Red alternator light came on, my plan, missing anything? - Mercedes-Benz Forum

 
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post #1 of 7 (permalink) Old 03-15-2014, 07:00 PM Thread Starter
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Red alternator light came on, my plan, missing anything?

My red alternator light (already converted from generator when I got the car) came on bright red suddenly today while driving. I did notice that it seemed to dim slightly when pulling away from stops but I didn't have much time to observe being close to home I just put her in the garage.

I plan to do these things first:

* Check that V-belt is tight.
* Test battery.
* Check Alternator connectors and battery leads.
* Test alternator.

So now I have a few questions;
Is there anything I left out you guys would check?
Can I check the alternator in place or does it need to be removed?
I doubt it's a connector issue since the intensity of the light seems to change which makes me think Alt is going out, battery is bad, or maybe the belt. Would the fact that it came on super bright immediately point to anything specific over another cause?
If it's the alternator, which is correct model or type for the M127 if I go with Bosch? I read somewhere else that the car may have come with a 35 amp but 55 amp is common replacement, is that correct? (I may buy new or factory rebuilt unit.)

Thanks!

-Marrs

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Last edited by DirectLA; 03-15-2014 at 09:25 PM.
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post #2 of 7 (permalink) Old 03-15-2014, 09:56 PM
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You could also check the condition of the voltage regulator if your alternator has one. Mine was held onto the alternator by two screws and the contacts were worn out. I replaced with a newer one from the junkyard and my light went off and all is well.

Check out these links: (copy and paste into your browser)

youtube.com/watch?v=WYvZRz5Truo

youtube.com/watch?v=NhAAIY5TLLI




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post #3 of 7 (permalink) Old 03-16-2014, 10:22 AM Thread Starter
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Fantastic links, thanks!

I have garage space where I can get down to business on this, but it is about 8-miles away from here, through the city. Would a fully charged battery be likely to get me there without the alternator working or is that a huge gamble? I've seen people say they have gone 60+ miles with all accessories off and others say they made it only 10-minutes, but that was a general search online across various types of cars (non-Mercedes) and years.

-Marrs

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post #4 of 7 (permalink) Old 03-16-2014, 02:32 PM
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Just a guess but 8 miles without using headlights should be fine. Can you charge up the battery before you go?

Simplest test - before you do anything is measure the voltage with the engine off should be 12.4 or so with the engine running just over 13.

Check the belt before you go. It's easy and if it's broke or missing you don't want to drive it more than a block or two if it's also driving the water pump.


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post #5 of 7 (permalink) Old 03-17-2014, 10:26 AM Thread Starter
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Originally Posted by frankflynn View Post
Just a guess but 8 miles without using headlights should be fine. Can you charge up the battery before you go?

Simplest test - before you do anything is measure the voltage with the engine off should be 12.4 or so with the engine running just over 13.

Check the belt before you go. It's easy and if it's broke or missing you don't want to drive it more than a block or two if it's also driving the water pump.


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Thanks for the advice, I didn't have my voltmeter with me (it was at the garage I was heading to, now stays in the car) so I had to make a blind faith attempt with your encouragement. The belt was present but did seem slightly looser than it should be, but alas no way to tension in the parking garage. I made it just fine, but halfway here, the light went out which gave me some assurance it was a slipping belt or failing connection.

Once here I finally had the inspiration to remove the A/C compressor since I'm deleting the system out of my car, which gave me great access to the alternator. I was surprised to see a Motorola alternator with it's original voltage regulator on the firewall. Tightened the belt, and noticed that a power wire was losing it's sheathing so I am adding heat shrink today to protect that bit.

I tested the battery with the car running when I first arrived and it was solid at 12.2v so clearly wasn't charging, after the fix the battery indicates 14.22v-14.30v running, and with my lights on (upgraded to halogen) it still shows 13.4v while running. Seems like a success!

-Marrs

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post #6 of 7 (permalink) Old 03-17-2014, 12:23 PM
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intermittent failure seems like worn out brushes. Maybe a loose connection but I think the connections would fail and stay failed. Loose belts make noise and typically revving the engine will cause it to work at least enough to turn off the red light. You can look at it while idling - is it turning and the red light is on? If so then it's not the belt.

In 1976 (I believe) MB changed the design of the alternator to have the voltage regulator on the back of the alternator with the brushes all in one thing - an under $20 part, you can replace it without removing the alternator. But for older models the alternator needs to come out to replace the brushes. That's why you have a Motorola and not Bosh. You can switch back to Bosh or go for any that fits.

Good luck.
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post #7 of 7 (permalink) Old 03-17-2014, 06:03 PM Thread Starter
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Originally Posted by frankflynn View Post
intermittent failure seems like worn out brushes. Maybe a loose connection but I think the connections would fail and stay failed. Loose belts make noise and typically revving the engine will cause it to work at least enough to turn off the red light. You can look at it while idling - is it turning and the red light is on? If so then it's not the belt.

In 1976 (I believe) MB changed the design of the alternator to have the voltage regulator on the back of the alternator with the brushes all in one thing - an under $20 part, you can replace it without removing the alternator. But for older models the alternator needs to come out to replace the brushes. That's why you have a Motorola and not Bosh. You can switch back to Bosh or go for any that fits.

Good luck.
The light would dim slightly when at idle and only bright when driving. That made me think the slower RPM allowed it to catch but at higher RPM the belt was too loose to be grabbed by friction and trigger the light. I could be wrong of course. It isn't really an intermittent issue, the light came on and was on the next morning when I started out then went off on the drive to the garage. It originally came on when I was going up a hill, higher RPMs so again maybe belt related, but maybe that means something too.

I was hoping I had the removable internal voltage regulator so I could do just as you described, but no luck. If it happens at all again I'm just going to replace the alternator with a new Bosch internally regulated model for peace of mind, I just didn't want to deal with brackets, making sure pulley fit, changing connectors, etc., last night.

-Marrs

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"Too many people are working at jobs they hate, to buy things they don't particularly want, to impress people they don't even care about." --Dr. Deepak Chopra
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