What does this alternator diode test tell me?
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2006 MB Clk 500
What does this alternator diode test tell me?
2006 Mercedes Benz CLK-500The battery is disconnected (pos and neg), and nothing is connected to my alternator. When I connect a multimeter's common (black) to the positive pin and the red to the alternator housing, I get a reading of around 500mv. If I test it the other way around, I get a reading of 0mv. I read this is correct and indicates the diodes in the alternator are working correctly.
But as soon as I attach the alternator's positive pin to its cable (the battery is still completely unconnected) and repeat the same tests, I get a reading both ways (the same 500mv and a higher 800mv to 900mv when I expect 0 for the last test), indicating that the diodes are bad.
I am sure this tells me something, but what does it mean? Why does that happen? (Because of this, I replaced the alternator, but another "new/used" alternator does the same thing.) I'm sure it is something down the line, but I want some help finding the problem. (The battery is draining, and electrical problems)
But as soon as I attach the alternator's positive pin to its cable (the battery is still completely unconnected) and repeat the same tests, I get a reading both ways (the same 500mv and a higher 800mv to 900mv when I expect 0 for the last test), indicating that the diodes are bad.
I am sure this tells me something, but what does it mean? Why does that happen? (Because of this, I replaced the alternator, but another "new/used" alternator does the same thing.) I'm sure it is something down the line, but I want some help finding the problem. (The battery is draining, and electrical problems)
Last edited by bggangel; Mar 13, 2024 at 02:28 PM.
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G550
Your first measurement of "500" is with the multimeter set on the diode function, correct? 500 is 500 millivolts which is an acceptable voltage reading indicating that none of the diodes are shorted. When you reconnect the alternator's positive cable, you get 0 in both directions because there is some other low resistance pathway in parallel with the the alternator. If you take a resistance measurement (ohms) between the alternator's positive cable with it disconnected and ground you'll probably get a reading of a couple ohms.
Does your CLK have an auxiliary battery?
Does your CLK have an auxiliary battery?
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2006 MB Clk 500
Yes, 500mv with the multimeter set on the diode function.
I updated my initial post to clarify that when connecting the positive to the alternator, I get a reading larger than zero in both directions. The first is the same ~500, the reverse where I had 0, now shows 800~900mv.
The CLK-500 does not have an auxiliary battery.
I can take a resistance measurement, but to be clear, you say: "take a resistance measurement (ohms) between the alternator's positive cable with it disconnected and ground"
I am not sure what you meant: Do you want me to measure the resistance between the alternator positive bolt and the alternator casing when the positive cable is not connected, correct?
I updated my initial post to clarify that when connecting the positive to the alternator, I get a reading larger than zero in both directions. The first is the same ~500, the reverse where I had 0, now shows 800~900mv.
The CLK-500 does not have an auxiliary battery.
I can take a resistance measurement, but to be clear, you say: "take a resistance measurement (ohms) between the alternator's positive cable with it disconnected and ground"
I am not sure what you meant: Do you want me to measure the resistance between the alternator positive bolt and the alternator casing when the positive cable is not connected, correct?
Last edited by bggangel; Mar 13, 2024 at 04:53 PM.
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Yes, 500mv with the multimeter set on the diode function.
I updated my initial post to clarify that when connecting the positive to the alternator, I get a reading larger than zero in both directions. The first is the same ~500, the reverse where I had 0, not shows 800~900mv.
The CLK-500 does not have an auxiliary battery.
I can take a resistance measurement, but to be clear, you say: "take a resistance measurement (ohms) between the alternator's positive cable with it disconnected and ground"
I am not sure what you meant: Do you want me to measure the resistance between the alternator positive bolt and the alternator casing when the positive cable is not connected, correct?
I updated my initial post to clarify that when connecting the positive to the alternator, I get a reading larger than zero in both directions. The first is the same ~500, the reverse where I had 0, not shows 800~900mv.
The CLK-500 does not have an auxiliary battery.
I can take a resistance measurement, but to be clear, you say: "take a resistance measurement (ohms) between the alternator's positive cable with it disconnected and ground"
I am not sure what you meant: Do you want me to measure the resistance between the alternator positive bolt and the alternator casing when the positive cable is not connected, correct?
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2006 MB Clk 500
Ok, I figured I'd measure what I can.
