I have a 1990 300E and the battery keeps dying. I have replaced the alternator. I thought it was the central locking system because I could often hear a motor noise coming from the back seat area when I would get in the car and the battery was dead. However, I disconnected the vacuum system and the battery still dies about once every 2 weeks. Anyone have ideas?

Hi, Jim Rippy,
you need to find the draining circuit with a multimeter.
Disconnect negative cable from battery and messure voltage between plus and minus side of battery, should be around 12.6 volts or better.
Then connect one probe to the minus pole and one probe to the ground, i.e. some shiny metal. If a circuit drains to the ground, you should read that small current flow now in amps or milliamps on the multimeter.
Now have somebody pull a fuse. If no change in current flow, this circuit is not leaking. Fuse back in and next fuse pulled until the current flow drops close to 0. That would be the circuit that has a faulty ground connection somewhere and that you then need to investigate.
Good luck
Reinhard Kreutzer

Aren't the door locks vacuum? So they would not cause the problem.

The motor noise might have been your antenna, which has an always-on connection to the battery so it can retract when you turn the ignition off. Try disconnecting it.

You might also check if your trunk lid is closing enough to turn the light off. I removed the bulb and my problem went away, although I need to figure out why it is happening. Believe it is a worn latch.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

If you do the amp draw/pull fuse test above and see nothing, suspect the alarm system if you have one. Of course, there could be others system draining that are not part of a path that included the fuses pulled from the fuse box.

how old is the battery ?

What kind of a stereo do you have? Is it stock?

Often poorly installed after market stereos with external amps are wired always on. Even in the standby state the amps deplete a battery in days.

Haasman

__________________
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)

The battery is new but the car does have an after-market JBL 12 disk CD changer in it. I appreciate all the suggestions!

Now that we know that the battery is new, .....then perform RKreutzer advice. Let us know what you found out... I hope that your charging system is intact...A cd changer or new stereo will take a week to drain the battery down....You might have an open ground....