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Long time guest lurker, finally having a big enough problem to need to make an account and actually post lol

Tl;dr at the bottom for a quick synopsis. I have a 1985 380sl on the East Coast for reference.

I went to start my car for the first time in a few months the other day but the battery was dead. Took the battery out and hooked it to a tender for a couple days. Went to put it back in the car and sparks flew. Turns out I put the clamps on backwards when charging and managed to reverse the polarity of the battery without it exploding (thankfully). Went out, got a new battery, and when I hooked it up. I noticed that the battery light is now on while the car is not running (I had the doors closed/locked and I could see the light was on through the window). I started the car, which fired right up, and the battery light turned off but the O2 sensor light is on. I took the new battery out again so the battery light doesn't

Having done some searching, I saw a couple posts saying that likely the alternator is shot and that's why the battery light was on when the car wasn't running. I'm *obviously* not too great with electrics but I was wondering if there was an idiot fuse for this kind of situation that might have been popped when I put the battery in backwards. Of course, it probably isn't as simple of a fix as that so if anyone has any ideas what I should look at/test first aside from the alternator and OVP relay I'll appreciate the help!

I tried looking in the startek online manuals for some wiring diagrams to see which systems would likely be effected first but it seems like the startek site is unavailable for some reason.

Worst case scenario, my mechanic is about 20min away and if it doesn't make it I have AAA. ANY advice helps and ridicule will be accepted so pile it on, I have it coming.



Tl;dr - I'm an idiot and essentially put negative to positive and vice versa. Car starts and runs but the battery light is one when the car isn't running and the O2 sensor light is on when it is. Any ideas what it could be aside from blown alternator?
 

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When the ignition switch is first turned on, the "battery light" will illuminate thus supplying a small resistance to the charging circuit to activate the alternator when the engine starts. This light will go out when the alternator begins charging. This sequence is normal. If your battery light is lit with the key removed from the ignition switch, it is quite possible that the diode internal to the alternator has gone bad and should be checked.
 

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My problem is the latter. Key out, the light is on. Your statement lines up with other things I saw while searching the subject.

I'm going to go out later today and test the charge at the battery to see if it is the alternator. I guess I won't know if anything else is effected until that gets settled.

Thanks!
 

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The voltage regulator sits on the back of the alternator and can be replaced separately. I'd start there.

Explain what you mean by "sparks flew". Where were the sparks?
 

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A new alternator isn't very expensive. I'm trying to figure out what amperage the alternator I have now is so I can match it but if I can't I'll take a stab at the voltage regulator.

On that note, anyone have any idea how many amps a 1985 380sl alternator puts out? The only alternators I'm seeing at my local part shops are 70/80 amp but I have a feeling that's a bit high for our cars.

And the sparks were at the battery as I was connecting the wires. Since the polarity of the battery was reversed it was essentially like touching live to ground.
 

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Sorry for the short answer; here's a longer one.

The alternator has two sets of three diodes internally-- one set are the power diodes, which are connected to each winding of the stator. When the magnetized rotor passes by it induces an alternating current in the stator windings: these power diodes clip off the bottom part of the wave and only allow a pulsed DC current to flow to the battery. Since there are three of them the pulses smooth out and you're left with a more or less constant DC current. (OK not strictly true but you EE's will have to play along for this part).

The rotor itself is engergized, and therefore magnetized, by current flowing through the D+/61 "blue wire" circuit which comes out of the battery, goes through the ignition switch, through the warning light on the dash, goes into the blue wire, over to the alternator and into the voltage regulator.

Remember I said there is a second set of three diodes, these are each connected in parallel with the power diodes- their purpose is to rectify the stator current into an alternator output level, which can be compared to the voltage regulator's "set-point" to know whether to switch the field on or off. So when the alternator's turning and the rotor's energized, there's a voltage on D+/61.

The Voltage regulator's job is to compare the voltage between what's flowing from the blue wire to the voltage coming from D+/61. When voltage on D+/61 is less than the voltage on the blue wire, then the current from the blue wire is sent directly to the rotor, through its windings and to ground. This completes the circuit, energizes the rotor windings and lights the warning lamp.

Now the engine starts. The rotor turns and induces a current in the stator which gets rectified into power to the battery and voltage at D+/61. Voltage builds up on D+/61 until the set point is reached, at which point the voltage regulator cuts off current to the rotor and D+/61 voltage falls. This continues happily keeping the output voltage at the set point. At the same time, because the voltage on D+/61 is the same as the voltage on the blue wire, no current can flow and the warning light goes out.

Now, when you hook it up backwards. . . the positive side of the power diodes is connected to ground, and the other side of the diodes gets power from the battery-- this short-circuits both the D+/61 diodes and then the power diodes, to which they are connected.

Now, once the diodes are shorted, current can flow from the battery, backwards through the power diodes and into the D+/61 diodes, through the blue wire, lighting the bulb, then back into the ignition switch and to ground.

So the warning light glows without the key even being in the ignition.

Solution: replace all the diodes in the alternator.
 

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Actually there are six power diodes, two for each phase of the stator, one each for the bottom and top of the waveform. Doesn’t change the answer but I can’t edit the post above.

Sometimes there is a single or a pair of diodes for the D+/61 “trio” circuit.

But you get the idea.
 
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