alank2

2008 xB

Battery was replaced about 9 months ago.

My son was driving it and the battery light came on. I put a cigarette lighter voltmeter in the car and driving it home, it seems to just be running on the battery voltage only mostly low 12V range, sometimes it would dip into the 11V range.

I bought a new alternator from rock auto and installed it this afternoon - all was well once I learned that it is easier to slide the belt off the water pump and then take it off the alternator.

With that done all seems well enough, but the voltage in the cigarette lighter fluctuates more than I would have expected. It bounced around no matter what (lights/ac/cabin fan on or off) and will be 12.4, 13.4, 13.0, 12.7, etc. Doesn't seem to matter if the engine rpm is up while driving or sitting idle.

If I measure the voltage at the battery, it fluctuates between 12.8 and 12.9V mostly with an occasional burst to 13.0 or 13.1V. I measured the voltage difference between the alternator body and the battery negative post and it was 0.050V or so. Resistance measurement when power off between alternator bdy and battery negative post was about 0.7 ohms. I put a clamp meter on the big lead coming off the alternator and it seems pretty consistent, about 10A just running/idling, and about 40A with the headlights on and AC on max blower. I measured the voltage on the alternator and it seems better than the cigarette lighter, but still fluctuating between 12.7-13.1V.

My 15 xB had its alternator replaced under warranty so it has an OEM one and the cigarrette lighter voltmeter stays around 13.7-14 either way if I turn the lights/ac/cabin fan on or off.

Is 12.8-12.9V at the battery enough? It seems like there is a problem here though replacing the alternator did stop the battery light. You can head the voltage fluctuations in the fan speed variation sometimes as well.

Any ideas?

RichBinAZ

It should be running close to 14.0V. I wonder if you have a loose or corroded ground somewhere

Here is a simple test

on my '09 XB

alank2

More testing - probably the most notable thing is the voltage drop on the positive side. If I measure the voltage difference between the positive battery terminal and the B connector bolt on the alternator, it is 600-700 mV which seems like too much. If I use a meter in current mode in the 10A range and make a connection with the leads between B and positive battery terminal, it tightens up the voltage swing and hangs more around 13.5V or so. (the current over the meter was anywhere from 2A to 4A, but it did seem to stabilize the voltage regulation. I've looked at the charging schematic for this it only goes through two fusable links ALT and FL MAIN. Could my fusable link block be at fault? It looks like a PITA to change.

Greg S

iTrader: (1)

You can't just check the positive lines, need to check the negatives as well. It's the other half of the circuit. Check the main engine ground under the air box and the battery ground. It sounds like a loose, or corroded ground somewhere.

alank2

Thanks Greg; I did check the negative lines as well - alternator body to the negative battery ground is a very consistent 50mV. I appreciate the help!

alank2

I took it to the dealership and they checked it out. They said that it is fine and that while they saw it drop into the 12's and even once into the 11's that the battery light doesn't come on until 10.1V and so not to worry about it. They think it is okay. I did put a better meter on the cigarette lighter to measure it and it was a more consistent 13.5V while running. It would adjust itself to the load and come back to that, so I am going to assume it is good enough. Maybe my cigarette lighter voltmeter is poorly designed and just does an instantaneous type voltage instead of an averaging...

RichBinAZ

It may be that you are getting a poor connection between a plug and a socket
Here's an example of where I found a poor connection on my 1100G

poor connection on my 1100G

.
It's a bike, but the electrical connectors are the same useless POS design. Cheap but...

alank2

RichBinAZ - I wondered about the connector that plugs on the alternator because it doesn't feel like it has much tension, so maybe I should look at it a bit more to see.

I do have one question though - I keep seeing people mention dielectric grease, but isn't it non-conductive? Is its benefit that it keeps more oxidation from forming essentially?

RichBinAZ

After a google search, Motortrend says Dielectric grease is an insulator.
In the video I used axle grease in an effort to stop corrosion and after tightening up the sockets, perhaps it helped the spades slide in a bit easier.
Just be careful not to over squeeze them.

Perhaps the logic goes that you don't want stray current going from one connector to another via the grease.
Another google search reveals that there are greases made for conducting electricity. They have carbon in them.
Another rabbit hole says petroleum jelly is just fine too...
It is possible to overthink the whole thing too...

alank2

I was wondering that too.

Agreed. The new alternator is in and seems to be working, so while I would have lied it maybe 0.5V higher, as long as it replenishes the loss from starting and runs everything while started, I suppose I can't ask for more than that.

Thanks!!!