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  #46  
Old 02-08-2021, 12:42 AM
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Quote:
Originally Posted by NZScott View Post
I was reading this thread last week and by Friday last week my alternator crapped out........('77 300D)

Finally got around to checking things today.
Obviously if I had paid attention to the weak and flickering battery light I would've known something was up a lot sooner, the battery light is for some reason on this dash cluster the worn brake pad light and I'd forgotten.

....
You sure about that? Thought that was my case as well. Turns out the battery light had simply corroded. Fresh bulb and haven’t blipped since.

.

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  #47  
Old 02-25-2021, 12:27 PM
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Originally Posted by Diesel911 View Post
In theory you are supposed to be able to do a voltage drop test.
If you put your volt meter on the negative battery terminal and you ground the other lead good to the Chassis and have someone crank the Engine you are not supposed to see any voltage on your meter.

The same test works on the positive cable but it is harder to get at. You put one end of the Voltmeter on the positive batter terminal and the other end on the stud and nut that holds the cabal end onto the Starter and have someone crank. Again you should see no voltage on the meter.

Well when I changed the negative battery cable it was because I needed a different length of cable and I got a surprised when with the slight voltage increase. So in my case there must have been some issue with the Cable.

When I worked as a Mechanic I found that ends of the battery cables with the crimped eye on the the copper cable inside of the crimp and inside and of course the inside of the tube on the Eye would get corroded over time.

Replacing the whole cable on a a big rig truck due to a bad crimped connector is a whole lot of work and expensive and I was there as a maintenance mechanic so my Boss nor myself was making money off of that sort of job.

I would take a propane torch and heat up the crimped end and fill it with Solder and that restored the good connection of the cabal inside to the crimped on eye. A fast and simple fix. (I also did this to the ground strap/cable on the Mercedes.)

Of course you could get a chisel and remove the crimped on Eye clean the cable ends and crimp it back on but when I did that I still filled it in with Solder.

With a bad terminal clamp end if it is brass you can often do the same where the cabal inserts into it. Stick the cable end in baking soda and water to deal with any acid and to a small extent with corrosion. Heat it with the propane torch and fill it in with solder.

If the terminal clamp is lead all you can hope is the cable is long enough you can cut the end off and install anther terminal clamp.

The pic is of my original Mercedes positive cable. In the pic I put red dashed lines where I took a hacksaw and cut a slot into the Terminal Clamp. I used a Screwdriver in the slot and spread it out enough to pull out the cable.
Once the cable end and the hole the cable goes into were all cleaned of the white corrosion I shoved it back in and again got out the Propane Torch and filled it in with Solder.
I got no extra charging voltage out of that. But it saved me from having to buy another Cable.



This has been a very entertaining and informative thread. The comments esp from 911 are spot on in my experience ie., eliminating corrosion in the connections is high on the list. Formerly I learned about alternators and charging on our commercial Alaskan fishboats. We had multiple alternators, generators and battery banks, isolators, inverters and the rest. I had to go to "alternator school" to understand the basics. To wit the field is in the rotor generally fed by that blue wire on "P" type alternators. Increasing the field current increases the magnetic field which increases the alternator output. All the tests you do center around that concept. Maybe I missed it but I have always believed that good, relatively new battery banks are very important. Once the batteries age testing alternator outputs can become less informative. It is also important to do the tests initially immediately after starting with the battery a bit depleted by the starting process as the first step. I learned early on not to keep old batteries around unless they were sound. We also had independent alternator output black boxes to increase output as needed but you need to know what you're doing or battery and alternator damage is inevitable. We are lucky with these old MB alternators because they are so easy to service, brushes replacement is a snap and in my experience you rarely need to put a new alternator in if the brushes are good and you don't make dumb mistakes and damage things like bearings and diodes. Thanks again for such a good discussion.
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  #48  
Old 04-03-2021, 11:29 AM
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I wanted to follow up on this thread. I’d previously posted that I’d fixed my charge light problem on startup was fixed by a new high amp alternator.

Well...the light came back yesterday after many months of flawless operation. It took a lot of blipping and about four minutes of idling to make it go away. I remembered this thread and cleaned my bulb. It was grungy. It would turn on when running but maybe it wasn’t carrying enough current.

When it failed (battery light stayed on) I tried restarting. Oddly, during glow I didn’t get a battery warning light which is abnormal. This would suggest a bad connection on the lamp circuit. In other words I got battery light when running, I shut down the engine, glow again and the battery light doesn’t turn on, then crank and after idling up the battery light glows. That was weird.

Also I replugged the relay to the warning buzzer up under the dash by the steering column. I’m under the impression the critical signals through the bulb go through here. My hopes were wiping the connector after forty years may clean it up.

The light at startup has gone away...for now. I’ll repost if it comes back.

Feeling a little dopey now buying the new alternator but my original was getting pretty long in the tooth or short in the brush per se. I will eventually haul it into Autozone and get it bench tested. Might be handy to have on the shelf.

