC, dump the car and take a more reliable, newer car back here. Then I read your post about checking the bulb. I had just ordered spare bulbs last week from Fastlane two weeks ago so I switched out the bulb and - the charging light came on!! I cooled off and relaxed at that point. Indeed, most of the time the best solution is the simplest one. At this point, can I assume my voltage regulator is good? Also, when they run a test on your alternator, does the functionality of the voltage regulator affect the results they obtain at all?

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver