MacCready

Hey guys, new account but not a new member... forgot my old account info and hadnt had any issues on my car so its been years since I've been on the forums. So my battery was getting pretty old and I got a new one, got a new alternator, and also got a new belt (figured might as well get that system a fresh start). She fired up no problem and I kept her running for a good amount of minutes, even drove it down the street... but my battery idles @12.18v. With the car off, voltage back at around 12.5ish. Gave it a few minutes, started and drove it again = same issue. Getting no error codes, checked fuses, everything sounds and appears normal but can't see what the problem could be. Am I missing something? Is there a hidden or special fuse that I'm overlooking? I've done the alternator and serpentine belt replacement on my 96 E320 no problem (same exact M104 engine) so I'm hoping its just me being an idiot and overlooking something minor. Any tips or tricks to pinpoint what the problem could be would be greatly appreciated.

khomer2

Welcome MacCready,

Try checking for a parasitic draw by testing each of the fuses.
Plenty of others available to review too.

fabbrisd1

If you replaced your battery - side note - Reset Steering Angle Sensor - Reset Radio - Reset Sunroof - and importantly Reset Each Window

Are you measuring voltage using the Climate Control Read Out or using a meter at the Charging Posts ? Climate Control Readout can be "normally" 1/2 to 1/12V low.

If you are asking about "issues" - with up off idle to me you "should" show charging at 13-14 - and to me your "off" voltage if from the Climate Control Read Out is fine..

MacCready

khomer2! Thanks for the welcome, I remember your name from alot of the help threads back then its good to see you're still around, glad to get input from a member like yourself. Gonna try checking those fuses for a draw first thing in the morning. It shouldnt take too long so if anyone else has any idea on what to add to my checklist of things to check let me know.

MacCready

@fabbrisd the reading is coming off of my meter directly on the battery posts. Forgot how to do the CC way so I just do it the old fashioned way. I'll also try re-doing the resets you mentioned, might've overlooked some of them while I was determining why my voltage was so low at idle.

raymond g-

i'm scratching my head wondering why voltage at the battery terminals are reading LESS while the car is running
than when the engine is off. that seems odd to me. isn't that similar to measuring alternator output through the
voltage regulator....and as such, should be quite a bit higher ?

jakel01

Things that popped into my head...
I could see a poor ground connection being an issue if the car has rust.
Also, a new battery might not have had a full charge when you bought it, so it's in process of charging and thus showing lower than normal voltage.

raymond g-

i'm just a amateur garage hack. in my head, this is what I've projected: when the DVM/VOM is connected to
the battery terminals AND the engine is running, the DVM is measure electrical potential between those two
points (- vs + terminals) at that point in time, ie as the alternator and voltage regular has created voltage at
that intersection in the circuit. IT DOES NOT mean that what you are measuring is the battery voltage.

to measure battery voltage, you would need to 1) measure the battery with ignition OFF, or 2) measure
battery disconnected from the car. is my thinking flawed?

MacCready

Same. I thought maybe there was a normal explanation to this that I didnt know of and thats why I couldnt pinpoint the culprit. Might've been a fluke (no pun intended to my multimeter) so I'm gonna check results on testing again. (Good to see you still around too bud!)

khomer2

I would make sure the alternator was tested (properly). and confirm the diodes in the alternator are working correctly (has to be done on a scope). If you rev the car and the the V is low, there is a problem.

Also, check that the alternator is grounded properly as well, check the -ve battery terminal is also connected tight. I've had glitchy issues after I swapped out the w163 battery and went back to realize the terminal was tight but NOT tight enough. (cleaned the -ve cable thoroughly again before reconnecting to solve the glitchy issue).