After replacing the alternator, and double checking wiring, I find that when I start the W114 and let it idle, the voltage reading at the battery terminals is 12.1 volts.
When I increase the RPM's the reading is 13.9v-14.1v. OK. So then I let it idle again, and the voltage reading is 13.7v.
After that, I shut the car down, disconnect the Negative, and the battery reads 12.8v.
This car has to be dependable for my daughter, so I'm asking: Does this sound normal?

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old

Sounds OK to me.

You start the car and let it idle. The battery is being recharged slowly since the alternator is not putting out much at idle. You run it up and alternator output increases. The idle drops down but the battery is fully charged and voltage is about right.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I worked on the car today, and kept checking the voltage. The voltage is 13.8+ and the battery stays charged.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old

Hello,
Just as a precaution, check the ground cable from the battery negative terminal to the bodyshell mounting. Remove the cable and confirm that there is actually copper wire inside the insulation, not just green dust and check the bodyshell end mounting for corrosion and a secure captive nut.
Have a good week and holidays

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Nachi11744

I ran an extra 2 gauge ground wire to the transmission housing. The original ground starp was approaching 'Green dust'

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old

Hello,
I do not mean to sound like to breath down your throat, but you need to run a NEW wire from the BATTERY negative terminal to somewhere on the engine block OR a secure, non rusted area on the bodyshell.
The ground strap between the body and tranny housing is the thingy that you have replaced?
Sorry if I misunderstood or come across as condescending
Have a good week and holidays

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Nachi11744

I left original in place and added a 2 gauge to the driver's side of the transmission housing. Thanks.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old

Maybe check the ballast resistors? Don't know if they have anything to do with your problem, but its a cheap and easy fix. I discovered that mine were virtually disintigrated - not surprising for a 30 year old part.