Alternator light on

[tuultyme]

I have a 70 280SL.  Last year I replaced my alternator with a 50 amp rebuilt one; The bearing went out on the old one.  I also replaced the voltage regulator at the same time.  Lately the Alternator light has been on dimly while waiting at a light; car has automatic.  Yesterday the alternator light was on at all times except at driving speed; tring to get car home.  The battery tested 12.4 volts; engine off.  While running the battery tested 12 volts. Checked the belts including the A/C belts and altenator belts all fairly tight.  Where do I start to figure out why I am not getting a charge to the battery.

Another issue that I have noticed is a rattling sound from the front of the engine, seems to be more external rather than internal.  Does not seem to be the fan.  Could it be the water pump?  How would I test that?  Could this be at all related to the charging system?

Bruce; 268Blaugrun(green) 1970 280SL; IL

[A Dalton]

First Check is to  see if you can turn the alt pulley by hand w/engine off...

[tuultyme]

Alternator is off right now and I can turn pulley.  Alternator was moving when still hooked up.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[A Dalton]

quote:Originally posted by tuultyme

Alternator is off right now and I can turn pulley.  Alternator was moving when still hooked up.

Bruce; 268Blaugrun(green) 1970 280SL; IL

 Sorry for not being  clear..
 What I meant to convey is that a simple test for a slipping belt
[ vs. a belt tension test] , is to try and spin the alt. pulley by hand..[grabbing the fins with ones fingers], while the belt is still hooked up.
 Many aged belts become glazed and , even though they are tight . slip due to loss of friction surface. This spin hand test will show this right up, as you should not be able to spin the pulley, no matter how hard you try by hand..
 A glazed/worn V surface  of the  belt will usually show up at low RPMs [ as idle/indicator lamp] and slip less as Rs increase. Even more-so when there is an electrical load on the alt [ light/blower, etc]..
 This a just a simple check before getting into the system and is sometimes the culprit..
 I mention this in your case as it, along with lamp indictor at idle, coupled with  noise in the water pump area, makes me suspect of a worn/dry belt.
  A quick trick here [relating to the noise] is to spray the belt with a quick squirt of water or WD as the car is at idle... if the noise immediatly stop, you know it is a bad belt..
I have seen alt/battery/reg./starter changes all in vain for a slipping belt , so I alway check there first..
 This is not to say this is your problem, it is just a first check I do...

[tuultyme]

As the alternator belt is new but the A/C belts are not I will have to check if those may be the culprit.  I knew I should have replaced them last winter when I had everything apart.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[Cees Klumper]

Another check would be whether the wire connection at the alternator is o.k.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[tuultyme]

I replaced the A/C belts; it did not correct the problem.  What connection should I look at next?  I took the Alternator back off so I can take it to an electrical shop to have it bench tested to make sure nothing is wrong with it.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[tuultyme]

I had to replace my year old alternator.  The PO had put on one of those negative battery switches; which was great for shutting off the battery while working on the car or during the winter storage.  Every now and then I would have to retighten the switch when I tried to start the car.  Then one day this summer while waiting at a light the car just quit;  I retightened the switch and I got it going again.   While I was getting the alternator tested this last week the mechanic said that one lose of power while running burned out the diodes on the alternator and strongly suggested I remove the switch.  Switch gone another rebult alternator and I am running again.  Is there a better switch that could be used to quickly cut off the power?
Has anybody in the group replaced thier alternator with one with the internal regulators?  Is this an easy conversion?

Bruce; 268Blaugrun(green) 1970 280SL; IL

[A Dalton]

http://113.mbz.org/hints/alternator.htm

[Malc]

Make sure that your engine/alternator earth (ground) straps are in place, clean and doing the job, check resistance from the engine to the battery it should be less than 2 ohmms
HTH
Malc

[George Davis]

Bruce,

a better switch would be a kill switch as used on race cars.  Using one would require fabricating some sort of bracket to mount it.  You might find one at an auto parts store that has performance parts, or you can find them at race parts suppliers.  Some types include provisions for protecting the alternator when turned off, some don't.

George Davis
'69 280 SL Euro manual

[Malc]

quote:Originally posted by George Davis

Bruce,

a better switch would be a kill switch as used on race cars.  Using one would require fabricating some sort of bracket to mount it.  You might find one at an auto parts store that has performance parts, or you can find them at race parts suppliers.  Some types include provisions for protecting the alternator when turned off, some don't.

George Davis
'69 280 SL Euro manual

You MUST use a switch that provides a "drain" for the alternator output when the power is cut otherwise you will blow the diodes, regulator, windings or all three, very expensive.
A good "kill" switch should include a large resistor(physical size) of relatively low ohmms which can dump the current from a spinning alternator as the battery supply is cut.
Mandatory on Stage Rally cars as per FIA regulations.

Companies such as Demon Tweeks in the UK sell them, they are relatively cheap and have the added bonus of a "key" that can be removed acting as yet another anti-theft deterent
HTH
Malc

[A Dalton]

I do not believe he is looking for a Kill switch, but rather, a simple power dis-connect switch for storage and theft.
 There are several of these available at any good parts stores  and
need not be concerned with Alt drain resistors as the disconnect  sw is only used after the car has been shut off, not killed while the charging system is operating..
 The best ones we use are in-line snap switches. We use them on all our heavy equipment . They are about $12...any place that sells tractors/equipment should have them.

[Malc]

quote:Originally posted by A Dalton

I do not believe he is looking for a Kill switch, but rather, a simple power dis-connect switch for storage and theft.
 There are several of these available at any good parts stores  and
need not be concerned with Alt drain resistors as the disconnect  sw is only used after the car has been shut off, not killed while the charging system is operating..
 The best ones we use are in-line snap switches. We use them on all our heavy equipment . They are about $12...any place that sells tractors/equipment should have them.

Ah, miss understood, however I would still concider it just in case there was an "accidental" flip of the switch!
Cheers
Malc

PS As to the water pump try waggling it from side to side (movement not good) and make sure the fan is attached firmly and no loose blades

[ChrisInNashville]

I've got the same problem that tuultyme noted.

Here's my question:   In looking at A Dalton's response, I tried to turn the belt and the pulley. but nothing budged.  Please clarify...this is good, correct?

In addition, the wires on the back look good, so I'm wondering what to do next.

In looking in Haynes, it describes a test...but frankly, to one as unmechanically trained as me, it looks scary.  Any suggestions?   Is it complicated or should I defer to my mechanic?

Any ideas on what to check next?   Could it just be a weak battery?

[ja17]

Hello Bruce,
With your 50 amp alternator, the internal regulator and brushes come off easily. Just two screws. Normally if the brushes are not bad the internal regulator is bad. Did they replace the regulator/brush  unit when the alternator was rebuilt?

Joe Alexander
Blacklick, Ohio

[n/a]

Want to trace the rattly sound? Simply use a long shaft of some kind, a screwdriver, even a piece of dowell, start the engine, make sure your long hair/beard/headband/whatever won't get caught in the fanbelt, and place the shaft on various items while placing your ear on the other end of said shaft. There is no better way of tracing the source!

Further to this, on re-reading your posting, your voltage with engine running should be at least 14V. Since a bad experience with a 'replacement' alternator decades ago I have always replaced bearing/brushes/slip-rings/whatever myself. It ain't rocket science!

Neil Rote, Melbourne Australia
Unrestored '64