Battery, is it time? - The Unofficial BMW M5 Messageboard (m5board.com)

rdm190 · 4th October 2006, 17:56

I have noticed that with all the interior lights on at night, when the car settles into idle, that the lights dim slightly. When you give it a little gas they brighten right back up. Is this typical of the chrageing system on the M5, or is my battery on its way out?

I checked the battery out, appears to be the factory unit. Still has a green "eye". I could not find a date on the unit.

TIA guys,
Ryan

Need4Spd · 4th October 2006, 19:27

Could be the battery. If it's the original unit, it's getting time to change it. But the alternator could also be going. It should pump out enough current to keep the lights from dimming at idle.

M5inGA · 4th October 2006, 21:08

Sounds like the alternator needs to be checked. Make sure that your idle speed is where it is supposed to be. If it is too low then your alternator will not be able to generate enough of a charge. Your battery will usually show it's age when you try and start the car. It will not be as 'enthusuastic'.

greg · 4th October 2006, 21:42

I agree with M5inGA - I think it is more likely its an alternator / RPM issue - unless you're drawing a huge amount of current, such as with headlights and seat heaters still on. So have it checked or check it yourself.

BUT (and I speak from personal experience here) if your battery is 5 years or older you're living on borrowed time. And when it goes, it will most likely go in a hurry.

Philip · 4th October 2006, 21:55

Idle speed, the alternator, and/or voltage regulator.

MAH · 4th October 2006, 22:00

I would be willing to bet it is not the battery. In fact you could take the battery out of the car after it is started and it would not have any effect on the electrical system at all. I agree that the issue lies else where.

Mark

M5inGA · 4th October 2006, 22:14

Technically the car uses the battery to start and power things when it is not running. Anything while it is running would be using the alternator (or battery as backup when the alternator goes bad).

As for Philip's response, I am not sure on the M but a lot of cars have the voltage regulator built into the alternator. Older cars had it as a separate unit. So make sure before you go hunting for it.

rdm190 · 5th October 2006, 17:13

Thanks for all your advice guys.

My idle speed never varries from ~600 (from view the tach position)

How do I go about checking my alternator? This is not a severe problem, just a slight dimming when all the interior lights were on, but still noticeable.

If this going to be a alternator/voltage regulator problem, what am I looking at for a repair/replace? The alternator on this engine seems to be very low on the engine and more difficult to get to

Need4Spd · 5th October 2006, 18:50

Oddly enough, the battery still could be contributing to the problem. Old batteries collect conductive junk at the bottom (scaling off the plates) and eventually short out. This could be causing the excessive drain or a failed alternator.

As for idle RPM, it's set into the ECU, so if the engine is not able to maintain idle to specified RPM, you need to look into what mechanically is causing the feedback circuit to be unable to maintain. Not sure of what the idle control mechanism is in our cars, specifically, but there is often a small idle circuit valve that regulates idle. If it's stuck (not enough Techron!?), it could be failing to maintain idle speed. I don't recall what the exact figure is for "normal," however. I do know that aftermarket software places like PowerChips can raise it for you, but that would be a band-aid. What you need to do if the RPM is under spec is to find out why the idle circuit is failing to maintain.

As for checking the alternator, you will need test equipment that will place a load on it while checking its current output. Most people don't have that in their private garages, but any place that sells batteries will (like even Sears). If defective, it's a replacement job, not a repair.

rdm190 · 5th October 2006, 18:54

Quote:

Originally Posted by Need4Spd

Oddly enough, the battery still could be contributing to the problem. Old batteries collect conductive junk at the bottom (scaling off the plates) and eventually short out. This could be causing the excessive drain or a failed alternator.

As for idle RPM, it's set into the ECU, so if the engine is not able to maintain idle to specified RPM, you need to look into what mechanically is causing the feedback circuit to be unable to maintain. Not sure of what the idle control mechanism is in our cars, specifically, but there is often a small idle circuit valve that regulates idle. If it's stuck (not enough Techron!?), it could be failing to maintain idle speed. I don't recall what the exact figure is for "normal," however. I do know that aftermarket software places like PowerChips can raise it for you, but that would be a band-aid. What you need to do if the RPM is under spec is to find out why the idle circuit is failing to maintain.

As for checking the alternator, you will need test equipment that will place a load on it while checking its current output. Most people don't have that in their private garages, but any place that sells batteries will (like even Sears). If defective, it's a replacement job, not a repair.

Thanks for your input. Do you know what the standard idle is for our cars? I do not think this is my problem.

Also is there anyway to idenify how old the battery is? Mine is the BMW unit, but I am unsure of how old it acually is. Thanks