Author Topic: Alternator red dash light  (Read 2333 times)

mauro12

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Alternator red dash light
« on: December 01, 2023, 21:43:30 »
Hi guys , I have a strange problem in my car . Recently I have replaced my 6 years old battery for a new one but I realized that when turning on the blower fan,  I see a very very dim red light from the dash which is visible only if you pay attention  .
I assumed it is an alternator issue but having checked  the voltage at idle with no electrical on , the voltage is about 14.6v.
With everything on ( fan , radio , and lights )it drops about 13.3 . Those are quite normal values I guess . Any explanation for this ? Maybe a faulty voltage regulator or some bad wire contact ? Thank you all.
Mauro Pisani
250sl 1967 5speed zf manual

mauro12

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Re: Alternator red dash light
« Reply #1 on: December 04, 2023, 12:31:27 »
Any ideas ?
Mauro Pisani
250sl 1967 5speed zf manual

Jordan

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Re: Alternator red dash light
« Reply #2 on: December 04, 2023, 12:51:43 »
So this is new since changing your battery?  Or you don't know if it was happening with the old battery?
Marcus
66 230SL  Euro 4 speed

mauro12

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Re: Alternator red dash light
« Reply #3 on: December 04, 2023, 19:49:13 »
I haven’t noticed to be honest  with the old battery . I don’t know if it’s a false contact with the fan wiring or something else . The alternator is giving the correct voltage . With everything on it gives around 13.3 volts at idle which is quite acceptable . Or second option is something related to the voltage regulator .
Mauro Pisani
250sl 1967 5speed zf manual

mauro12

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Re: Alternator red dash light
« Reply #4 on: December 04, 2023, 23:02:36 »
It is quite strange because I see this very dim red light  only when the blower fan is on . Having seen this red light, I’ve checked the voltage and is 13.2 with radio on , fan , and high beams . It means the alternator is working . I’m thinking about some loose ground cable or something related to the heater motor and the alternator . Any ideas ?
Mauro Pisani
250sl 1967 5speed zf manual

Benz Dr.

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Re: Alternator red dash light
« Reply #5 on: December 04, 2023, 23:41:05 »
Try changing your head light switch.
1966 230SL 5 speed, LSD, header pipes, 300SE distributor, ported, polished and balanced, AKA  ''The Red Rocket ''
Dan Caron's SL Barn

1970  3.5 Coupe
1961  190SL
1985   300CD  Turbo Coupe
1981  300SD
2013  GMC  Sierra
1965  230SL
1967 250SL
1970 280SL
1988 560SEC

DavidAPease

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Re: Alternator red dash light
« Reply #6 on: December 05, 2023, 01:38:19 »
Mauro,

Do you see the red light only when the engine is idling?  If so, then I personally wouldn't worry too much about it.  If you continue to see the light when you increase the engine RPMs, then you can ignore the rest of my post.

Warning: I am not an automotive electrician, so the following is my opinion:

When I was young and driving (American) cars from the 50s and 60s, it was very common for the charging light to come  on (usually dimly), at idle with the headlights and radio on.  It just meant that at idle, the cars electrical accessories were pulling more amperage than the alternator (or generator) was capable of supplying (at idle).  So the difference was made up by the battery, and once you increased the engine RPMs the battery would begin charging again. 

The 14.6 volts at idle with no accessories on means that your alternator is charging the battery under light load, even when idling.  The 13.3 volts even when the light is on means that your battery is healthy and is making up for the small usage deficit.   The draw on your battery is probably very slight, and it would take a long time for the battery voltage to drop significantly.

Again, just my humble opinion.

          -David
-David Pease
 '66 230SL (Originally sold in Paris)

mauro12

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Re: Alternator red dash light
« Reply #7 on: December 05, 2023, 07:26:22 »
The red light is very dim , almost unnoticeable. I have to check if is still on when I accelerate.
At the beginning I thought that the blower fan was putting too much load on the system but even with it on , the multimeter was reading 13.2 at idle which is perfect reading  .
I read some old post with similar problem and some people solved the problem by cleaning the ground cable at the alternator .
Second thought is that the voltage regulator is not working properly . Like I said , this light comes on only when I turn on the blower fan . I have to admit that in 20 years of ownership I’ve never used the fan .
Mauro Pisani
250sl 1967 5speed zf manual

mauro12

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Re: Alternator red dash light
« Reply #8 on: December 05, 2023, 07:33:46 »
Why the headlight switch ? I’ve experienced that these cars are really delicate regarding wires , fuses and so on . Everything is very old and you can get mad about solving issues .
Mauro Pisani
250sl 1967 5speed zf manual

Benz Dr.

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Re: Alternator red dash light
« Reply #9 on: December 05, 2023, 16:46:35 »
Why the headlight switch ? I’ve experienced that these cars are really delicate regarding wires , fuses and so on . Everything is very old and you can get mad about solving issues .

Because that's a very good place to look. These cars send all of battery power through the head light switch even before it goes to your ignition switch. Corrosion can build up in the switch and even though it may still work cause all sorts of problems like poor ignition performance, dim lights, and glowing charge light.

 
1966 230SL 5 speed, LSD, header pipes, 300SE distributor, ported, polished and balanced, AKA  ''The Red Rocket ''
Dan Caron's SL Barn

1970  3.5 Coupe
1961  190SL
1985   300CD  Turbo Coupe
1981  300SD
2013  GMC  Sierra
1965  230SL
1967 250SL
1970 280SL
1988 560SEC

mauro12

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Re: Alternator red dash light
« Reply #10 on: December 05, 2023, 19:33:58 »
I cannot understand the relation between the headlight switch and the knob to turn on the blower fan .
Replacing the entire head light switch is not so easy and you have to mess up all those wires and it’s not job for me .
For what I can understand the problem could be around alternator or voltage regulator, or a ground wire that gives the input to have the red light on the dash.
Maybe I will be wrong .
Mauro Pisani
250sl 1967 5speed zf manual

Cees Klumper

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Re: Alternator red dash light
« Reply #11 on: December 05, 2023, 19:52:36 »
What Dan is saying is that all power first goes through the headlight switch, including what is needed by the fan. Not an electrician by any means, but it makes sense to me that if the headlight switch is not 100%, all the rest of the system will suffer a drop in voltage.
Cees Klumper
1969 Mercedes 280 SL automatic
1968 Ford Mustang 302 V8
1961 Alfa Romeo Giulietta Sprint Coupe 1600
1962 FIAT 1500S OSCA convertible
1972 Lancia Fulvia Coupe 1.3
1983 Porsche 944 2.5
1990 Ford Bronco II

Jordan

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Re: Alternator red dash light
« Reply #12 on: December 05, 2023, 20:32:44 »
Because that's a very good place to look. These cars send all of battery power through the head light switch even before it goes to your ignition switch. Corrosion can build up in the switch and even though it may still work cause all sorts of problems like poor ignition performance, dim lights, and glowing charge light.

I'm curious if that would be the same for all cars of that era, not just MB?
Marcus
66 230SL  Euro 4 speed