hi
above idle(2000rpm) the voltage 13.50 with every thing on as soon as you put it at idle the voltagestartsto drops slowlly in about 5 min the volt goes to 11.99-12.00 v
the cable i removed from the alternator to the battery because the old one (70amps ) had a connector to the alternator .with the newalternatorit has 2 studs one size 10 and the other size 8 .

 

....above idle(2000rpm) the voltage 13.50 with every thing on.
OK. as it should be.
as soon as you put it at idle the voltage starts to drops slowlly in about 5 min the volt goes to 11.99-12.00 v
....With every thing on? If so then this is showing a heavy discharge at idle. Shows that alternator is working OK.
What is the voltage with a normal load. i.e. ign, radio, wipers, side light. I'd be worried if this was still as low as 12v. 12.5v would most probably be OK.
...the cable i removed from the alternator to the battery because the old one (70amps ) ...
Sure the new cable is the correct size? (Cross section/No. of strands)

Just for info....My S Class alternator is 150 amps. this is not unusual on "modern" cars with a load of electrical equipment.

Stuart

 

hi snanceki
here is some of the tests i took after installation of the new alternator(80 amps alternator):-
battery voltage at rest---------------12.75
at idleno load ----------------------14.21at normal tempreature 80c
at idle fully loaded ------------------12.00 -11.99 after 5 min with every thing on ( headlight,hi blower,rear defroster & radio )
at 2000 rpm fully loaded ------------13.50-13.55
while driving i took some reading with the multemeter hooked to the cigeratte lighter :--
speed 70-80 mile fully loaded ------------13.00v
speed 70-80 headlights on only------------13.8v
speed 70-80 no load -----------------------14.00-14.1
the cable i installed was the dealer p/n 124-543-45-26for the 80amps alternator
the alternatori installedibelieve was 80 amps wasfor 380 sel v8 engine
the other thing i did not mention is i replaced the battery with new one bigger size interstate( mt-93 )
my concern is is it normal for the voltage to drops at idle with load on below the 12.00v after 5-8 min at idle
you have 150 amps what is the voltage reads with every thing on incloaded the rear defroster and wait for 5 to 8 min at idle.
on my carit seems like only when the defroster on it start to drow the volt quickly.

 

I still think that the volts on full load are lower than I would expect. However if the volts pick up with a little throttle it shows that the alternator is working fine.
Since the lead is an MB one I'm sure that that will be OK!!
I wonder whether you have the correct pulley fitted.
Did you use the original pulley for your car or the one that came for the alternator. Were they the same size? What are there diameters? (OD will do)
Maybe the alternator is NOT turning fast enough at idle.

Stuart

 

yes that what i was thinking at first but i installed the same one on my old alternator (50 mm ) measuerd from the midle of the pulley. and i found a smaller pulley dia (48mm) but i was worry to install itin hi speed could burn the alternator if it the alternator no t desgin for that speed.
every thing is ok now i have no problem with dead battery . the only thing worry me is this normal for all cars to drop voltage at idle below 12.00v.or should i go for higher amps alternator .

 

A higher outpit (amps) alternator will not necessarily address your problem.
Output changes with speed of the alternator and the rated figure is the MAX (high speed) output.
A smaller pulley is most probably the answer but you are correct in stating that you mustn't overrev the alternator. 48 vs 50mm doesn't sound too big a difference in OD to over rev the machine.
Do you often use max revs on the engine. I guess not.
However you may do a lot of idling aroundtown in the winter etc.
A GOOD battery is designed to cover these periods where the consumers are using more power than the alternator is producing, so there is not a real problem.
Its all about how long you are in deficit vs surplus and that is down to your route and the consumers you have switched on.
Stuart

 

It depends on the make/model/ mercedes alternator brand of your car for the parts. Labor should not be bad if you have a front engine car. It should not require more than one hour to install unless you have a really cramped engine bay like a late model.