2001 4.4i 14 volts under hood, 11.7 at battery? - Fixed
 

Okay, here goes. Picked up an '01 X5 4.4i a while back from a friend.

Last week battery light starts blinking. Changed the alternator. Still blinking.

The alternator output seems fine, I've got 14 volts at the + jumper post under the hood.

But I've only got 11.7 volts at the battery + terminal.

I added an additional temporary engine ground strap to the body, no change.

Anyone run into anything similar?

(Update: after a few wrong turns the answer was a loose/arcing batt + jumper stud at firewall)

Bad battery possibly? I've never tested that large a voltage difference at the front than in the back...

when you tested the battery in the back did you go + post to chassis ground or + post to - post on battery?

+ post to the -cable attaching stud on the side of the well.

Right after key-off I get 12.1 volts at the battery. A little low, but not terrible. I've tried two different batteries (one 3 months old, one four years old) and get the same behavior in both cases.

It cranks easily, after it starts it will go up a bit (12.7) and then start drifting back down to somewhere around 12.1 volts, the 11.7 seems to be about a worst case but it seems to me this is 1-1.5V down from where it should be.

Still have a solid and consistent 14 volts at the jump-start stud under the hood.

Bad connection somewhere? Or some big load somewhere shorted out? The HVAC fan speed control works normally.

I'd probably check for corrosion anywhere the main + cable is outside the cabin... or a loose connection at the back of the alternator?

Where did the alternator come from? Not likely to be a voltage regulator with a consistent 14+ at the front terminal... but who knows. I went through 2 replacements from Oreilly's last month before one that worked right.

I would take the battery in and get it tested at an auto parts store. Most test them for free. After that, look for bad/corroded connections on the battery cables and the ground lugs (engine and battery ends).

The alternators on a car have a bolt circle, but it has no index, so it is possible to install it rotated the wrong way and then the output cable is very close to the frame and will occasionally touch and cause electrical glitches. Still embarrased to say, but a couple of years ago, after doing some heavy engine work, I reinstalled the alternator rotated one hole off. Took me a while to figure out the source of my glitches, but easy to fix. Didn't even loose any coolant, just remove all the bolts and slowly rotated to the right orientation. So, did you mark it or otherwise confirm the orientation when you swapped yours out?

I use deoxit products to clean/protect electrical connections. Very popular in the hifi world, and very helpful for our digital cars... P100 is 100% cleaner. S5 has 5% cleaner and some protectant. The local auto parts stores sell "electrical contact cleaner" but that is solvent based (good at removing grease and grime) but doesn't really attack oxidation the way deoxit does.

I'd remove both cable connectors (at battery), clean them and re-attach them.
Please report back...

The latest:

Checked battery cable continuity/resistance from engine compartment to trunk, no problems.

Put in a fully-charged, 12.8V battery (we've got an C5 S6 Audi and an E39 540i, so got plenty of H8/Group 49 around here...) 12.78 volts at posts with key off.

Start engine, voltage drops to 12.5, settles for a moment, then KEEPS DROPPING down to 12.1 at which point the light starts blinking.

Okay, shut off, disconnect battery. Figure nothing that's fused under 20-30A could create enough load to drop the voltage that far, so start looking for loads to disconnect.

Unplug aux radiator fan (known dead, have spare sitting here, was hoping to do it when I do the valley pan, love them M62s...) and the HVAC final-stage resistor (just 'cause it usually seems to play in things electrical of that BMW vintage) and pull the big 50A BLOWER fuse 64.

Hook up battery, start car. 13.7-13.9 volts at the battery at idle. Now to go back and put the fuse back, then plug the FSR back in and see what happens. The fan can stay unplugged until it's changed if it's the problem.

Happy, happy, but wife who's been reading Merc 450/550GL ads might not be. Not like we need to spend the money after the last couple weeks...may just change the FSR on general principle. At least it's on the side of the HVAC housing, not halfway 'round the back, I think I've ended up dripping blood every time I changed one on the E39s.

Update - looks like it's the fan. FSR plugged in and AC on and still got 13.6+ at the battery.

Moral to the story: if your AC condenser fan dies, and you're not going to replace it right away, unplug it.

Glad you figured it out. Pretty amazing it can be drawing that much juice and not blow the fuse.