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Awesome. Ground is typically the negative lead. Unlike a diode that allows current to flow in one direction, measuring voltage or resistance does not depend on which lead you use for the 12V power or the ground. If you have them backwards, the voltage will show as a negative number.
Okay, two last questions for you if you happen to know the answer. What level of resistance is normal and what would raise some concern when I check tomorrow? And lastly, just out of curiosity what would happen if I checked the “trigger” pin against a neutral? Or would nothing happen since I won’t be attempting to crank the engine over?
Last edited by hmclaughlin67; Feb 25, 2025 at 12:31 AM.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
I don't know the values for resistance, but they are not in the test. However, you can always compare with a couple of others to see if they are consistent. I don't think anything would happen if you checked other pins against ground. You are working with a DC circuit and there is no neutral. The internal resistance of the volt meter is high enough to appear as an open circuit to any electronics. I will say, however, that I tested a circuit in trying to solve a false positive error against ground and it had the effect of "resetting" the error. I can't explain it electronically because the voltmeter should have resisted any flow of current, but it seemed to fix the problem. Or maybe it was coincidental because I also had been futzing around with the IC control module. Here's the thread where it is discussed: https://mbworld.org/forums/w212-amg/...fueling-2.html
I am completely lost now. Resistance measured an absolute zero, tried multiple different pin orientations to make sure it wasn’t a mistake on my end but it was definitely zero, so I doubt there’s a torn wire anywhere. The voltage measured anywhere from 11.9V to 12.5V, which is within spec and it measured the exact same as 3 other cylinders I checked so I don’t think that’s my problem. I’ve already swapped coils around several times now and those haven’t been the problem either. There’s no obviously broken wires or any sign of contamination at all, and the fuse checked out okay too. I got enough amps going to it. I’m going to try swapping it with a different 15A fuse in the box in case that’s the issue, but I don’t expect it to be. I am stumped on this one.
Last edited by hmclaughlin67; Feb 25, 2025 at 10:01 AM.
I once had a stored code for the drivers side heated mirror, forget what the actual code was. I used my Foxwell scanner and found a section for testing/actuation of different things. It let me test the heater function, basically just turn it on and off. it worked fine so i didnt need any further diagnosis though. Im not sure if it would have told me what was specifically wrong. On mine I think the mirror heats up with the rear window defrost button on the climate control unit.
I once had a stored code for the drivers side heated mirror, forget what the actual code was. I used my Foxwell scanner and found a section for testing/actuation of different things. It let me test the heater function, basically just turn it on and off. it worked fine so i didnt need any further diagnosis though. Im not sure if it would have told me what was specifically wrong. On mine I think the mirror heats up with the rear window defrost button on the climate control unit.
My actuation test passed, so I know there’s something wrong in-between the module and the mirror. I’m going to investigate how to pop that mirror cover off and start there and work my way over since there really shouldn’t be too much wire to sort through. I imagine that the module is probably up front in the dash somewhere.
On another note, out of pure curiosity I got my battery and alternator tested at AutoZone. My battery is reading 650 CCA but the alternator failed their test. I’ve been suspecting that my alternator pulley has been bad anyways so it was on the list of future replacements. Could that be causing my weird electrical issue with the ignition coil?
Edit: mine is also activated through the rear defroster button, and I confirmed my passenger mirror heater is not working
Last edited by hmclaughlin67; Feb 25, 2025 at 03:30 PM.
Thank you. I am going to go ahead and replace my alternator with a refurbished Bosch 180A and if that code is still there I will give that a shot to see if it helps smooth it out.
This is definitely an issue I have not been enjoying chasing down lol. I hope it’s as simple as the alternator being faulty or else I am going to have to really start tearing this car apart, which I’d rather not do since it’s my daily.
Okay so here is a question for all you out there who are a lot smarter than I am. I checked 3 or 4 different ignition coil plugs, measured with my negative on pin 3 and positive on pin 1. They mostly all stabilized at 11.9V, all the same to the hundredth decimal place.
With the possibly faulty alternator, could this still be my issue? Given that the car had been sitting overnight even?
I’m new to the Benz world but on all my old cars anything under 12V was concerning and was most likely the problem. Is 11.9V really ok? And if it’s not, do I have a faulty battery as well? I’m just struggling to understand why I only get the code on cylinder 4 of all things if the voltage is the same across all cylinders.
