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
