Help. Electrical Fault At Large

[tiggins]

Hi all you merc fans. the good lady has a 1991, 500sl. lovely little toy for her but since mid summer it has started to eat batteries. bu eat batteries i mean eat batteries. it had a brand new heavy dutie battery fitted, about 4 weeks later the battery was flat and would not re-charge. so suspecting a dodgy battery i took it back to the shop and they replaced it. ever since then every 4 weeks or so the battery goes flat. luckily i know the lad in the spares shop so he keeps ex-changing them for me so that aint an issue but it would be good to be able to rely on the car starting when i want it to and not when the battery wants it too. can anyone help. has anyone experienced anything like this. please help me its-dooin-mi-ed-in.

[tecqboy]

Well, I can't explain why the batteries fail to recharge, but I do know of several "issues" which will cause the battery to drain down in a couple of days although the ignition switch is off and the key removed.

1. The trunk lamp. The switch for the trunk lamp fails and the lamp stays on all the time. Of course you can't see it "on" in a closed trunk.

2. A leak in the vacuum locking system can cause the vacuum pump to run constantly ( or intermittantly). It's a quiet pump in the trunk and you usually can't hear it run.

3. The power antenna motor. The mast cord breaks or the limit switch fails and the motor runs continually. You should hear this one.

4. The timer "relay" for the interior lamps. The lamps stay on after the doors are closed and the key removed ( rather than timing out ). This should be obvious.

5. Others have reported that a faulty vehicle alarm module will drain the battery, but I haven't experienced that personally.

6. Bad voltage regulator. It's part of the alternator. A shorted diode will drain the battery and prevent proper charging.

In my car, the positive battery cable was badly corroded up and inside of the red plastic insulation at the battery end. It looked OK from the outside, but inside it was shot. It didn't cause the battery to fail, but it's worth a look.

Good Luck.

[danwashuk]

Make sure your charging system is working properly first. It's the easiest thing to do anyway. Just put a DC voltmeter on the battery, with the engine running, and the headlights on. At this point the battery should be charging and you should read 13.8 to 14.2 volts. If you read 13 volts or less your battery is not charging and if you read more that 14.2 volts your battery is over charging. Overcharging will kill your battery. If you don't have a voltmeter then find a place that will check it for you. My local Checker Auto store does this routinely for free. As a general rule an alternator will put out somewhere between 18 and 20 volts, which is then regulated down to the proper charging voltage for the battery................Dan