for context: i bought my 1998 mb3 in november of 2025 as my first car, before that i haven't had any experience whatsoever with working on cars, nor do i know anyone who can teach/help me so it's pretty much just me and the internet trying to figure this one out. (what i'm trying to say is that my knowledge is somewhat limited, so keep in mind that what might be obvious to you may not be obvious to me). also wanna mention that my current financial situation is quite bad to the point where €20 is a noticeable chunk of my wallet, therefore i would very much appreciate any help with this issue as it's literally impossible for me to just buy a new alternator and see if that fixes it.
first of all i wanna make very clear that i had a mechanic replace the alternator + belt in january of 2026 because i was having charging system issues so i'm currently trying to rule out every other possible cause of this issue before having to put myself in debt to get ANOTHER new alternator.
so i've been trying to figure this out for a week straight now and it's driving me crazy, not joking when i say i've spent every night of this week researching until atleast 4am, but i'd say my average sleepy time this week is around 7am. i've then woken up 5 hours later and went straight out to the car to try and make my every braincell work together to fix this. (obviously haven't worked out great)
a couple weeks ago i was out driving with my friend and i was in second gear accelerating quite hard. as i was approaching 7000rpm i noticed the battery light flickering a little bit. thought it was weird but didn't really think twice about it since it didn't stay on or anything. fast forward about a week and i was out driving by myself. accelerated quite hard once again (not gonna act like i don't abuse this car, because i really do drive it harder than i probably should lol) and i was probably somewhere around 6000rpm when the battery light came on and didn't go out. i stopped the car on the side of the road and called my dad since he lived closeby and decided to go to his place because he has a booster pack and tools etc incase i was gonna need it. the drive to his place was around 15km, and during the drive there i noticed the battery slowly being drained to the point where i literally JUST made it outside his place before it was gonna die completely. however when i parked the car the battery light suddenly went out? i let the car idle for a bit and eventually drove like an elderly lady home hoping that the battery light wouldn't come on again, which it didn't.
day after that i was out on a drive enjoying the moment and once again the battery light came on but this time i was only at about 4000rpm when it did. decided to go straight to a family member who lives closeby and also has some tools etc if the problem would escalate. i parked the car and went inside and talked for a bit, then went out to the car to see if it would start, which it did, and no battery light.
the next 3 or 4 days this cycle would continue where the battery light comes on during a drive, always somewhere between 3500rpm and 4500rpm, i shut the car off and turn it back on with no battery light.
since then i've done the following things to try and fix the issue:
- replaced the very worn out ground cables from the valve cover and transmission housing with thicker 21mm2 cables aswell as sanding down to expose bare metal underneath the grounding locations on the chassis
- replaced the battery ground cable with a much thicker one (not sure exactly how thick) and also relocating the ground location in the chassis + sanding it down
- checked the fuses in the engine bay + under the dash
- checked the alternator connectors and cleaned out the green connector since there was some kind of mayonnaise looking gunk in there (i'm now realizing that it might've been some kind of grease and maybe that was intentionally put in there but oh well)
- checked the spark plug wires since i noticed one of them kept getting pushed out slightly and also noticed some kind of clicking noise which i suspected was the spark possibly arcing to the metal and somehow making the alternator act up since the alt is grounded through the engine block (if i understand it correctly) but after changing all the spark plug wires back to the original ones that were in the car when i bought it, the battery light does still come on so i guess that was a dead end (and the clicking noise is still there)
- checked the spark plug that was pushing the wire out and found it was a little bit loose, there was some brown-ish residue at the top of the porcelain part but i don't know if that's relevant or not
- checked the battery voltage with multimeter; 13,3v with engine off, 12,5v with engine running and battery light on, 14,5v with engine running after shutting the car off and starting it again. my battery has a indicator light which is green meaning it's good so i'm just assuming it's good lol
- sanded down the ground location for my subwoofer amplifier (2000w amplifier with a 900w rms subwoofer) just in case that has anything to do with it. amp is grounded through a factory bolt in the trunk where the factory "rear speaker holder/thing that covers the trunk" thing meets the rest of the chassis, i really have no clue whether that's a good or bad ground location but yeah. probably also worth mentioning that whenever i crank the volume up real high and the sub is working hard, the headlights dim down a little bit aswell as the dash lights but it's really only noticeable when it's dark out
- checked the alternator belt which seems to be in ok condition however it looks kind of dry-rotted and has some tiny cracks all over, and from what i can tell it's not slipping unless it's doing so silently. my AC belt likes to slip alot and that sounds like 345 pigs screaming for their lives in symphony, which my alternator belt does not.
