What is your charging voltage? My 300D is around 13.4 v cruising on the hwy and that's quite a bit lower compared to my TDI which charges at 14.1 v . Has anyone tried raising the charging voltage by modifying the voltage regulator?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have a Scanguage on my vehicle, and in stop and go traffic, it registers anywhere between 13.6-13.9 depending on what accessories are on. On a long drive, it usually stays between 13.4-13.8

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96 E300d

This is normal but as you think , it's too darn low IMO .

I am beginning to experiment with BOSCH brand regulators ending in 023 & 028 , those should do the trick , no other brand (and I've tried lots) will give above 13.9 VDC and they usually only have 11.5 @ idle when the lights , HVAC and radio are on .

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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

My 123 cars charge in the 13's like you mention. My BMW has a 14.1V regulator (hella or bosch, cant recall), and is 14.1 at some temperature around freezing, goes lower at higher temps, higher at lower ones. Still at steady state under load it is in the 13's too.

My saab is consistently at 14.4-14.6, regardless of load. On long drives with no real auxiliaries and a charged battery, it may go down to 14.1.

Id sure like a higher voltage...

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

13.6-14.4 is the normal range for a 12v system.

I came across this article if you have the same type of Regulator:
Bosch Alternator Regulator Modification with diode to increase resistance
http://www.detomaso.nu/~thomast/alternator/

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84 300D, 82 Volvo 244Gl Diesel

Try the diode modification diesel911 found. It makes good common sense.

Looks simple to execute as well.

Thanks for the link. Looks easy enough. Hope I have that type regulator.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have found that my '87 300D (W124, OM603) charges at a higher voltage when it's cold (both outside temperature and engine temperature). As the temperature rises, the charging voltage drops. In any case, it's usually around 14 volts depending on engine rpm and electrical load. The OP's 13.4 volts sounds a bit low to me.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The charge voltage needs to be sufficient to provide the necessary charge rate to the battery. if it runs too high once the battery is fully charged it will boil the electrolyte & damage the battery. With charging systems more is not always better. If you have a 65 amp alt, 13.4 or 13.6 is fine. If you have a 120 amp alt you can expect to see higher voltages. Fast charge & discharge is not a good thing. The 13.4 or 6 is a RMS voltage as well, as the alt is producing a rectified sin wave. It will be peaking much higher. If you put a CRO on the output of your alt you will see peaks up around 20v. I would be putting my effort into having heavy leads rather than high output voltage from the alt.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I just had an oscilloscope on my car's output to find that I had a failed diode.

The reason I mention it is that there wasn't all that much variance in the voltage, probably due to it being a rectified 3 phase sine wave. With that many phases, and since it's rectifying both the high and low sides of the sine wave, you end up with a fairly smooth waveform on the output. (At least you should if you don't have a diode out, causing a huge valley in the waveform!) Picture HERE

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

Please measure the voltage (at 2000 rpm) before and after you fix the bad diode.

I am gonna put a switch across the diode mod so I can switch it in/out. At idle, my voltage is around 12 v which means it is not charging and I don't think that is normal. Raising the charge voltage will help in my case.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

My voltage stayed at 13.9-14.1 max, both before and after, when running at 2k-2.3k
(The 2.3k figure was mentioned in the ETM for the alternator max voltage test, so I was looking there.)

My idle voltage was 13.6 if I remember correctly.
With your voltage running at 12v, I have to wonder if there is something else wrong with your alternator, other than the cautious voltage regulation they are built to do.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

FUNNY YOU SHOULD ASK. I have a friend who made his own adjustable 'on the fly' regulator. (He was snowed/iced in and the reg burned out in his work van, so a good bit of it might have been cabin fever.) He's a retired EE from Ma Bell and a ham-call sign N4ZD. He also works on avionics, aircraft radios & electrical systems.

He might be persuaded to build you one but you'd have to ask nicely and offer $25 or so. His name is Art. Aerofabco@juno.com.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!

I installed a new battery to replace the weak battery in the 300DT. Took a reading today and it is charging at 13.9 volts with no loads (at the battery) at 2000 RPM. If I turn on the headlights and blower, it drops to 13.6 volts at 2000 RPM. I may still do the diode mod and add a switch to short it out so I can select high or normal charge.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked