On mine the Ground Strap is actually a cable no insulation and an 2 crimped collars with an eye on it which is different then the old US cars I used to own that had a flat braided wire ground strap.

There is different selections and lengths ground straps on places like eBay.

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84 300D, 82 Volvo 244Gl Diesel

Yes, this is also the case for both of my 300Ds.

The socket was fine but I cleaned it up anyway.
Wild… This light would come on from time to time so I assumed it worked OK.

Fresh bulb and everything appears to be fine.

This has me reading a bit about upgrading ground wires around the battery/starter. Switching to 2 or even 1 AWG. I run a few power-hungry stereo components, amps and what have you, starting to think it might not be a bad idea

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1981 240D 4sp manual. Ivory White.

If you did a proper amp installation, the power should be coming straight off the battery positive through a fuse to the amp. Upgrading the rest of the wiring in the car will net you absolutely nothing. The factory ground wire to the chassis is good to pull a continuous 150 amps or so without worry. If you're running more than 1 amp, I'd strongly suggest upgrading your alternator if you're worried about power consumption.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Of course I did a proper amp installation.

I’m agnostic. The reason had more to do with the electrical system as a whole.
Specifically referring to d911’s posts.

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1981 240D 4sp manual. Ivory White.

I was supersized to find that the stock Positive Cable wire part is 1/4 of an inch and with the insulation is .410 of an inch.

For some reason the cable chart I found shows the diameter of the cables with insulation. When you look at that the closest to the Mercedes .410 diameter is 2 gauge which is .418 but I have no spec on the wire diameter.

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84 300D, 82 Volvo 244Gl Diesel

I know that you like to ignore advice I give, please know that it comes from experience. My entire career revolves around power generation, voltage regulation, and electronic design/repair. I won't bore you with any more advice or tips, it seems to fall on deaf or unappreciative ears anyway.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

A 2 gauge (AWG) conductor is ~ .25".

I think you might have me confused with another poster.

If you check our correspondence in the past, you'll find I've been immensely grateful for you guidance. And still am. I've learned a great deal from you. I found your phrasing a little abrasive, but I do apologize if I've given you the wrong impression.

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1981 240D 4sp manual. Ivory White.

Sorry if I missed some posts. A few quickies:

The alternator in my 1985 300D is termed "Motorola-style" on a web-site about them. Also used by GM at that time. It has self-generated field power, which has to "boot up". It uses current on the return side of the dash bulbs for that. Originally just the "BATT" bulb, but looks like from 3 dash bulbs by 1985. If your dash bulb(s) doesn't light on key-on, the Owner's Manual states to immediately drive to a dealer for "emergency bulb swap". Today's owners are hopefully less helpless.

If you use LED bulbs in your dash like me, that doesn't provide enough "boot-up" current. However, there is generally enough residual magnetism in the rotor to boot-up if you rev >1000 rpm. The dash bulbs go out once booted-up. The OP sees that, so has no real problem.

If you want an emergency fix, the blue exciter wire has a connector clipped to the aft side of the coolant reservoir (1982-85 300D). You could unplug that and apply current to the alternator side of the connector, with engine idling. I wouldn't apply 12 VDC directly since might smoke something. You could carry a ~100 ohm resistor between BATT+ and that connector to provide a reasonable boot-up current. Of course, just rev'ing the engine is much easier if that works.

Re. BATT- ground, there is a thick ground strap between transmission and frame on the driver's side, off a bell-housing bolt. Corrosion there was a problem for me once, which I noted because I saw smoke from the speedometer cable as I cranked the engine when the dash was pulled out, so it was using that metal as the ground path. Since aluminum oxide is a poor conductor, I removed that connector, sanded all surfaces, coated with silicone grease re-bolted. This was in a CA car, so I imagine much worse in the salty North. Also, clean the BATT- to frame connections right by the battery. Cleaning battery terminals is an annual task in the East, which is why even grocery stores sell a battery brush.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Thanks Bill. In the end, it was a corroded battery bulb causing the issue.

I cleaned my ground strap connection a few months ago, but didn't know enough to inspect the strap itself for corrosion. Good point re: cleaning battery ground.

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1981 240D 4sp manual. Ivory White.

This has been an enlightening thread.

I think I should have checked my bulbs before buying the new alternator last month. But, my original was 11 years old with worn down brushes so maybe it was time.

The bulb issue and “boot up” Bill mentions makes a ton of sense. I had no idea a working idiot light was actually necessary for a functional charging system. Btw my bulbs always worked. I had measured the low charging voltage before blipping with a meter at the battery.

On my todo now is put in a new charge light bulb. And, check my ground straps...I haven’t looked at the tranny one in a few years. I remember it was the wire rope type on the SD. I guess it’s cheap insurance to just get new ones.

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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

Suppose it’s been documented, but I still find it quite surprising.
I posted this in the other thread, I got an additional .2v alternator output by simply cleaning my battery ground cable. It was not visibly corroded. I inspected my ground strap about a year ago and cleaned the connections, though to be honest, I wasn't totally sure what to look for. I too have the braided rope type. I think you might be right regarding cheap insurance.

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1981 240D 4sp manual. Ivory White.

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.

See photo # 4 in post #2 in the link above.
A more direct negative cable connection (about 3 feet) is an improvement over the stock (about 10 foot) path from the alternator housing to the negative battery terminal.

Good grounds help keep the electrical gremlins away.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

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.
I had only installed a new voltage reg with new brushes 3 years ago and it had not done much mileage since, being parked up for 2 years.

Just to make sure, I check the connections at the alternator plug:
12.81V at both the output wires, 12.52 at the excitement lead. Obviously this voltage drop is expected with the bulb in the circuit.

So I yanked the alternator out, and pulled the reg brush unit out and found a brush had broken off. The remnants of the broken brush had worn like it was touching the slip ring but obviously not enough to work properly.
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.
I don't know what caused the brush to break as the slip rings were fine however there was probably a mm radial play in the rearmost bearing which isn't good.
Ended up taking the alt off my '78 which is a Motorola unit. Cleaned the terminals, checked the brushes etc. 4.3 ohms on the field coil. (Old alt was 3.6, FWIW)

Works great, but! Back to having to blip the throttle to energise the alternator again (same happened with the '78 I removed this alt from). The old one, before it played up, wouldn't. Although I know many other diesels do it.

So, in this instance, the alternator itself is to blame....

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD