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That pin on the DME is an input - the DME cannot switch anything there. If your test light turned off when hooked up, that means your alternator was not outputting voltage. I do not have an M5. I do have experience with them. And I have pretty extensive knowledge of the DMEs (and have several MSS52s on my desk). I do have an M3, which has the exact same wiring scheme that you cite as proof of the DME communicating with / switching the alternator: Alternator (as compared to the M5: Alternator)

I have no clue what fits the S62. All I said was reportedly the S62 alternator fits the I6 (I saw claims of the M52TU which dimensionally is the same as the M54), and also stated that if the reverse is true, there are higher output alternators available. M54s are dime a dozen and the alternators don't seem to fail as frequently, so I do think it might be worth checking to see if it fits (perhaps with a pulley swap).

I'm not assuming things. I am explaining things based on software I disassembled and hardware I manipulated. Perhaps you're witnessing this behavior: https://www.newtis.info/tisv2/a/en/e46-m3-cou/wiring-functional-info/power-train/voltage-current-control/voltage-current-generation/on-delay/XfXxoLV (I cannot find that blurb in the M5 docs, but it's probably the same) - but don't get confused, that is not DME mediated. That's purely a function of the circuitry internal to the alternator. It also doesn't turn off the alternator, but instead limits the rate of change of current.
 

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The TIS wiring diagram says it applies to 2000/09+ for the M5. It only shows two wires out of the black connector X6524 even though the VR has three pins. Maybe the OP can look at the wiring harness to confirm how many actual wires are in it.
If there are two the DME has no control. Third wire is for PWM control.
 

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This video of an E39 alternator replacement shows only two of the 3 pins being occupied

Early car diagram is pretty much the same anyway. Main difference is it shows the alternator signal going to a distribution block before going to the instrument cluster: Alternator
 

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Found this on Cosmos Auto Parts website. Shows the middle pin (the D+) going to the cluster warning lamp and third pin (which is empty on the S62 as shown in Ryan's video) going to the DME. I wonder what models are actually using it this way. RealOem shows no models after 2003 with this part number.
934541



If I had an alternator issue I think I might go this route.


It's Bosch and same part number.
 

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Digging through old wiring diagrams (earliest use of that voltage regulator appears to be E36 318 and E31 840i), I can't find any car that uses the 3rd pin
 

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That pin on the DME is an input - the DME cannot switch anything there. If your test light turned off when hooked up, that means your alternator was not outputting voltage.
Edit this is an after thought you say input but I prefer to call things like that a controlled ground and that is how most things work on the car.
Sorry for the delay but I am traveling, in a few days I should be at my East Coast house with my M5 document loaded computer. Then I will have WDS at hand and I will find the passage I have vaguely quoted.
The short story is if you cut that wire the alt would have 0 output. In other words it would turn off. To run it needs that connection to the DME to control it's output. I think you can also just hook it to the battery side and the alt will run full power and skip this thing it does when you take your foot off the gas, I need my notes to see though, I just don't remember well enough about details.That gives the DME control of the Alt. It has no need to talk to the Alt it can get all the info it needs from other terminals it monitors, like 30 or 15 and likely the one in the trunk that reads different to the rest. Whether it turns it off completely or just drops it's output to very low is not really moot. Charging at .2 amps means little and will have no effect on the battery voltage you read. Likely any amps produced by the alt, less than the current draw of the car will show battery voltage. That is how old idiot lights worked they registered the difference between the battery side and the charging side which are effectively separate, even thought that is hard for most to grasp.
Just going backwards you said the alts output is controlled during startup, if the DME can control it then it can control it at other times.
The next thing that the car does or has a habit of doing is if someone has a dying battery that only charges to say 12.5 for a period of time. Then they get a new battery, but the place does not charge that battery up to 12.9 as BMW asks then runs the car with say it's first max charge voltage of 12.6 or 7 the battery will never charge more than that. However if the person then manually charges the battery to 12.9 for a couple of starts then the system will start charging that battery to 12.9 or at least try to. This again shows some control of the Alt, but this has not been significantly tested.
 

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Edit this is an after thought you say input but I prefer to call things like that a controlled ground and that is how most things work on the car.
It's the voltage regulator that is doing the ground switching. The DME is monitoring that pin and not switching anything itself. And if you take the DME out of your car and check with your multimeter, you will find that the pin receiving the signal from the alternator and the one outputting the signal to the instrument cluster are continuous.

