Dimming Lights

I’ve got a '99 Mercedes E320 and the lights have been getting brighter and dimmer depending on the RPM of the engine. For example, while in park and idle the interior lights will seem a little dim, but then if you rev the engine or start driving it will get a bit brighter.

This started suddenly last week, but I’ve never had any problems starting it. The water level in the battery was a little low, but I added distilled water and it’s not improved.

If that battery is over 3 years old, replace it.

If that fixes it, fine. If not, then measure the charging voltage at idle. Should be 14V - 14.5V

I made a mistake. If that battery’s over 5 years old, replace it.

Okay, I think I need to try changing the battery. It’s older than 5 years.

You just hook it up and jump it, right?

A fresh new battery should not need a jump. Most places that sell batteries replace them for free.

Along with checking the battery condition also check the condition of the battery connections. You should clean them to make sure they aren’t the cause of the trouble due to corrosion. Also check for a loose fan belt and loose connections on the alternator. If you replace the battery make sure you don’t reverse the connections when installing the new one as some folks have done.

Have the battery and charging system tested before changing anything.

The battery is original, so I’m going to get a new one and we’ll see if that works.

The battery is original, so I’m going to get a new one and we’ll see if that works.

A '99 still on original battery?

Ehh, yeah. It still starts. . . I’m not a particularly well informed car owner. I figured that the battery was recharging and when it was time for a new battery I wouldn’t be able to start it.


The battery is under the rear seat. I can envision a scenario where a mechanic might be too lazy to remove the seat to check the battery . . . or maybe he doesn’t know where it is located.

So, even if it’s not the cause of the problem is there any question that the battery needs replacement?

@threephi No doubt at all. Batteries are expected to last 4-6 years. You are way overdue.

Make sure the shop installs the correct group 49 battery, not whatever happens to “fit” . . . and make sure the battery gets vented. There is a vent hose that goes from the battery to a small hole in the floorboards. That is to vent any gassing to the outside.

Make sure you have your radio code, if you still have the original battery.

After replacing the battery, you need to resynchronize a few things

Power seats (all the forward and all the way back)
Sunroof (all the way open and all the way closed)
Windows (press and hold switch all the way up and all the way down)

If one of the window regulators fails, DON’T BLAME ME. It was bound to happen. It is a common failure item on your car, as the plastic cable guides are a weak point. I wouldn’t be surprised if you’ve already had the rear regulator(s) replaced at some point in time.

And good luck!

I’ve got the radio codes in the documentation from when I bought the car.

Are the window regulators more likely to fail because of the battery replacement?


It’s not the battery replacement that causes them to fail. The design is the problem. I’m just speaking from personal experience.

If it’s any consolation, this “problem” is not unique to Mercedes cars.

New battery is in. Took it to a new shop and they really seemed more on their game and organized than the last place.

I popped the radio code in and did all the adjustments mentioned and everything seems fine. The problem with the lights got better with the new battery but it still does it.

@threephi glad to hear about the new battery and the resynchronization.

The next step is checking the charging voltage at idle. Could just be a bad voltage regulator.
From my personal experience as a former Benz mechanic, faulty alternators were very often fixed by replacing the regulator.

But that should be diagnosed correctly.

@db4690 Yep! Voltage regulator.


Ummmmm…are there other things that should have been addressed years ago that haven’t? Have you read the recommended maintenance schedule that came with your owner’s manual?

No disrespect meant, I’m just wondering…

@threephi Congratulations!

And I’m glad to hear that you only needed a reasonably priced part.