Red Charging Light On With Key Off

[mBdrvr]

This all started with noisy bearings in the alternator in my '70 280 SL.

I had the alternator rebuilt by a local shop and reinstalled it with a new Beru regulator.

Before the rebuild it was probably charging since the red charging light was off with the key out or inserted and only on in the process of starting.

Now the red charging light acts as follows:

Key out         Light ON
Key in           Light ON
Position 1      Light ON
Position 2      Light OFF
Starting        Light OFF
Running        Light OFF

The rebuilder insists there is a short in the light circuit. Any ideas where it might be?

FYI: The voltage across the battery terminals is 12.6 at idle and increases to 13.7 volts at 2500 RPM. The voltage between Alternator B+ and ground reads about the same. The rebuilder showed me it was able to make 14.6 volts on his bench test.

Thanks,
Paul


PS I just found the alternator page in the tech manual and will follow the tests indicated there.

[lpeterssen]

Dear Paul

If you have an alternator with external voltage regulator, redo (rebuilt) the lines between your alternator and that external voltage regulator.  In that portion of your harness is your most probable culprit.

Those are three, thin lines.  One is red, another black and finally one brown.

Consult the wiring diagram posted on the technical section of this forum.

Any further question send me a PM or visit my site www.wiredoktor.com

Best regards
Eng.Leonardo Peterssen
Wiredoktor

[Pawel66]

I remember several posts related to this kind of issues where Beru regulator was involved.

Simple check is to swap beck your old regulator and see if it helps.

I would also check if the connectors on the Beru one are the same - maybe different wires go to different slots.

[mBdrvr]

I’ve now replaced the wires between the regulator and alternator with the same result. The red charging light stays on with the key out. I’ve also swapped in the old regulator with no change.

I have however seen 3 different Pagodas on bringatrailer that each show the red light ON with the key OUT. Am I chasing shadows??

I’m attaching a picture of one of these cars.

Thanks,
Paul

[Garry]

Paul,

Are you sure it is the alternator light and not the brake fluid low level warning light coming on. Just a suggestion.

With ignition key out and door open the red light comes on as a warning that the brake fluid level is low.  This usually happens when the float in the brake reservoir gets a pin hole in it so the reservoir is full but the float sinks giving the warning light.

There is a fair bit of write up on the forum on this quirk 

[mBdrvr]

I’m not with the car right now but the light goes out when I disconnect the alternator wires. But that may be the light I see on those other cars. I’ll check it though.

[BobH]

This all started with noisy bearings in the alternator in my '70 280 SL.

I had the alternator rebuilt by a local shop and reinstalled it with a new Beru regulator.

Before the rebuild it was probably charging since the red charging light was off with the key out or inserted and only on in the process of starting.

Now the red charging light acts as follows:

Key out         Light ON
Key in           Light ON
Position 1      Light ON
Position 2      Light OFF
Starting        Light OFF
Running        Light OFF

The rebuilder insists there is a short in the light circuit. Any ideas where it might be?

FYI: The voltage across the battery terminals is 12.6 at idle and increases to 13.7 volts at 2500 RPM. The voltage between Alternator B+ and ground reads about the same. The rebuilder showed me it was able to make 14.6 volts on his bench test.

Thanks,
Paul


PS I just found the alternator page in the tech manual and will follow the tests indicated there.

Hello, if it is your charging light that's on, you must be getting a permanent +ve supply at the lamp somehow, with the ignition key out, which is wrong.  You should only see +ve there with the key turned. I wonder if you have a problem with the ignition switch, and fuse 2 is getting a +ve feed all of the time.  Fuse 2 also feeds the horn, wipers and cigar lighter, are these working without the key in?  Having said that other fuses would also then have a permanent feed without the key in, including the fuel pump, and i would have thought you'd notice that, but worth checking in the first instance

[mBdrvr]

Thanks for the reply. I’ll check that out.

[mBdrvr]

I finally took my rebuilt alternator to a very experienced shop; Am-For Auto Electric in Souderton, PA.

The problem was diagnosed as a bad diode as many had suggested. I bought another rebuilt unit from him and had the original shop replace the bad diode in the original.

Thanks for your help getting this sorted out.

[mauro12]

Hi , so it was a bad diode the main cause of the red light on when the ignition was off ?
I had the opposite situation , I had a very dim red light on when the blower fan or radio were on at the same time . The voltage was jumping around from 14 to 16 at idle . I have replaced the voltage regulator with a new beru and the problem was solved .
My mechanic removed and checked the alternator and is ok and charging properly but probably some diodes are defective too .
Should I expect to have the same problems  as you ?
When the key is off and the red light is on , it means there is a + on that bulb somewhere . Where this + comes from ??

[mBdrvr]

If you look at the wiring diagram you'll see that the red dash light gets power from two places. One is from the regulator, the other from the ignition switch if I remember correctly.

[mauro12]

So the problem is the regulator or the ignition switch .

[mBdrvr]

In my case it was the alternator's bad diode(s).