So, with the battery still completely disconnected:
Alternator positive CABLE to the ground while not connected to the alternator: slowly runs up for a while... seems to stop around 4 (on 20k Ohm setting).
Alternator positive BOLT to the ground while not connected: sometimes 10 (on 20k Ohm setting) and sometimes almost 20. Can't seem to get a grip on it, every time I go into the garage I seem to get 10 or 20
So, with the battery still completely disconnected:
Alternator positive CABLE to the ground while not connected to the alternator: slowly runs up for a while... seems to stop around 4 (on 20k Ohm setting).
Alternator positive BOLT to the ground while not connected: sometimes 10 (on 20k Ohm setting) and sometimes almost 20. Can't seem to get a grip on it, every time I go into the garage I seem to get 10 or 20
Last edited by bggangel; Mar 13, 2024 at 03:14 PM.
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2006 MB Clk 500
So do those readings provide some new ideas? Is the problem related to the starter that is between the alternator and the positive battery? And if so, how can I test that?
Last edited by bggangel; Mar 13, 2024 at 05:34 PM.
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The alternator seems to check OK, so I'd start checking fuses. If all the fuses are OK, then I'd use the multimeter to measure current -- put the multimeter in current mode and in series with the battery positive cable With the battery reconnected, you shouldn't be drawing more than a few hundred milliamps with the ignition off. If you're drawing more than 1 amp, start pulling fuses one at a time to see which circuit makes the current drop. Pull a fuse, if that one doesn't drop the current, put it back in, and move on to the next one, until you've covered them all. Hopefully you'll find the offending circuit, and be able to move on to components on that circuit.
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2006 MB Clk 500
I did that last summer. (This is a summer car, and I haven't driven it or worked on it for at least eight months.)
That is how I ended up at the alternator because, as far as I could tell, none of the fuses that were pulled caused a change in the drain that was there. Fuses in Engine compartment, next to the steering wheel and in the trunk.
(or at least I remember there was a drain... like I said, eight months ago).
I read somewhere that it then can only be the alternator or starter. I tested the alternator and saw the "defective diode". So I decided to change the alternator.
Changing the alternator on a v8 MB CLK 500 is a real pain, as you must remove the radiator fan and disconnect some cooling hoses. So, I saved that job for when the Chicago weather gets better, which is now.
But then, after replacing it (the cooling fan is still not on the car), I found out that the alternator I pulled out was perfectly fine once it was out, which led me to the test of unhooking it.
And that led me to write this post before I put the whole fan and bottom back.
I didn't want to dilute my original question with all this extra info, but there it is.
I think both the old and "new" alternators are good, but I can't explain the diode behavior and am wondering if I should start looking into the starter. I was hoping that the readings would indicate something in that direction, but I am just guessing and reading everything on the internet without actually knowing what I am doing
I guess I will start reassembling the front of the car again and maybe start all over with the analyses. I was hoping the weird diode behavior would indicate that I was closing in on the problem instead of having to start all over again.
That is how I ended up at the alternator because, as far as I could tell, none of the fuses that were pulled caused a change in the drain that was there. Fuses in Engine compartment, next to the steering wheel and in the trunk.
(or at least I remember there was a drain... like I said, eight months ago).
I read somewhere that it then can only be the alternator or starter. I tested the alternator and saw the "defective diode". So I decided to change the alternator.
Changing the alternator on a v8 MB CLK 500 is a real pain, as you must remove the radiator fan and disconnect some cooling hoses. So, I saved that job for when the Chicago weather gets better, which is now.
But then, after replacing it (the cooling fan is still not on the car), I found out that the alternator I pulled out was perfectly fine once it was out, which led me to the test of unhooking it.
And that led me to write this post before I put the whole fan and bottom back.
I didn't want to dilute my original question with all this extra info, but there it is.
I think both the old and "new" alternators are good, but I can't explain the diode behavior and am wondering if I should start looking into the starter. I was hoping that the readings would indicate something in that direction, but I am just guessing and reading everything on the internet without actually knowing what I am doing
I guess I will start reassembling the front of the car again and maybe start all over with the analyses. I was hoping the weird diode behavior would indicate that I was closing in on the problem instead of having to start all over again.
Last edited by bggangel; Mar 13, 2024 at 06:08 PM.
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2006 MB Clk 500
Last time I was working without help. Maybe this time I will post my findings to see if I can get more educated guesses
Thx so far.
