Frustrating...alternator light

[///M3Mike]

The facts:

New battery 3 months ago
Alternator rebuilt 3 months ago
(unrelated, hopefully, but)
New starter 2 years ago
New starter solenoid 1 year ago

Problem:

After starting engine, the alternator/battery light stays on until revved over 1500-2000 rpm.  Sometimes, starting sounds labored like the battery is dying but always starts.  At idle, turn signal makes dash/OBC lights dim in sync with signal.

Observation/measurements:

After sitting overnight, battery voltage 12.4V.  
Immediately after starting (engine @ idle), voltage measured at battery/alt output: 12.8V (alt light on at this point)
Rev engine to 1500-2000rpm, alt light goes out, 14.4V measured at battery
Return to idle after revving past 1500: 14.4V measured at battery and alt light stays out

I also within the past year replaced the alternator adjustment mount and the associated toothed bolt.  I get that damn thing as tight as I can get it, and half maybe an inch of belt deflection (?)

What am I missing?  What else to check?  What am I doing wrong?  

BTW: Car is bone stock electrically except for a Sirius receiver I hardwired to the cig lighter wiring.

[monty23psk]

When you had the alternator rebuilt, did you have a new voltage regulator installed? Also, could be a bad ground at the battery or alternator. Looks to have low voltage at idle.

[cristimm]

I think the alternator is your problem.

[///M3Mike]

Thanks for the replies, guys.....they actually got me thinking.  Earlier this summer, I replaced the ground cable from the alternator to the engine block (or mount?), the engine block to the chassis on the passenger side, and the battery to the chassis (trunk-mounted).  The woven grounding strap that goes to the hood was pretty much only hanging on by a thread and I never replaced it which got me looking around realoem.  In reference to this pic:



#3 has two part numbers.
03   Earth strap for engine hood      
03   Earth strap for rocker cover

I know I'm missing the first one, but does anyone know where the valve cover grounding strap is supposed to go?  I don't think my car ever had one, and I haven't noticed one on any other M42.

#7 is also listed as an "Earth strap".  Where is this one?

In the Engine Electrical section, there is the following:



#16 Grounding cable.  Where is this located?

Thanks for the help!

[monty23psk]

I have to look at mine, but dont believe there is a grounding strap on the m42 in the rear of the engine. Part #16 might be the alternator one if under the alternator section, there is none for a ground cable.

[DesktopDave]

If I had to guess, I'd you have some very large electrical load on at idle.  IIRC there was a TSB about the starter not disengaging.  The key lock was the usual culprit, the switch would spring back normally but still keep the ignition in position III.  If the starter remains energized you'd see a large voltage drop off idle.  Not sure how I'd go about testing that...maybe push-start the car in 2nd gear and see if the low voltage problem still exists?  You could troubleshoot by isolating both the starter & key switch that way.

I'd figure that the rest of the car's electrical system is OK; it charges properly at higher RPM and the light stays out after you rev it.  Maybe it's kicking the starter armature back out at high RPM?

I'm not sure how likely a condition this is (in a 20-year-old car  anything goes), but it's worth consideration if BMW published it.

My alternator belt is nice and tight, perhaps 1/2" deflection on the longest span.  I noted that it's nearly at the limit of the alt bracket even brand new, that's not right.  Seems to work for now.

[monty23psk]

Wow, Dave just said something about the belt that hit me. On my old alternator which I have in a box as a backup, the hole at the top had turn into an oval from the vibration and probably not being tight enough. This cause the belt to squeal once in awhile. Also, sometimes the light would come on, when going over puddles. Also, if you rebuilt the alternator, the casing stays the same, might want to double check that. It bugged me for awhile unit I got another used m42 alternator.