If your alternator is maintaining 14-14.4V shortly after a cold-start, it is delivering adequate power and adding a higher amperage alternator will net you nothing more than extra inertial load on the belt drive system.

In the case of my 350SD I mentioned above, the stock alternator is 80A and the afterglow really makes it strain at idle. Once afterglow kicks off, there's no real difference other than slightly brighter headlights due to 14.4V vs the 13.5V or so of the older alternator design.

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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!)

On a stock car, I don't see much of a reason to put in a higher rated alternator. This article covers subject better than I can:

https://www.onallcylinders.com/2015/03/27/alt-text-5-things-you-should-know-before-using-a-high-output-alternator/

As the link says, a 120A alternator probably doesn't put out more than 90A max. But even that, is only at max rpm. At idle much much less. I don't think you would want your battery to be charged at 90A!

I recently put a new starter into my 85 300D. I also replaced one glow plug that had failed. The starter spins so fast! Night and day compared with old starter. Car starts immediately even on a day like today (-7C).

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I couldn't get the anemic 55a stock alternator to keep up on my swap. Swapped in an enormous 22SI with the intent to power inverter, and don't regret it at all. If I've got to build a new wiring harness and new mounting system, why not go big anyway? Maintains charging voltage with everything on, at idle. Couldn't be happier.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Sorry for the slow reply. I bought my 120A alternator from an eBay seller called bnrparts. They have a lot of ratings and apparently are very reputable. They said it is a rebuilt Bosch unit.

The one I bought was a direct bolt on with no changes required.

Dkr.

Thanks for the heads up. I just ordered one from bnrparts. 120A. The photo shows a single belt pulley. I specified I needed the double belt.

It seemed like a pretty good deal at $150 with shipping. Autozone costs about that for the Duralast Gold. The Bosch 65A rebuilds run a little more.

My voltage has been sagging a bit for no apparent reason. I swapped a regulator with a spare Chinese unit I carry in the trunk for emergencies and it made no difference. I think my alternator is flaking out. I start it and the trouble light stays on with the other neighboring indicators. I measure 12.5v at startup then when I blip the throttle it goes up to 13.5v and stays there. Sometimes it goes up as high as 14 after revving.

It’s like the light turns on at startup then I blip the throttle to shut it off. Not a good sign of things to come. The existing Bosch AL117X unit was replaced by the PO at the dealership for $590 in 2010. Ouch. I’m curious to see if it failed. I guess ten years of use isn’t bad.

Same with either regulator. I checked my battery/ground connections and there is negligible voltage drop across them. Ground from my negative terminal to my alternator case is also good.

We’ll see what the 120A alternator does.

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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

Well, I put the BNR alternator in. My voltage is up to 14v. No blipping of the throttle required. I’ll take my old bosch down to autozone for a test. Curious to see what is bad.

I wonder if it’s worth keeping as a core or rebuild candidate. It is 11 years old and I guess it has lived it’s life. I also noticed that my lower radiator hose had a slow drip probably when I was doing a pandemic speed run through LA to OC. It usually doesn’t drip but I stressed her out pretty nice. There was a film of coolant on the alternator. Perhaps I should split it apart and see if there’s a pool on the diodes. This may explain my sagging voltage that goes away with revving.

The BNR was a headache to put in. The connections weren’t clocked to the right angle in the back so the harness didn’t reach. I tried to disassemble it and rotate the case but I stripped a screw head. I eventually drilled out the screw and stole a screw from my old alternator.

Once in it charges well. But the rotated case added an extra hour to the job. Oh well, we’re stuck at home with the pandemic crisis so I guess it’s ok.

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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

I've always had this issue. Last alternator I had and new rebuilt bosch I installed a few months ago.
Need to blip the throttle to "excite" the alternator.

I assumed this was normal?

-

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1981 240D 4sp manual. Ivory White.

Not normal, but my 83 sedan is the same way. I have an AL129X 115A alternator from a Saab.

83 wagon, stock alt, no dash lights or throttle blip required.

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1983 300D Midnight Blue
4 speed, W115 intake mani, non-EGR exhaust mani, KKK K26, 3 pc Euro bumpers, Lo-spec Euro headlights, AL129X, hubcaps, custom A/C

1987 300TD Smoke Silver
Euro headlights, thermostat relocation, coolant bypass mod, rebuilt Becker + Jehnert + Helix audio, OEM Oris roof boxes and surfboard racks, Euro towbar

1983 300TD Reed (Moss) Green / Dark Olive MB Tex Euro spec

1983 300TD Silver Blue / Blue cloth Euro spec, OM648 and 722.6, 15" hubcaps, W126 S2 brakes

I would say not normal.

My sd came with the newly replaced bosch alternator in 2010. It never lit the charge light when started. Just this month it started doing it.

It’s totally gone now and the voltage comes up to 14v

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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

Well what could be causing that? My alternator appears to be perfectly healthy (and new).
Is this a ground thing? Secondary fuse?

Edit: It's the "exciting" wire. Nothing to do with the alternator itself... now to trouble shoot its path

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1981 240D 4sp manual. Ivory White.

A bit more info I found:

"if the alternator is not charging, the blue (exciter) wire becomes ground, and the bulb lights up (since it's connected to 12v+ on the ignition switch side). When the alternator starts charging, the blue wire has a positive charge applied against it... no more ground = no more bulb lit up."

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1981 240D 4sp manual. Ivory White.

Just wanted to follow up. Less than four months later the BNR alternator has gone belly up. Open circuit between the slip rings. No battery light at startup or running. Charge voltage 11v.

Yuck. Just thought I’d let everyone know. Now I have to pull this junky part and ship it back to them my dime.

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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

That's too bad. Did you ask if they would let you have it repaired at a local alternator shop and send them the bill? Seems like that would be a good deal for them.

No, I just sent it back. They turned one around to me really fast. I haven’t installed it yet.

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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

Eagle Electronics was B.S. ~ I went there in person and they didn't have any way to test the actual amperage out put...

The original 55 ampere unit was never more than adequate, for the 1985 model year they upped it to a 65 ampere unit that's a direct bolt in ~ same plug connection .

The 80 and higher amperage units found on SAABS and Volvos are a direct fit -but- they have the better threaded stud connector ~ you really don't want a push on connector with the higher out put .

In any case, none of the none BOSH / HELLA regulators will charge to 40 volts unless the battery is seriously discharged .

I'm currently running the '85 model 65 ampere unit with a BOSCH regulator / brush holder and it's normally 13.5 + volts, goes up to 14.2 occasionally depending if I'm doing a lot of in town stops and re starts with the AC going...

Just replacing the regulator / brush holder will get the 55 & 65 ampere units up to 14 VDC off idle .


I've also used the AL129X units to good effect but was sad to find none charged over 13 VDC u less I replaced the HUCO or other off brand regulators .

If you can find the brushes they're easy to replace, I often find older dirtier Mercedes' have oily and black rotor slip rings, easy to polish up for optimum contact .

Never, EVER use steel wool, crocus cloth or other abrasives on any electrical contact surfaces ! .

If you're near a self service typ of junkyard, go looking ~ take a picture of your current alternator and you'll often find a nice clean low mileage BOSCH branded rebuilt for cheap .

90 + % of the time only the brushes wear out, easy to fix DIY .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better