W123 correct alternator output?
 

Hello all, I searched the archives and got some answers.

My folks 1979 240D which is in San Diego had the battery changed today. The battery was about 6-7 yrs old and went bad. The mechanic reported that the alternator was at about 12.9 volt output but should be at 14 .

He said that for the time being, for city driving it was OK but later it may be a problem.

That alternator is a rebuilt Bosch that was replaced by the previous owner.
I'd have to look up the service records.
It is visually new looking.
So, in case the warranty is out, could it be the brushes? Is the quoted voltage OK for this car?

I declined the replacement because I knew I could do a fix once I visited later in the summer.

Expertise please?

Thanks

Denis :sun_smile

12.5 volts
 

i am not sure what is acceptable. if it is the brushes the charging will be erratic for a while before failing. brushes are not too expensive and take about ten minutes to change. the regulator is integral with the brush assembly. two screws to remove and replace. the wires plug in all in one big plug. bosch is best brand if you can find it. others less expensive but dont last as long.

unkl,
12.5 volts is just about battery voltage and won't charge the battery. You need 13.5+ volts to charge the battery.

Take a voltmeter and place it across the battry terminals before U start the engine. You should get about 12.5 volts. Then start engine. The voltage should now read higher: 13.5 + volts.. If it doesn't, the alternator isn't working.

There is no setting on the voltage regulator. Its solid state and if it doesn't work, it must be replaced.

P E H

Do these things have diodes like in the old days ? That used to be something which could give these kinds of symptoms and be corrected by cheap parts and careful soldering.

leather,

Yes, these things have diodes. THat's what gives the DC output. Normally alternators produce alternating current.

P E H

voltage
 

at idle most of the diesels and other mercedes appear not to be charging, do you know if the mechanic revved the engine while he was checking the output voltage? that makes a big difference. i work at a mercedes specialist shop in san diego, send me an email if you would like me to look at it, no charge of course, to see what i think. my email is mercedessd@cox.net i check my emails often every day, let me know. Ray

correct output is 55 amps and everyone is correct in saying it ought to put out at least 13.5 at 1000rpm.

MTU,

55 amps is the maximum current output of the alternator. It will only put out 55 amps if the battery is low on charge and/or a lot of electrical accessories are being used.

P E H

Thanks
 

Thank you all for your insight !

Ray, I will communicate with you when I visit Carlsbad (north county) hopefully in August late.

I'll bring my multimeter and check it out as per this thread's recommendations.

In my mind I also suspected that perhaps the belt was not adjusted right.

In any event, measuring the electrical ought to give better infos.

Then proceed from there.

Thanks again. :sun_smile


p.s. the alternator had been replaced Sept, 17, 2001 at 112,304 miles. It now has about 139-142K miles on the odom.

Thanks for the timely conversation. The battery on my recently purchased 85 300TD is of unknown vintage and doesn't seem to hold a charge well. At idle with no options on the output from the alternator is 12.5 volts. I'll rev it up and see if it improves. I don't have high hopes though. Based on this post and the Volts post it sounds like it is time for a voltage regulator as well.

55 amps?
 

i thought on a 240 the factory alt is 45a.

240d
 

Just to add more details on my 240D, it has manual windows and no sunroof. The a/c is the old manual control type. 141K miles

I will do tests on it hopefully in late August.

edited, I just received the service receipt by mail. The alternator was tested as charging at 12.9 volts.

I just read the Haynes repair book and it voices the previously stated tips. It states that correct voltage should be about 2-3 volts over 12 Volts at fast idle or if the voltage progressively rises and tops at say 18 volts, then the regualtor is bad.

Denis