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  #1  
Old 01-16-2002, 08:31 PM
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Battery or brushes?

Jumped in the 4.5 this afternoon, was going to pick up the kids. Hit the key and the engine fired up. I thought it sounded like it spun up slower than normal, but not much. As I sat there and watched the oil pressure come up, it dawned on me that there was more than the brake light glowing in the dash - the generator light was glowing as well.

If I run the RPM's up a bit the light almost goes out, but is still dimly lit. As soon as I let off the gas the light comes back. I drove the 4.5 last night and noticed nothing unusual, all lights were bright and no idiot light illuminated.

The battery is a 6 year old Interstate, and I have been expecting it to give up the ghost. The generator light confuses me, I would expect it to lit (if it decided to) when there was a problem with the charging system (although judging by the many posts here, that is not necessarily the case), not a dying battery.

So what is the likely cause of the light - dying battery or brushes?
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

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  #2  
Old 01-17-2002, 08:01 AM
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Gen Light

It is my understanding that this light monitors only the output of the alternator, not the status of the battery. Check your alternator belt for proper tension, battery connections, alt connections. But six years on a battery is getting your moneys worth. Consider Dead battery= you are either at home or a destination. Dead alternator=you are somewhere in between. Replace both for the peace of mind.
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  #3  
Old 01-17-2002, 08:58 AM
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Brushes are cheap. I would try them first.
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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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  #4  
Old 01-17-2002, 09:16 AM
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probably not brushes

My understanding of the 'battery' idiot light on the Benz (and my experience based on a recent similar problem on my 300SD) is that the light is grounded through the brushes; when they wear out the light goes out altogether (one of the very few examples of dumb design in a Benz). I guess this means you should look to your battery first. Too bad - brushes are dirt cheap!
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  #5  
Old 01-17-2002, 10:11 AM
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I agree, 6 years on a battery is pretty good. I think I'll look around, see if I can find an Optima big enough to fit the tray, if not, another Interstate will be sitting there. I'm also going to pull the brushes and check them out for possible replacement...just because.

I decided the brushes and alt (or generator) are OK as the light came on and stayed on at idle. All of the dead alternator posts I've read, no one saw anything but a brief warning light, if they saw it at all.
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.
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  #6  
Old 01-17-2002, 10:48 AM
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Here is how those idiot lights work.

In an alternator there are two sets of rectifiers. One is the high power set and delivers the high current to the battery and the other is a low power set and it delivers DC voltage to the regulator circuits. The light is wired from the ignition circuit to the alternator to provide voltage to get the alternator started.

In theory, an alternator will not produce an output until a magnetic field is present. Well the problem is that you need some current flowing in the rotor to make the magnetic field. Since the alternator needs this field before power can be generated, you need a source of current to get the alternator started. Once the alternator is producing voltage it can sustain itself. But you need a source of current like the battery to get it going. In some alternators there may exist a resuidial magnetic field as a result of the rotor becoming magnetized and they will "self start". But the industry does not rely on this possibility.

So the role of the idiot light is this. It is wired from the ignition switch to the alternator and provides the power to the regulator circuit. When you turn on the ignition, the light is on because the regulator circuits is trying to apply current to the field magnet (rotor) through the lamp and thus it is on. When the alternator begins to rotate, it begins to generate enough voltage to drive the regulator circuit directly and the current through the lamp now goes to zero. If the alternator is malfunctioning and not able to generate normal voltage the light will remain lit because there is current flowing through it to the field magnet.

However these idiot lamps are not a very good indicator of malfunction because if the low power rectifier is good but the high power rectifiers have failed the light will not be on even though the alternator output is zero. Whats happening is the low power rectifiers are working fine and the the regulator curcuits are driving current into the rotor as they should, but since the power rectifiers are not working there is no output. Which I have seen this senario many times.

The best test is to look at the voltage on the alternator output when it is running. In your case Mike, I would say that some or all of the rectifiers have failed.

Hope I have not confused anybody.
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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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  #7  
Old 01-17-2002, 01:51 PM
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The only time I saw a similar situation was on a 250C and it needed a new alternator. What are you waiting for on the battery, though? You can replace it in your garage on your terms, or wait for it to strand you somewhere.

BTW - that was the best explanation of how alternators and associated components work that I have ever seen.

Thanks to dpetryk for a great post!
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Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #8  
Old 01-17-2002, 07:12 PM
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Well, I checked out the system with the multi-meter this afternoon. The alternator is putting out ZERO. Looking as hard as I can, I cannot see anything that resembles a removable brush pack. I'll have to look at the manual and see if there is a remote voltage regulator.

Guess I'll be buying an alternator.

Haven't bought a new battery because the old one was working fine. No fear about being stranded. This car has that Mercedes trait of having problems at home, or close to home. Hope that never goes away.
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.
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  #9  
Old 01-17-2002, 07:33 PM
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What year and which car is the alternator on?
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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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  #10  
Old 01-18-2002, 12:51 AM
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Dave, this is on the '73 280SEL 4.5. I've been out for the evening, just got home and will be looking at the shop manuals before hitting the rack. Looks like this might be a sealed alternator, but hard to tell as it sits pretty close to the valve cover/ head. Looks to have one three prong connector out by the edge of the case, another connector at the center and a single lead bolted 180 degrees out from the three prong.
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.
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  #11  
Old 01-18-2002, 10:30 AM
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Can you determine if the voltage regulator is built in or external.
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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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  #12  
Old 01-18-2002, 11:04 AM
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I spent some time last night with the manual and I found very little on the alternator. There is some testing procedures, but little else that I could find. In one of the test diagrams it shows the alt light is between the alternator and regulator, but no mention of where the regulator is located.

After work I'll look through the parts book, see what that shows.
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.
Reply With Quote
  #13  
Old 01-18-2002, 11:29 AM
dpetryk's Avatar
Electrons can do anything
 
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Location: Houston, Texas
Posts: 1,071
Thats the right place for the bulb. Follow the wires from the alternator to find the regulator. I know thats hard to do at times. You either have bad diodes or a bad regulator. So you gota find it.
__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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  #14  
Old 01-18-2002, 11:54 AM
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I've decided to take the easy way out. I just got off the phone with Phil (Partsman), ordered an alternator and voltage regulator (turns out it is external). I'll just replace both and be done with it. Even if it's only the regulator that is shot, I'll feel better replacing the alternator too, because I thought I heard a slight rattle from the alternator when I was testing output yesterday. Possibly the alt bearing heading south.

Also, thanks for a great explanation of alternator dynamics. I always knew that if the idiot light burned out the alternator wouldn't work, now I know why. BTW, DC volts with engine off were 9.35, running at idle and ~2K were 9.35.
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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.
Reply With Quote
  #15  
Old 01-18-2002, 12:57 PM
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Electrons can do anything
 
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Location: Houston, Texas
Posts: 1,071
You cant go wrong with new stuff. Glad its a done deal.
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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
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