Sorry for the delay but I am traveling, in a few days I should be at my East Coast house with my M5 document loaded computer. Then I will have WDS at hand and I will find the passage I have vaguely quoted.
The short story is if you cut that wire the alt would have 0 output. In other words it would turn off.
100% wrong. Try it (pull the pin from the DME end). Your alternator will still output a regulated voltage. Your instrument cluster may light up though.

To run it needs that connection to the DME to control it's output. I think you can also just hook it to the battery side and the alt will run full power and skip this thing it does when you take your foot off the gas, I need my notes to see though, I just don't remember well enough about details.That gives the DME control of the Alt. It has no need to talk to the Alt it can get all the info it needs from other terminals it monitors, like 30 or 15 and likely the one in the trunk that reads different to the rest. Whether it turns it off completely or just drops it's output to very low is not really moot.
Sorry, but the DME isn't doing jack ****. It has zero control over the alternator.

Just going backwards you said the alts output is controlled during startup, if the DME can control it then it can control it at other times.
Yeah, the circuitry internal to the alternator modulates current output during startup. The DME has no role in that process.

The next thing that the car does or has a habit of doing is if someone has a dying battery that only charges to say 12.5 for a period of time. Then they get a new battery, but the place does not charge that battery up to 12.9 as BMW asks then runs the car with say it's first max charge voltage of 12.6 or 7 the battery will never charge more than that. However if the person then manually charges the battery to 12.9 for a couple of starts then the system will start charging that battery to 12.9 or at least try to. This again shows some control of the Alt, but this has not been significantly tested.
Again, the DME has zero control. The behavior you're observing is either a soft-failed alternator or the current ramp limiting that the voltage regulator performs.
 

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Sailor the system you are describing fits the diagram I posted above. But the S62 is only using two of the three pins as you can see it the E39Source video. One pin is voltage input from ignition. The second pin does connect to the DME but it is the D+ output from the alt to the DME which only serves the purpose of lighting the warning lamp. I'm guessing they went to this to improve accuracy of when the lamp lights.
Even with only two pins the system is still semi-smart. The voltage regulator has an internal chip containing numerous logic gates, op amps, etc, that allows it to compare battery voltage (input pin) and generator voltage (D+) and make adjustments by modulating rotor field strength.
 

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For X
When I get to the house with my M5 and computer that has all my notes on M5 stuff, I will have the details. The reason the lamp is controlled by the DME is because when the power is turned off then the light would go on. If you look at your diagram above which is old school and drew a line down the middle you can see the battery side and the alt side, basically and in very simple terms when power flows to the alt the light comes on, the light is off if power flows to the battery from the alt . It is a very simple system that has been used for many years. That is as accurate as you can get. If the system discharges the light is on if charging the light is off, even if it is just a little bit.
For T
I have owned 8 of these cars and when I was working on them I serviced 12 so that is 20 cars that all behaved the exact same way. 20 bad alternators? Then there are all the posts on the board stating the same behavior and not by me. Usually they go something like this, just installed an new alt and the volts drop to 12.8 when I am coming to a stop but then go back up. They just never looked before.
Further you stated that the alt's output is controlled during start up, I agree with that but your explaination of how the system works would prevent that? Do you mind enlightening me on how it would be controlled?
 

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If it's not a failed alternator, then it's just the alternator not being able to provide enough current to charge a weak battery - i.e it's underspec'd by BMW.

Startup control would be done by circuitry internal to the regulator without any influence from the DME itself.

The DME does not control the light, it does not control the alternator. It has no means of doing so.
 

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The diagram I posted is specifically the VR used in the S62.
The lamp requires approx. 3V to light with current in either direction. When you turn the key on you have power from the battery, none from the alt so the lamp lights. When you start the car and the alt is functioning correctly the alt output will be within 3V of the battery, so not enough delta V to light the lamp....it goes out. If the alt/VR is producing too little or too much voltage and the delta V gets over or under 3V of battery voltage the lamp goes on.
With old school, the D+ wire from the VR went directly to the cluster. In the S62 the wire goes to the DME then to the lamp. Not sure why this was done. But as far as I can tell this is the only function of this wire.
 

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The next issue is the semi smart system and the voltage regulator. The DME controls the alt. it turns it off and on frequently. You can see this in a normal drive when you take your foot off the gas under a certain rpm the alt is turned off and then it stays off until a stable idle is reached but then only gives part power.
What you are seeing is load response and it happens inside the VR chip. From TIS:

Annotation 2020-02-03 103532.png
 
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