Normal output for W115 alternator
 

I suspect mine is toast. Bearing noise too. Just wondering what voltage to expect with accessories off at idle measured from the battery.

In a perfect world, 13.8 to 14.2 V assuming a fully charged battery and no electrical load. If it takes a high idle to get up to the range that is ok. Any load/drain from an accessory or a shorting battery will lower the voltage. Realistically, 13.4 to 14.4V is fine

That’s what I thought coming from the gas world but didn’t know if it translated to diesel. I loaned out my multimeter and am using a backup that doesn’t show decimal points and it fluctuates between 12 and 13 volts. Harder to start after a run involving the headlights etc. so I figured it’s time to rebuild it.

Dumb question but what alternator should have come with this car? I can’t tell but it looks original. And I want to order the bearings and voltage regular to attempt a rebuild.

The most reliable way is to ask your local friendly Mercedes dealership if they'll look up which part you need via your VIN information - they'll be able to give you a part number (and probably a small heart attack for the price). Alternatively get EPC access yourself.


If you don't / won't / shan't then have a look on sites such as the site sponsors to see which part number they recommend.

There is a fair chance the alternator has been replaced once in the life of the car. What year and model is it? Internal or external regulator? Fortunately I have a great local rebuilder to take items to and get the original correct unit back with a thorough rebuild at a great price. Perhaps there is one in your area to source parts.

Good luck!!!

W115 Alternator
 

I was thinking the 115' still had the external regulator , your signaoure says 1975 so here's the most likely one :

MERCEDES-BENZ 230 1974-1978
MERCEDES-BENZ 240D 1974-1983
MERCEDES-BENZ 280 1973-1976
MERCEDES-BENZ 280C 1973-1976
MERCEDES-BENZ 280CE 1978-1981
MERCEDES-BENZ 280E 1977-1981
MERCEDES-BENZ 280S 1975-1976
MERCEDES-BENZ 280SE 1977-1980
MERCEDES-BENZ 300CD 1978-1985
MERCEDES-BENZ 300D 1975-1985
MERCEDES-BENZ 300SD 1978-1985
MERCEDES-BENZ 300TD 1979-1985
MERCEDES-BENZ 450SE 1973-1976
MERCEDES-BENZ 450SEL 1973-1980
MERCEDES-BENZ 450SL 1974-1980
MERCEDES-BENZ 450SLC 1974-1980

This is the 55 ampere one with internal regulator, if the bearings are bad it trash, order in the 65 ampere one for a 1985 300D, it'll bolt right up and have more out put .

Be aware that the brand of the regulator has everything to do with the charging voltage.

I only use BOSCH branded regulators for this reason .