- checked the battery terminals and noticed the positive cable wasn't really very tight so i jammed a little nail inbetween to make it sit tighter (i am the literal opposite of rich so if a ghetto fix is possible then that's what i'm gonna do)
- spent countless hours googling/youtubing/checking forums/checking service manuals/brainstorming with chatgpt/overall researching trying to find anything that might be of help but so far nothing has worked and i've only found one single forum post of someone having a somewhat similar issue but it didn't help my situation
so yeah. i don't know what to do next. the only thing that i haven't investigated so far (to my knowledge) is the wires in the 3p connector on the alternator, maybe there's a break in one of the cables somewhere but even if there is, idk how to find it let alone repair it. and also i don't see how that would be affected by the engine being at higher RPMs.
everything seems to be pointing towards the alternator or voltage regulator or something but like i said, i really don't want that to be the cause because i probably won't be able to fix it for atleast a couple months and i need my car every day since it's my only car.
some things worth mentioning:
- i'm in sweden so my car is left hand drive, if that makes any difference.
- the wire harness at the top right corner of the engine bay where the brake fluid reservoir is located has 3 connectors (male connectors i think) but only 2 of them are actually connected to something, while the third one is just hanging there next to them.
- while i was underneath the car and inspecting the alternator, i found a cable that was disconnected (it was the same cable but had a female and male connector inbetween them that were plugged out) but i believe that was just the cabin warmer outlet thingy, like the thing where you can plug in a heater fan in the passenger side footwell. i don't use one of those anyway so i don't really care that that's disconnected but i'm just mentioning it in case someone knows something that i don't.
- i'm pretty sure the wires going to the passenger door are damaged and shorted, because when i upgraded the door speakers i noticed that all the speakers in the car (i have 4 speakers) besides the subwoofer stopped working whenever the passenger side door speaker was connected to the headunit, so i'm guessing the headunits internal amplifier went into protect mode. another reason for suspecting the wires are shorted is that every time i unlock the car with the key, the passenger door mirror adjusts itself a little bit so i keep having to adjust it back up to not be just seeing the pavement lol
- almost every time i do anything that requires electricity (rolling the windows down, turning the wheel so the power steering engages, turning AC or headlights on, etc) while the car is idling, the rpms drop to the point where the car starts shaking from the vibrations for a second and then goes back up
i'm probably forgetting some stuff but it's now 2:30am and i've been writing this post for 2 hours so this will have to do for now. if you need any other information from me then i'll gladly provide it, i just really need this to be fixed ASAP and i really really REALLY hope it's not an expensive fix.
any help would be greatly appreciated !!!!!!!!!!!!!!
Question
oaktreehill.
for context: i bought my 1998 mb3 in november of 2025 as my first car, before that i haven't had any experience whatsoever with working on cars, nor do i know anyone who can teach/help me so it's pretty much just me and the internet trying to figure this one out. (what i'm trying to say is that my knowledge is somewhat limited, so keep in mind that what might be obvious to you may not be obvious to me). also wanna mention that my current financial situation is quite bad to the point where €20 is a noticeable chunk of my wallet, therefore i would very much appreciate any help with this issue as it's literally impossible for me to just buy a new alternator and see if that fixes it.
first of all i wanna make very clear that i had a mechanic replace the alternator + belt in january of 2026 because i was having charging system issues so i'm currently trying to rule out every other possible cause of this issue before having to put myself in debt to get ANOTHER new alternator.
so i've been trying to figure this out for a week straight now and it's driving me crazy, not joking when i say i've spent every night of this week researching until atleast 4am, but i'd say my average sleepy time this week is around 7am. i've then woken up 5 hours later and went straight out to the car to try and make my every braincell work together to fix this. (obviously haven't worked out great)
a couple weeks ago i was out driving with my friend and i was in second gear accelerating quite hard. as i was approaching 7000rpm i noticed the battery light flickering a little bit. thought it was weird but didn't really think twice about it since it didn't stay on or anything. fast forward about a week and i was out driving by myself. accelerated quite hard once again (not gonna act like i don't abuse this car, because i really do drive it harder than i probably should lol) and i was probably somewhere around 6000rpm when the battery light came on and didn't go out. i stopped the car on the side of the road and called my dad since he lived closeby and decided to go to his place because he has a booster pack and tools etc incase i was gonna need it. the drive to his place was around 15km, and during the drive there i noticed the battery slowly being drained to the point where i literally JUST made it outside his place before it was gonna die completely. however when i parked the car the battery light suddenly went out? i let the car idle for a bit and eventually drove like an elderly lady home hoping that the battery light wouldn't come on again, which it didn't.
day after that i was out on a drive enjoying the moment and once again the battery light came on but this time i was only at about 4000rpm when it did. decided to go straight to a family member who lives closeby and also has some tools etc if the problem would escalate. i parked the car and went inside and talked for a bit, then went out to the car to see if it would start, which it did, and no battery light.
the next 3 or 4 days this cycle would continue where the battery light comes on during a drive, always somewhere between 3500rpm and 4500rpm, i shut the car off and turn it back on with no battery light.
