Battery under voltage warning
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From: Fallon, NV
BMW and Mercedes
Battery under voltage warning
So about a week and a half ago, the C240 wagon would not start. Tried all my tricks and nothing. Had it towed to the service folks, paid $1750 to replace the fuel pumps/sender. Probably a bit of a rip off, but I didn't have time or desire to do it myself. Fast forward to today, driving home from the airport after having been gone all week, I notice the "UNDERVOLTAGE SWITCH OFF CONSUMERS" caution. I figured, hey, the car has been sitting around a lot recently, maybe the battery charge was just low. 40 min drive home from the airport didn't make it better. Re-started the car, and had the caution again. It seems to come on after the car has been running for about 2-3 mins, with or without the A/C, fan/blower or stereo on, i.e. all the time. I'm thinking possibly dying battery, but more likely alternator or regulator? I know there are some other threads about this, but is there a more common cause of this amongst those 3 possible culprits? Also, anything that could have possibly been knocked loose or improperly re-installed while the back seat was out for the fuel pump work, that might have caused this? Its the first I have ever seen this problem, and it just seems like suspiciously bad luck to have this pop the 3rd drive after I got it back……not saying they did anything wrong, but I'd like to rule out any simple solutions.
I'm really getting pretty sick of dealing with this car. I have literally never owned a vehicle that had so many problems, and specifically problems that just leave you stranded by the side of the road.
I'm really getting pretty sick of dealing with this car. I have literally never owned a vehicle that had so many problems, and specifically problems that just leave you stranded by the side of the road.
Last edited by FLYNAVY; Apr 15, 2016 at 10:47 PM.
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For sure…….wasn't planning on taking it anywhere until I have a good idea what is going on with it. My guess is that it is the most expensive problem (i.e. alternator)…..why wouldn't it be? But on the side of optimism, how likely would it be this is just a bad battery? It seems like a no load voltage check of the battery wouldn't really be conclusive in terms of the health of the battery itself if it wasn't getting recharged by a faulty alternator. I suppose I could just replace the battery and see if it fixes the problem, but that is kind of a waste in my mind if that isn't the root problem. If I understand correctly, the engine on/load check of the battery should tell if the alternator is supplying sufficient charge, though in theory, it also wouldn't prove that a bad battery just wasn't accepting the alt output? I'd also imagine that if it were some system drawing way more than spec power, this test would also be inaccurate? Thoughts?
Last edited by FLYNAVY; Apr 16, 2016 at 01:31 AM.
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Easy brother. That is the advice/first troubleshooting step potentially of many, one that is notably in any manual or thread on this topic. I'm just trying to clarify the process and make sure I'm not making bad assumptions along the way.
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Multimeter voltage checks good, both with engine off, as well as engine running with as much of a load I could give it. Somewhat difficult to verify a difference of a couple volts using an analog multimeter, but my best schwag put the battery voltage at about 12.5-12.7 VDC, and engine on was showing somewhere between 14.2-14.5 VDC. Caution also didn't pop during that time. Only thing I can think of off the top of my head is whether it could be engine bay temp related? Cold start when I tested it, while each time it happened yesterday, it was a warm engine. I'd imagine that could have an effect on either a weak battery, or a weak alternator. Thoughts on that? BTW I do appreciate the help, I just was hunting for a bit more info.
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Small update, with hopefully some better info….
Decided to take the car out and see if I could cause it to happen again. Cold start, warmish day, and it took about 7-8 mins of driving between 35-70 mph to make it happen. No headlights, no A/C or climate control, no stereo, etc. Shut it off, unloaded the car at my destination, then started back up about 3-4 mins later. This time, the warning came back on within 2-3 mins. Got back home a few mins later and kept the engine running, turned on the A/C, headlights, stereo and then checked the voltage. Read out was 12 VDC. Turned the car off, checked the battery charge, which showed a normal 12.5 VDC. Let it sit for a couple mins, then started the car back up with all the same loads on. Initial voltage check showed somewhere between 14.0 and 14.5 VDC. Based on that, I'm thinking it is either the alternator or alt regulator. It would seem like the battery is pretty healthy, and the alternator is giving me an under voltage for some reason after the car has warmed up. Thoughts on that assessment? Thanks again guys.
Decided to take the car out and see if I could cause it to happen again. Cold start, warmish day, and it took about 7-8 mins of driving between 35-70 mph to make it happen. No headlights, no A/C or climate control, no stereo, etc. Shut it off, unloaded the car at my destination, then started back up about 3-4 mins later. This time, the warning came back on within 2-3 mins. Got back home a few mins later and kept the engine running, turned on the A/C, headlights, stereo and then checked the voltage. Read out was 12 VDC. Turned the car off, checked the battery charge, which showed a normal 12.5 VDC. Let it sit for a couple mins, then started the car back up with all the same loads on. Initial voltage check showed somewhere between 14.0 and 14.5 VDC. Based on that, I'm thinking it is either the alternator or alt regulator. It would seem like the battery is pretty healthy, and the alternator is giving me an under voltage for some reason after the car has warmed up. Thoughts on that assessment? Thanks again guys.
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Got back home yesterday from work travel and finally had a chance to address the problem. Based on the above reasoning I posted, I went ahead and ordered a new alternator last week. Installed it today, and that fixed the problem. Weird side note, now my CD changer doesn't work and shows "Magazine Empty"….I'd assume this had something to do with disconnecting the battery while I was doing the alternator work. Anyway, pretty straight forward, took about an hour start to finish.
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Thread Starter
MBWorld Fanatic!
Joined: Nov 2005
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From: Fallon, NV
BMW and Mercedes
Yeah, beyond the fix being a new alternator, I am not savvy enough to know exactly what the problem was. Were you getting over voltage in your case?
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c240 2002
yea but not too high.. it was reading 14.9 whenever i had to boost my car. not some outrageous 18v or something. the main issue i ended up with was an incredible battery drain. The warning indicator came early on and i ignored it as it went away and never returned for quiet sometime but all kind of charging system problems happened. it took about a year before i seen the warning light again. but when i did not have to boost my car i had gotten no over voltage readings whatsoever.
Last edited by kanadian; Apr 28, 2016 at 10:42 AM.