I got some photos of my battery terminals. First is the negative and then the positive. These look pretty scratched up to me, also there’s a weird substance built up around the top of the battery. Hard to tell what it is. It’s some brand I’ve never heard of but it’s dated early 2023 so it’s doubtful it’s bad? May just replace it anyways given the state of the terminals. Also may try a wire brush to clean it up a bit as well as the connectors.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
That's weird stuff on the battery terminals. It would probably be a good idea to replace the battery. Make sure it is an AGM battery. Do you have a second battery in the trunk? You will if your car has ECO Start/Stop. But it doesn't show on your datacard. If you happen to have a second battery, consider replacing that too. Second (auxiliary) batteries are usually needed to run a lot of the electronics in the car, but mostly for ECO Start/Stop on newer cars.
How long did AutoZone test the battery and alternator? A load test usually takes a while.
That's weird stuff on the battery terminals. It would probably be a good idea to replace the battery. Make sure it is an AGM battery. Do you have a second battery in the trunk? You will if your car has ECO Start/Stop. But it doesn't show on your datacard. If you happen to have a second battery, consider replacing that too. Second (auxiliary) batteries are usually needed to run a lot of the electronics in the car, but mostly for ECO Start/Stop on newer cars.
How long did AutoZone test the battery and alternator? A load test usually takes a while.
It was maybe 30 seconds long. I’m taking it with a grain of salt but I don’t have my own tester as of this moment. Two of my OBD readers say both battery and alternator are good so I’m not sure.
I don’t have the start/stop, I don’t even think they had it in 2008 to be honest but I still think I have a battery back there? I’ll have to check it out tomorrow when it’s light out again. I’m going to try a scotch brite pad on the terminals tomorrow and see if maybe that helps clear up this intermittent issue. The 0987 code is always stored, even when I clear it and refresh constantly, while driving, the first time it shows up it says stored. Never pending or current or anything.
Also, I tested the Torque Converter Slip Speed and graphed it out and noticed the pretty massive fluctuations in it. I’ve been chasing a vibration while driving, only when engine is warmed up, for a while now and many of you who replied have helped me along the way. I believe this all but confirms that the transmission is my issue and hopefully the fluid change will help sort it out.
Anyways, do we think maybe the transmission sort of flipping out like this is drawing power away from the ignition system enough to intermittently set off a code?
Edit: Should have mentioned that the two big fluctuations you see there were when I was purposely making the car vibrate to see how the torque converter reacted.
Is the correct battery installed? My battery is an H7 AGM. My old battery was a Mercedes battery so I replaced it with the same one from the MB dealer, but later i realized they sell H7 AGM batteries right at Walmart for cheaper with better warranties.
You mention 180 amp alternator, is that whats in the car? I think mines a 150amp.
When I was having the red battery warning pop up once in awhile I bought two inexpensive battery testers. One is the Ancel BM200 which is a small bluetooth device that mounts right to the battery and has an app to see the readings. I like it because i can drive around and see exactly what the battery voltage is showing. Its basically a multimeter I think. Then I got a Konnwei battery tester. Im not saying these things are super accurate, but i think they are good enough to show me if there is an issue with the battery or even the alternator. Oddly enough the red battery warning stopped showing up and its fine now.
Also I noticed on the Mercedes battery if you peel the sticker away there is a view port that is supposed to glow green I guess if the battery is bad.
Is the correct battery installed? My battery is an H7 AGM. My old battery was a Mercedes battery so I replaced it with the same one from the MB dealer, but later i realized they sell H7 AGM batteries right at Walmart for cheaper with better warranties.
You mention 180 amp alternator, is that whats in the car? I think mines a 150amp.
When I was having the red battery warning pop up once in awhile I bought two inexpensive battery testers. One is the Ancel BM200 which is a small bluetooth device that mounts right to the battery and has an app to see the readings. I like it because i can drive around and see exactly what the battery voltage is showing. Its basically a multimeter I think. Then I got a Konnwei battery tester. Im not saying these things are super accurate, but i think they are good enough to show me if there is an issue with the battery or even the alternator. Oddly enough the red battery warning stopped showing up and its fine now.
Also I noticed on the Mercedes battery if you peel the sticker away there is a view port that is supposed to glow green I guess if the battery is bad.
Honestly I don’t have a clue what battery it is. The handles are the same as the MB battery, but weirdly enough there isn’t a single identifying sticker on the battery. Only the logo and date it was manufactured. No CCA, standby amps, battery size or type, nothing. It’s quite odd. And when I check FCPEuro, it says the Bosch 180 is a perfect fit and OE supplier. I was under the impression that the 4Matic’s have the 180 amp and the RWD models have the 150 amp, but honestly not sure how true that is. I know you have the 4Matic as well Tim, but I haven’t checked for a part number elsewhere yet.