Funny on all of my diesel Benzes one of the commonly replaced items was the alternator. My 300sd has had 2 done by the PO at the dealership. Maybe it’s just that they throw a new alternator at a variety of problems. My 240d had one, my 300d has had one. My 300TD didn’t have any records.
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82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
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  #49  
Old 04-09-2021, 06:34 PM
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My car had trouble starting today and I checked the charging voltage. 11v.

Groan.

I took off the regulator and measured the resistance between the slip rings. Open circuit. Not a big fan of the BNR auto parts high current alternators on eBay anymore.

I’ll update my other post.

This is a 4 month old alternator.
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82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
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  #50  
Old 04-09-2021, 07:26 PM
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Quote:
Originally Posted by ykobayashi View Post
My car had trouble starting today and I checked the charging voltage. 11v.

Groan.

I took off the regulator and measured the resistance between the slip rings. Open circuit. Not a big fan of the BNR auto parts high current alternators on eBay anymore.

I’ll update my other post.

This is a 4 month old alternator.
A usb/voltmeter that plugs into the cig lighter is a good thing to have in these cars.

Im guessing that your battery light didn't come on
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  #51  
Old 04-10-2021, 02:31 AM
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Quote:
Originally Posted by ykobayashi View Post
Not a big fan of the BNR auto parts high current alternators on eBay anymore.

Why not just buy a Bosch AL129X? 115amps not enough?
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  #52  
Old 04-10-2021, 08:39 AM
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Alternator light didn’t come on. It needs the field coil to complete it circuit to ground.

I didn’t buy a Bosch because I was hoping the $140 BNR would be a better deal. Apparently not if I count the hassle factor.
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82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
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  #53  
Old 04-18-2021, 11:04 AM
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Okay so some backstory. I put in the 120A bnr alternator and four months later the magnet coil burned out. Open circuit on the slip rings. In he meantime I’ve reinstalled my bosch unit that had the glowing charge light that would only go out if you blip the throttle.

Basically I rebuilt my system to what it was before and I got the old problem back. Start up and the light is on. Blip the throttle and the light turns off. Before the blip, no charging, after the blip charging.

Very odd and frustrating.

So I figured it was time to start digging. I did the following and non of it worked.

1) cleaned the connection at the alternator blue wire at alternator. Still needed a blip to cancel the charge light.
2) cleaned the blue wire at the screw bulkhead on the passenger side fender. Still needed the blip.
3) cleaned the two prong connector on the blue line up by the washer reservoir. It was really corroded but it still needed the blip. It was better now in that once warm a restart wouldn’t require a blip to turn off the battery light. But it still needed a blip on cold starts.
4) changed to a new 3W bulb on the charge light. Still needed he blip.

So I read on the pelican porsche groups guys were putting a 91 ohm 5W resistor in parallel with the lamp to get their cars to boot the alternators without a blip. I worked the numbers and that equated to about a 7W light bulb equivalent. Putting in a resistor sounded too hard.

So I went to Walmart at found Sylvania 2835 5W bulbs in the same form factor as our charge lights. I put it in. Now no blip required at startup.

Hooray!!! But now I own an extra 120A BNR alternator. Wasted my dough. A higher wattage bulb solved it.

Is it possible the OE lamp is 5W?

Anyone got a parts manual?

There are also some power resistors on the board next to the lamp. Maybe they are a 91 ohm load in parallel with the lamp and they’ve gotten old? Solder joints cracked and overheated after 39 years of starts? I didn’t have the patience to tear down my cluster and look at them closely. I just saw them in photos of disassembled instrument clusters online. There are some big resistors mounted around that lamp by design.

It seems like a silly design decision to use a lamp as a resistor. The resistance changes with filament temperature. This was a very old Hewlett-Packard trick from the 50s on their first signal generators. It is not good design. Modern halogen peanut lamps may be more efficient and a 3W today may not draw as much current at a 3W in 1982. Just a wild thought.

Hopes this helps you guys with glowing lamps at startup that need a throttle blip.

Try a 5W battery charge light bulb!
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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
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  #54  
Old 04-18-2021, 11:22 AM
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Getting closer

Ok now this is getting clearer in my mind. My bet is that there is a cold or cracked solder joint in the charge light circuit inside the instrument cluster board. Right at the 10 ohm resistor or diode branching off from the brake wear warning light.

That is why releasing the brake after the car is started and idling made the light shut off on my car. It diverts more current to the battery charge bulb once you turn off the brake warning light. Also as Shern has mentioned, waiting for the glow plugs to heat longer helps. I think this is because the pre glow circuitry on the timer is powered off the same source as the battery warning light. Again, more power diverted to the battery power light once the glow cycle is done. The glow plug power is another circuit, I’m just talking about the pre glow system power as shown below.

Ok. Now you got me going. Time to tear down my binnacle and find the burned 10 ohm resistor, burned diode or old cracked solder joint. It makes sense, those joints are getting stressed thermally with every cold start. Eventually they’ll crack. Stay tuned. The Sylvania 5W bulb is just a bandaid repair.

