Author Topic: Alternator Indicator Light Problems - Faulty Alternator?  (Read 2486 times)

KHolloway66

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Hello All,
I am brand new to this forum. I was very excited when I discovered this group as I am a new owner of a 1966 230SL 5-speed and can use all the help I can get!

I am having an issue with my alternator indicator light where the light remains on when the car is off and drains the battery. I keep the battery on a trickle charger and for most of my trips around town its not an issue but I decided to investigate further. I believe my alternator is faulty after measuring the DC voltage at 12.4 when off and idle, still measures the same when revved to 2000RPM. The indicator light turns off when the ignition is in position 2 (this seems like a red flag) and remains off when the car is at idle. I have attached a few photos of the dash and the ignition for reference. I have searched the forum for an explanation on the different alternator indicator light configurations and can't seem to find the correct info.

I figured I would replace the alternator and VR. I ordered what I thought was a direct match (Bosch AL-64-X) but it turns out I have the old 3 pin alternator. I found several forum postings discussing the 3 prong wire harness that can be wired in to allow for the more modern 35A alternator but can't seem to find the wire harness in stock anywhere. I replaced the VR and still have the same issue. I found a local company that will rebuild the old alternator but find some folks in this forum don't think that's the best route as it may not be done properly.

Has anyone seen this light display with this indicator bulb? Any thoughts on what it might be and where to go from here? Thanks!
1966 230SL ZF 5-Speed

rwmastel

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #1 on: July 25, 2023, 02:39:13 »
Welcome to the Group!  We've been online for 20 years so there is a lot of information in the forums and tech manual.  Congrats on the 230SL 5-speed!!  Is it a Euro delivery?

I haven't searched, but I wonder why people would hesitate to rebuild one of these old 35a alternators?
Rodd

Did you search the forum before asking?
2017 C43 AMG
2006 Wrangler Rubicon
1966 230SL auto "Italian"

BobH

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #2 on: July 25, 2023, 10:36:21 »
Hello, i guess it could be a number of things, including the wiring breaking down, but it might be worth checking the diodes in the alternator rectifier, a bad diode could potentially leak a voltage to the charge lamp when the ignition is off

I can see no reason why an alternator can't be rebuilt, if you find it necessary, i've searched and can't find any particular recommendations, but the general opinion is that any reputable shop should be able to refurbish an alternator

I would check the diodes first and take it from there, have a look at this post:

https://www.sl113.org/forums/index.php?topic=34276.msg250752#msg250752

Please let us know how you get on
February 1965 230SL Automatic
UK delivered RHD
Papyrus white, blue hard top & hub caps
Blue soft top
Blue leather

KHolloway66

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #3 on: July 25, 2023, 13:37:30 »
Thanks Rodd! The car is believed to have been originally bought and brought over by a US serviceman stationed in Europe.

And thanks Bob, I'll move forward with the alternator rebuild. I'll report back with findings
1966 230SL ZF 5-Speed

lpeterssen

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #4 on: July 26, 2023, 11:53:42 »
Dear Holloway66

Strange the case you have. 

The charging light at the instrument cluster works this way:

1. When you switch the ignition to the ACC ON, or RUN position a BLUE/red cable which is connected to the main fuse box on port no.2 input, energizes and gets 12V.  That cable goes to one end on the charge bulb on the instrument cluster

2. While the car is on the ACC position or RUN, BUT with the engine still not started up, a BLUE cable connected to the charge port on the back of your alternator brings a NEGATIVE signal to that charge bulb on the instrument cluster.

That completes the circuit and makes tha bulb to light up, as has on one cable end a POSITIVE signal, and on the other cable end a NEGATIVE signal.

3. As soon as you crank the engine, the ALTERNATOR starts to produce electricity.  In that case the BLUE CABLE CHANGES ITS POLARITY from NEGATIVE that had with the engine not running to POSITIVE. 

Then the charge BULB at the instrument cluster as is receiving POSITIVE SIGNALS on both cable ends does not Light up.

In that way you know your alternator is doing its thing.



Now let’s see your case…..

You have mentioned on a previous post, that when the ignition is off, your charge bulb indicator lights up in the instrument cluster.

First, with a multimeter on hand set to measure DC voltage go to your alternator and measure the voltage between the post where the BLUE CABLE is connected (thin cable) can also be Red/blue stripe in some cars, to a ground.  Disconnect previously the red/blue or blue cable that comes to your alternator. If you have 12.x voltage there in that condition then….. definitely the alternator is faulty. It has some kind of problem with the internal circuitry.

If you measure no voltage there, then the story is totally different, and the first culprit that comes to my mind is your harness.

Repeat test this time still with charge cable disconnected, but test voltage on that cable end to ground with the engine off.

The 230SL has a main T30 conductor that brings power from the alternator to the main light switch at the dashboard, that has and external cable insulation that tends to fail. It gets cracked with age and stops doing its insulation purpose.

The blue cable for the instrument charge light travels along that cable inside the pvc tubing all the way from alternator to cabin, to the point mentioned before (main light switch) which is a kind of power distribution hub on the pagodas and many other Mercedes’ prior to w114.

My best bet is that there is a faulty insulation conductor that is passing power to your blue charge cable.

I have plenty of experience in rebuilding wiring harnesses.

Do the first test indicated, to check power between blue cable and ground. That first with cable ends connected to alternator.  Then repeat the test disconnecting cables from alternator. Then if fwith cables disconnected from alternator you still get 12v from blue cable then your wiring harness needs my service.

Best regards
L.Peterssen
« Last Edit: July 27, 2023, 11:10:01 by lpeterssen »

lpeterssen

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See this classic deterioration example
« Reply #5 on: July 26, 2023, 12:07:50 »
Dear friends

This is what I think that wiring harness has….

Look carefully at photo.  Main T30 cable with insulation jacket cracked and its proximity to the red/blue cable which Carryover the signal to the charge bulb indicator.

Gremlins have always the root case on an aging wiring harness.

Best regards
L.Peterssen

KHolloway66

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #6 on: August 22, 2023, 14:04:14 »
Ok, after several weeks I have finally got back to this issue. Thank you for your abundant knowledge on the topic L. Peterson!

I tried your initial test of measuring the voltage at the indicator post on the alternator and sure enough there was about 10V reading. I pulled the alternator and took it to a local auto electric shop for rebuild. The guy there said it was in very rough shape, shorted in multiple sites. After an extensive rebuild I reinstalled the alternator and everything seems to be working like normal. Indicator light operates just as you explained, L.Peterson, and I have 13V+ at battery when the engine is on.

I am very pleased it was not the wire Gremlins this time... Thanks again for all the help!

1966 230SL ZF 5-Speed

lpeterssen

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Re: Alternator Indicator Light Problems - Faulty Alternator?
« Reply #7 on: August 22, 2023, 14:26:01 »
Dear Holloway

Good to hear news from you and that damage was not so extensive. Nevertheless pay attention in near future to your harness.

Best regards
L.Peterssen
« Last Edit: August 22, 2023, 14:30:52 by lpeterssen »