Charging light question

[Jonny B]

It is a nice day in NE Ohio, so I decided to drive the 113 to the office. On the way in, the charge light was faintly glowing. It was almost off at idle, but would stay slightly lit the rest of the time. I did not take the time this morning to check the belt, but it is less than two years old, as is the alternator. I also will need to recheck the battery level. No nasty noises, or problems with the lights, blinkers etc. I would guess that during the day, you might not even see that the light was glowing.

Any other ideas? Voltage regulator??

Jonny B
1967 250SL Auto

[bpossel]

Hi Jonny,

I would check the obvious first:
1. Cable connections, clean & tight (battery end, starter end, ground to body)
2. Ensure belt is tight (slight flex ~1/4")
3. Drive to AutoZone, or Advanced Auto and have them check alternator, battery...  (free $0)
Good Luck, Bob

bpossel
Memphis, TN.
1971 280SL
1997 E320

[Naj ✝︎]

Jonny B

In my experience, loose belts squeal when reving the engine because of the high torque demand from the alternator during charging.

I would check the voltage. Should be around 14.4 volts above idling speed.

naj

65 230SL
68 280SL

[TheEngineer]

I observed the very same thing. Then I connected a volt meter. The voltage comes up to about 13.8 volts at higher RPM, so, for the time being, my battery is still being charged.

[mdsalemi]

Agree with the rest: alternator, voltage regulator, belt tightness.  You can get a free test or try all three cheapest to more expensive.

Caveat: On a 1978 VW Rabbit (all Bosch, so no snickering) I had once, had the same problem.  The free checkout at Sears Roebuck told me it was the belt tightness.  So I tightened it.  Some weeks later, same problem.  Tightened again.  Some weeks later, same thing.  Then I really tightened it--only to trash the alternator bearings shortly thereafter! :evil:  Live and learn...I'm sure there's a spec to the belt tightness, would be a good idea to behoove it!

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Jonny B]

I checked the belt and it was a bit loose, and I tightened it some. I was and am mindful of the advice from Mike regarding putting too much tension on the bearings. The light is still very faintly on, very hard to see in the daylight. The car started and ran fine. I checked the battery level, and it was alright. I checked the voltage across the terminals and it reads at 13.5.

I checked the maintenance book on how to test the output of the alternator, and I do not have the resistance loading device they specify. I take it, that is something that the auto places can check. Or is there another way to check this?

Jonny B
1967 250SL Auto

[ja17]

Hello Jonny,

Looks like your voltage reading is marginal. Should be ok for a while. However you may have some worn brushes in the alternator which will fail soon.

You can easily check the voltage regulator by simply un-plugging your old one and plugging a new or borrowed one in place to see if the problem is cured.  The voltage regulator is easy to get to in the engine compartment (right side).

Joe Alexander
Blacklick, Ohio

[Jonny B]

Joe,

The voltage reading was across the battery with the car not running. I was not sure about the proper way to get the output of the alternator, since all I have is a volt/amp/ohm meter

Is the voltage regulator an expensive piece?

Jonny B
1967 250SL Auto

[mdsalemi]

quote:Originally posted by Jonny B

Joe,

The voltage reading was across the battery with the car not running. I was not sure about the proper way to get the output of the alternator, since all I have is a volt/amp/ohm meter

Is the voltage regulator an expensive piece?

Jonny B
1967 250SL Auto

Jonny,

The voltage regulator is a relatively inexpensive Bosch part:

http://mercedes.thebenzbin.com/?year=1967&make=MB&imageField22222.x=15&imageField22222.y=9

and I guessed correctly, about $40.  Available most anywhere from the usual suppliers, or most "Bosch" dealers which would mean most foreign auto parts suppliers.

Alternator output is checked on a machine, go to Pep Boys, AutoZone, Sears, and they should be able to do it in a few minutes.

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Jonny B]

Thanks Mike,

If the weather is good, I may try to get this check done tomorrow afternoon, if not then it will have to wait on some travel.

Jonny B
1967 250SL Auto

[ja17]

Hello Jonny,

This is a decent reading with the car not running. You should re-check with the car running.

Voltage check is only one indicator. A repair shop will also check amperage. The voltage regualator could still be defective. The AC cars have the alternator right on top (as yours if I recall). I agree the simplest cure is to take it off and have it tested.

Joe Alexander
Blacklick, Ohio

[Jonny B]

I think the finger might be pointing. I took the car over to Auto Zone this afternoon, and they hooked up their magic machine. The voltage appears to be alright, 14+ range. The amperage however was at about 20. I am probably going to leave this alone for now, but it looks like an alternator might be in the offing. Argh, and this one is only 2 years old!

Jonny B
1967 250SL Auto

[DPaulski]

Looks stupid, but can someone let me know where the regulator is located on the 67 250 SL?
On the alternator? On the left or right side of engine, mounted where?
Thanks,
DPaulski

[jacovdw]

A 250SL should have an external fender mounted voltage regulator.
You will find it on the right (near the inlet manifold plenum) inner fender near the shock tower.

[DPaulski]

Thank you jacovdw,
I located the regulator as you indicated, hopefully it is the cause of my charging indicator staying on.
DPaulski

[hands_aus]

Hey DPaulski

This has been discussed.
I had my charging light come on unexpectedly and would stay on when the ignition/engine was not running.
As a temporary solution I was having to disconnect battery while away from the car.
Turned out to be a blown diode in the alternator.

As I wanted to keep my original alternator, I had it rebuilt for ~ AUD $200.00