Edit - okay just got in from the garage. Tore down the binnacle. Checked all the traces and connections on the charge light circuit. The resistor is actually 18 ohms. (Wasn’t clear in the image). All the solder joints and traces were in good condition. The resistor and diode are good. No problems there. I guess I don’t know what is wrong. Maybe the lamp is supposed to be bigger than 3W? I measured the two peanut lamps at the top of the binnacle which do the backlighting. One was a 3.5W and the other a 3W. Maybe I changed them over the years.

Also I should check the warning module. If there is too much resistance between 1-2 on that box then the field coil will get less power.

It might be fun to go to the dealership and ask about the battery charge light bulb. Or maybe call Pelican.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Last edited by ykobayashi; 04-18-2021 at 01:18 PM.
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  #55  
Old 04-18-2021, 01:24 PM
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Bravo!

I’m quite looking forward to seeing what you find in there.
If nothing else, the five watt bulb isn’t a bad fix. I much prefer it to putting another bulb in the circuit like some have done or installing a separate non-bulb resistor.
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  #56  
Old 04-18-2021, 02:57 PM
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Originally Posted by Alec300SD View Post
Part of my alternator upgrade was to add an ancillary ground cable from the negative battery cable/chassis ground bolt connection to one of the unused tapped holes in the alternator housing.
Good idea and many alternators have a "GND" bolt on the case for that. My 1985 300D alternator appears to use the alternator to bracket connection for the return path. My 1969 Dodge Dart was the same and those days had a dash ammeter to let you know when it wasn't charging. I installed a new alternator in the parts store parking lot and still didn't show charging. I got the bright idea to jumper cable from alternator case to BAT- and the dash gage then showed charging, so I knew the return path thru the bracket was the problem. Returned the new alternator, sanded that interface and reinstalled the old alternator and it then charged. In my current 3 1960's Mopars, I added a ground wire from alternator case to ground to avoid that. The later Chrysler alternators (square-back, also fit) have a ground terminal on the case. Indeed, later cars had many more dedicated ground wires and terminals. My 1985 300D is otherwise good on grounds, having many welded ground studs scattered about the car, as do modern cars. In the 1960's, most thought a rusty sheet-metal screw would suffice, causing many problems but after the 3 yr warranty so "good enough".

BTW, if you wondered what the connector on our M-B alternators do, the 2 larger spade terminals both go to BATT+ (via barrier strip fwd of battery). I'm guessing that was because another application sent the alternator output 2 different places. The smaller 3rd terminal is the "boot-up" wire which goes to the dash bulbs via the 1-terminal round connector strapped to the aft of the coolant reservoir, which is a convenient place to break the wire for testing. Short the side going to the dash to gnd and the dash bulbs should light (w/ key in "run") if the wiring and bulbs are good.
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  #57  
Old 04-18-2021, 03:01 PM
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I went under the dash and checked the warning relay pins 1-2. I decided not to pull it. It’s way too hard. I need to get the cruise amp out first and the nuts are too hard to get to. Hate to sound lazy but I’ve had enough of this. I want some lunch.

I measured the pins and I see a good diode in there. The circuit is hooked up. When I turn the key the brake light turns on even if the brake is released which means the 18 ohm resistor and diode are carrying current. I cleaned the contacts on the bulb.

This is interesting, if I hit the e-brake at that point the brake light gets brighter...I guess the e-brake switch goes directly to ground while the leg on the warning relay goes through the 18 ohm resistor, a diode, then the alternator field coil which I recall was about 30 ohms and then to ground.

I’m going to punt and say it’s good in the warning relay. Too hard to take it all apart and inspect the solder joints.

So I’m not too sure why the exciter wire needs the extra current. Something has changed.

Oh...I am running a low idle speed compared to the spec. My car had a shaking problem and I wound it way down to 700 rpm. I notice if I just speed it up a tiny tiny bit on the throttle the battery light goes out. Like 750 rpm.

This may have been a stack up of bad connections and low rpm. Maybe a modern efficient bulb contribution. I dunno. All I know is the 5W bulb seems to have fixed it.
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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
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  #58  
Old 04-18-2021, 03:09 PM
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Originally Posted by BillGrissom View Post
Good idea and many alternators have a "GND" bolt on the case for that. My 1985 300D alternator appears to use the alternator to bracket connection for the return path.

The smaller 3rd terminal is the "boot-up" wire which goes to the dash bulbs via the 1-terminal round connector strapped to the aft of the coolant reservoir, which is a convenient place to break the wire for testing.
I got coolant spray all over my alternator and bracket to the block. I wonder if I messed up my ground. I measured the voltage between the alternator case and the negative terminal of the battery and the potential difference is minuscule at idle. Basically 0.

On my SD the connection near the coolant reservoir is two pin. I should check my 123 to see if they do it differently. I’ve splashed a lot of windshield washer solution on it over the years and it was pretty corroded.

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82 300SD 300k miles
85 300D Turbodiesel 170k miles
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