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W212 E63 S AMG
Holy crap... has anyone tried this on a W212 AMG?
I disconnected the batteries and did the CTEK conditioning... then I went to unplug the ALT LIN, but there is no way to access the alternator from either the top of the bottom. From the top there is a turbo in the way along with a ton of coolant lines, etc... from the bottom, there is no gap anywhere near large enough to get beyond the steering rack and associated frame plates to get to the back of the alternator.
There is a plate that bolts to the subframe and also the steering rack which, if removed, may allow a tiny hope for access... but those are some big bolts. I don't know the torque specs, and I don't know if the steering rack could be damaged without having this plate in place. To me it looks like a shop would have to remove the rack to get to the alternator.
Has anyone done this on the same platform?
I disconnected the batteries and did the CTEK conditioning... then I went to unplug the ALT LIN, but there is no way to access the alternator from either the top of the bottom. From the top there is a turbo in the way along with a ton of coolant lines, etc... from the bottom, there is no gap anywhere near large enough to get beyond the steering rack and associated frame plates to get to the back of the alternator.
There is a plate that bolts to the subframe and also the steering rack which, if removed, may allow a tiny hope for access... but those are some big bolts. I don't know the torque specs, and I don't know if the steering rack could be damaged without having this plate in place. To me it looks like a shop would have to remove the rack to get to the alternator.
Has anyone done this on the same platform?
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W212 E63 S AMG
Holy crap... has anyone tried this on a W212 AMG?
I disconnected the batteries and did the CTEK conditioning... then I went to unplug the ALT LIN, but there is no way to access the alternator from either the top of the bottom. From the top there is a turbo in the way along with a ton of coolant lines, etc... from the bottom, there is no gap anywhere near large enough to get beyond the steering rack and associated frame plates to get to the back of the alternator.
There is a plate that bolts to the subframe and also the steering rack which, if removed, may allow a tiny hope for access... but those are some big bolts. I don't know the torque specs, and I don't know if the steering rack could be damaged without having this plate in place. To me it looks like a shop would have to remove the rack to get to the alternator.
Has anyone done this on the same platform?
I disconnected the batteries and did the CTEK conditioning... then I went to unplug the ALT LIN, but there is no way to access the alternator from either the top of the bottom. From the top there is a turbo in the way along with a ton of coolant lines, etc... from the bottom, there is no gap anywhere near large enough to get beyond the steering rack and associated frame plates to get to the back of the alternator.
There is a plate that bolts to the subframe and also the steering rack which, if removed, may allow a tiny hope for access... but those are some big bolts. I don't know the torque specs, and I don't know if the steering rack could be damaged without having this plate in place. To me it looks like a shop would have to remove the rack to get to the alternator.
Has anyone done this on the same platform?
https://mbworld.org/forums/w212-amg/...ml#post8793850
@CaliBenzDriver -- Do you happen to know where the other end of this line is routed - maybe which pin on the ECU? Maybe there's an easier way to interrupt that signal.
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W212 MY'14 M276-3.5NA @75kMi
concealed switch
Found it... ugh... "drop the steering rack"
https://mbworld.org/forums/w212-amg/...ml#post8793850
@CaliBenzDriver -- Do you happen to know where the other end of this line is routed - maybe which pin on the ECU? Maybe there's an easier way to interrupt that signal.
https://mbworld.org/forums/w212-amg/...ml#post8793850
@CaliBenzDriver -- Do you happen to know where the other end of this line is routed - maybe which pin on the ECU? Maybe there's an easier way to interrupt that signal.
The ALT-LIN homeruns straight into ECU under its connector M where you can install a $1 On/Off toggle switch concealed under the cover.
See the main ALT-LIN thread... MB Forum has disabled "Edits": I cant index the content
Last edited by CaliBenzDriver; Jan 25, 2025 at 07:04 PM.
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W212 E63 S AMG
Great question for a great shortcut... no need for pain & suffering!
The ALT-LIN homeruns straight into ECU under its connector M where you can install a $1 On/Off toggle switch concealed under the cover.
See the main ALT-LIN thread... MB Forum has disabled "Edits": I cant index the content
The ALT-LIN homeruns straight into ECU under its connector M where you can install a $1 On/Off toggle switch concealed under the cover.
See the main ALT-LIN thread... MB Forum has disabled "Edits": I cant index the content
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