Your battery looks like a no name lead acid battery. That substance in the terminals use corrosion. You can clean it with just about anything and some paper towels just becareful not to get it on ya hands. If you do just wash it off with soap and water. You can get a battery post and connector cleaner That's a small wire brush and the terminals will clean right up. If you keep a led acid battery then cleaning should bee a yearly thing you do. With AGM style battery's they don't curious like a led acid does so less maintenance on a AGM but a bigger price tag too.
Check voltage at the battery with the car running and it should be between 13-15 volts.
If there are spikes above and below that your alternator is prolly okay and 11.9 volts after sitting all night is a good reading IMO
Also if you use Grok AI you can send it a picture and it will identify almost any picture. It will also give you repair advice just ask a specific questions. Utt should be able to identify your battery.
Grok app is in the app store
Last edited by RobertR728; Feb 26, 2025 at 06:07 AM.
Your battery looks like a no name lead acid battery. That substance in the terminals use corrosion. You can clean it with just about anything and some paper towels just becareful not to get it on ya hands. If you do just wash it off with soap and water. You can get a battery post and connector cleaner That's a small wire brush and the terminals will clean right up. If you keep a led acid battery then cleaning should bee a yearly thing you do. With AGM style battery's they don't curious like a led acid does so less maintenance on a AGM but a bigger price tag too.
Check voltage at the battery with the car running and it should be between 13-15 volts.
If there are spikes above and below that your alternator is prolly okay and 11.9 volts after sitting all night is a good reading IMO
Also if you use Grok AI you can send it a picture and it will identify almost any picture. It will also give you repair advice just ask a specific questions. Utt should be able to identify your battery.
Grok app is in the app store
Thanks for the tip on the AI app. I figured it was just another no name battery as well, it’s just odd to me that it doesn’t have its amperage rating anywhere on there. I’ve never seen that before even on the cheapest of batteries. For now it will have to do, but definitely looking to replace with an MB AGM battery sooner rather than later.
I will check with my multimeter later to verify, but on my drive home last night my scanner said the alternator was consistently between 13.9V to 14.3V.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
You should review the thread on disconnecting the ALT-LIN wire and it's effect on stabilizing the voltage. Since disconnecting, I have a steady and consistent 14.1V after initial start. The argument is that a steady voltage provides a stable electrical environment and better operating environment for electronics. It seems to work. Since our cars are extremely temperamental when it comes to voltages, you may want to try it. If there is no improvement or you detect any adverse conditions, you can always plug it back in.
You should review the thread on disconnecting the ALT-LIN wire and it's effect on stabilizing the voltage. Since disconnecting, I have a steady and consistent 14.1V after initial start. The argument is that a steady voltage provides a stable electrical environment and better operating environment for electronics. It seems to work. Since our cars are extremely temperamental when it comes to voltages, you may want to try it. If there is no improvement or you detect any adverse conditions, you can always plug it back in.
Whats the best way to access it on the M272? Remove the alternator and unplug it from there? I took a quick look around and I don’t really see an easy way to reach that plug or anything.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
It's on the back side of the alternator. On my M276 I was able to reach it from the top, but had to remove the air filter housing. Not sure how to do it on the M272.
Update for everyone: Today I cleaned the terminals, connectors, and every grounding point I could find in the engine bay. The nuts for all of them were still painted with no scratches so luckily (or unluckily??) they seem to have never been touched since the factory. They had a little corrosion, nothing crazy but I wouldn’t be shocked if that’s the cause. Not expecting it, but wouldn’t be shocked. I have a transmission service to do this weekend, and I suspect that will fix the vibrations while driving I mentioned. Next up after that will be to inspect the charging system more closely, and definitely starting with getting a high quality AGM battery in there. Even if it doesn’t fix anything, it will give me peace of mind lol. I don’t mind throwing parts at this car as with as many miles as it has, I know every part I’ve replaced so far would have failed soon anyways. I will update later tonight on if the code returns.
On another note, anyone know how to remove the mirror cap to investigate the mirror heater? It seems like it just pops out as I can’t find anywhere there could possibly be a screw, but don’t want to break anything.
That website has a shocking amount of stuff I really didn’t think it would have. I need to play around some more with that link. Thank you!
Also, code did end up returning on my drive to work tonight. I’m going to try disconnecting the alt-lin as suggested and in the mean time I have a few other projects to work on. The code isn’t causing any problems so it’s not at the top of my list currently, just been a headache to try to track down.
I’ll try things that I think of as I go, and if one of the things I try work I will be sure to update. Thank you @TimC300 and @JettaRed for the help and advice like always! I run in to you two on this forum a lot and you’ve both got great knowledge and resources.
Last edited by hmclaughlin67; Feb 26, 2025 at 04:31 PM.
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