since then i've done the following things to try and fix the issue:
- replaced the very worn out ground cables from the valve cover and transmission housing with thicker 21mm2 cables aswell as sanding down to expose bare metal underneath the grounding locations on the chassis
- replaced the battery ground cable with a much thicker one (not sure exactly how thick) and also relocating the ground location in the chassis + sanding it down
- checked the fuses in the engine bay + under the dash
- checked the alternator connectors and cleaned out the green connector since there was some kind of mayonnaise looking gunk in there (i'm now realizing that it might've been some kind of grease and maybe that was intentionally put in there but oh well)
- checked the spark plug wires since i noticed one of them kept getting pushed out slightly and also noticed some kind of clicking noise which i suspected was the spark possibly arcing to the metal and somehow making the alternator act up since the alt is grounded through the engine block (if i understand it correctly) but after changing all the spark plug wires back to the original ones that were in the car when i bought it, the battery light does still come on so i guess that was a dead end (and the clicking noise is still there)
- checked the spark plug that was pushing the wire out and found it was a little bit loose, there was some brown-ish residue at the top of the porcelain part but i don't know if that's relevant or not
- checked the battery voltage with multimeter; 13,3v with engine off, 12,5v with engine running and battery light on, 14,5v with engine running after shutting the car off and starting it again. my battery has a indicator light which is green meaning it's good so i'm just assuming it's good lol
- sanded down the ground location for my subwoofer amplifier (2000w amplifier with a 900w rms subwoofer) just in case that has anything to do with it. amp is grounded through a factory bolt in the trunk where the factory "rear speaker holder/thing that covers the trunk" thing meets the rest of the chassis, i really have no clue whether that's a good or bad ground location but yeah. probably also worth mentioning that whenever i crank the volume up real high and the sub is working hard, the headlights dim down a little bit aswell as the dash lights but it's really only noticeable when it's dark out
- checked the alternator belt which seems to be in ok condition however it looks kind of dry-rotted and has some tiny cracks all over, and from what i can tell it's not slipping unless it's doing so silently. my AC belt likes to slip alot and that sounds like 345 pigs screaming for their lives in symphony, which my alternator belt does not.
- checked the battery terminals and noticed the positive cable wasn't really very tight so i jammed a little nail inbetween to make it sit tighter (i am the literal opposite of rich so if a ghetto fix is possible then that's what i'm gonna do)
- spent countless hours googling/youtubing/checking forums/checking service manuals/brainstorming with chatgpt/overall researching trying to find anything that might be of help but so far nothing has worked and i've only found one single forum post of someone having a somewhat similar issue but it didn't help my situation
so yeah. i don't know what to do next. the only thing that i haven't investigated so far (to my knowledge) is the wires in the 3p connector on the alternator, maybe there's a break in one of the cables somewhere but even if there is, idk how to find it let alone repair it. and also i don't see how that would be affected by the engine being at higher RPMs.
everything seems to be pointing towards the alternator or voltage regulator or something but like i said, i really don't want that to be the cause because i probably won't be able to fix it for atleast a couple months and i need my car every day since it's my only car.
some things worth mentioning:
- i'm in sweden so my car is left hand drive, if that makes any difference.
- the wire harness at the top right corner of the engine bay where the brake fluid reservoir is located has 3 connectors (male connectors i think) but only 2 of them are actually connected to something, while the third one is just hanging there next to them.
- while i was underneath the car and inspecting the alternator, i found a cable that was disconnected (it was the same cable but had a female and male connector inbetween them that were plugged out) but i believe that was just the cabin warmer outlet thingy, like the thing where you can plug in a heater fan in the passenger side footwell. i don't use one of those anyway so i don't really care that that's disconnected but i'm just mentioning it in case someone knows something that i don't.
- i'm pretty sure the wires going to the passenger door are damaged and shorted, because when i upgraded the door speakers i noticed that all the speakers in the car (i have 4 speakers) besides the subwoofer stopped working whenever the passenger side door speaker was connected to the headunit, so i'm guessing the headunits internal amplifier went into protect mode. another reason for suspecting the wires are shorted is that every time i unlock the car with the key, the passenger door mirror adjusts itself a little bit so i keep having to adjust it back up to not be just seeing the pavement lol
- almost every time i do anything that requires electricity (rolling the windows down, turning the wheel so the power steering engages, turning AC or headlights on, etc) while the car is idling, the rpms drop to the point where the car starts shaking from the vibrations for a second and then goes back up
i'm probably forgetting some stuff but it's now 2:30am and i've been writing this post for 2 hours so this will have to do for now. if you need any other information from me then i'll gladly provide it, i just really need this to be fixed ASAP and i really really REALLY hope it's not an expensive fix.
any help would be greatly appreciated !!!!!!!!!!!!!!
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