AP1Driver

So, for the past few days I've noticed my voltage dropping to 12.3-12.6 at idle and fluctuating between that and 14.3v. I know this is a problem, I just can't decide if I want to replace the battery first. It seems to do it more when it's hot outside (70+) than cold, as it doesn't do it when I drive home from my office at night (yet). I don't think my scangauge is throwing bad numbers at me, what do you guys think?? No battery light or strange behavior to speak of...

Polaris0352

You could have the battery tested. The first question I'd ask is how old the battery is. Alternator charging efficiency drops off a cliff at idle usually. (Note that I'm speaking in general terms as a Mercedes technician) When your voltage is wigging out, get the rpms up to about 1500-2000. If it doesn't immediately stabilize, the alternator is the likely culprit. You can also vary the load on the electrical system by turning on lights, defroster, blower on high. A good alternator should be in the neighborhood of 13.4v to 13.8v. Yes, up to about 14.3 is acceptable. I worry much higher than 14.5. A fully charged lead acid battery should be 12.8v. If you're below that while idling, you have a VERY weak battery pulling the alternator down, or a weak alternator. But, like I said in the beginning, start with having the battery tested. Hope this helps.

AP1Driver

The battery is less than a year old. If cruising or running 3k rpm+ it seems to stay above 13.9. The strange part is the fluctuation, not being consistent. I'm gonna take the battery back since it's still under warranty and exchange it. I would be a bit shocked if the alternator is dying at only 68k miles, as my 02' had 148k on it when I sold it and still had the original alternator. I hope it's the battery.

Polaris0352

The fluctuations are definitely a concern. One other thing to check that just came to mind. Battery cables, tight and free of corrosion. Both at the battery and where they attach to the body/fuse box. Connection at the alternator. Is the nut tight? Is the ground strap for the engine in good shape? Tight connections, not corroded too heavily. Dang it, I always forget the bigger picture. Hope this helps some more.

Slowcrash_101

Check the hoses on the intake manifold over the alternator, make sure they're not leaking.

zeroptzero

I've got a scangauge and I am pretty sure I get the same numbers. The voltage output does change with rpms and load.

The reason I know this is because I have LED's in my instrument cluster and they dim down a bit at idle when I get just over 12 volts, and then they brighten up around 4000 rpms and higher as voltage climbs up to around 13.5 -14 volts. If I turn on the headlights the voltage increases and remains pretty flat at the higher output. I think above 2000 rpms the voltage output increases to around 13.5 volts or so ?. It's been a while since I checked my scangauge readings but I am sure that they are the same as yours.

wadzii

crank it up, unhook the battery then drive and see if it still fluctuates. If it doesnt then battery is your problem, if it does then alternator is your problem.

JUSTINTHECOASTIE

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