Alternator red dash light

[mauro12]

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.

[mauro12]

Any ideas ?

[Jordan]

So this is new since changing your battery?  Or you don't know if it was happening with the old battery?

[mauro12]

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 .

[mauro12]

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 ?

[Benz Dr.]

Try changing your head light switch.

[DavidAPease]

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

[mauro12]

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 .

[mauro12]

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 .

[Benz Dr.]

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.

 

[mauro12]

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 .

[Cees Klumper]

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.

[Jordan]

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?