hi there, my alternator puts out a voltage of just over 12,5 volts. I've already had it rebuilt (diodes, windings) and the regulator replaced. My battery seems to be OK with 12,2V and an 80% charge. and the voltage is still only 12,5V
a couple of days ago I experienced a lack of electrical power when I turned on the lights, climate control and radio - switching on the hazard warning lights caused the radio to switch off. later that day I had problems starting the car - the battery was almost dead.
what's the problem with my alternator - help please.

__________________
1987 300E 185k miles

Hi there,
You must have gotten hold of a defective regulator. The alternator's output should be at least 13.8v in hot weather with a charged battery. In cold weather it will put out more like 16 volts. Make sure the regulator is grounded well to the case. this is done through one of the screws that hold it in place, and it's possible there could be some corrosion underneath the regulator on the frame of the alternator. Also check for a good ground between the engine and the frame of the vehicle. If you have access to an oscilloscope, look at the output of the alternator, it should show a nice waveform with no missing peaks. You could have a defective new diode pack causing low current output, but the voltage should still be good, just missing a phase or two. My money is on the regulator or regulator ground.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

this might be worth trying. mine is a 92 e300-24 with an oversized bosch alternator 120 amps usually used in 8 cylinder benz but still having low voltage ~ 11.5v. with lights and climate control at idle ~600rpm.

modifications was made by my electrician and it has been a month now.

even at the highest load with a/c at high blower spped, high beam and
car TV on with built in battery charging. checking voltage at battery and at ignition, here are the values

battery ignition
before modification
13.5v 13.5v no load, 600 rpm
11.5v 11.5v at full load, 600rpm


after modification
14v 14v no load, 600 rpm
14v 11.5v at full load, 600rpm


modifications: we directly connect a fused(high amp fuse as a protector ) wire from positive terminal of battery to positive terminal of alternator.
they have done a lot of these modification.

even if im having low voltage at ignition, i am sure that my battery is getting fully charged and at low risk of no start situation from a discharged battery.

i dont know the principle behind but maybe those who have knowledge in car electrics may have an answer

The regulator is good - I've tried two different (both new).

This might be a good clue. I'll have to check.

The output is a perfect waveform.

Thanks!

__________________
1987 300E 185k miles

You might try search...1986-89 300E's used an alternator designed for the 190's...too small only 65A...the 1990 & later cars have an 80A version..that is what we always install on the earlier engines as well.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

1984 300d Trbo 139k
Replace The Battery New 12.4 Volts, This Information On New Alternator Install

Never had an alternator problem with any of my MBs...knock wood, so not too much experience digging around inside them. However,

Alternators usually employ some internal method to energize the field so the unit will produce AC power. Your main diode assembly converts the production AC into DC to charge the battery. There may be another diode assembly inside the case that is used to energize the field coils. On older GMs this was called a diode triplet. I have had one of the diodes in the triplet go bad and that caused a weak field and only 12.5 volts of output, just barely enough to keep the system running, but not enough to prevent the battery from running down over time. It was a condition similar to yours, so maybe a similar cause?

Just a thought...FWIW and good luck.

230/8

my oem alternator for e300-24 is a 90 amp but got busted twice on its diodes but since the reliability of a replaced diode lasts only for at most 2 years and still experiencing low voltage, ive decided to upgrade it to 120v since there are no modifications needed. but with the upgraded alternator, i still have low voltage problem with full load, thus the modification was suggested by my electrician which at first im hesitant of having it done.

i have a 90 vw golf with similar bosch alternator with also similar problem, and also planning to have this modification soon.

tnx

I've upgraded to a 90 amp alternator and the voltage is perfect.

__________________
1987 300E 185k miles

make sure that the serpentine belt is tight enough.the belt tensioners were an issue on these cars and if loose will slip on alternator without making noise.david poole european performance dallas tx.