Hello. My Mercedes 300SL 107 1989 (last of 107) starts on key no problem. I leave it ticking over and it gradually dies. I then turn key again and the engine will not turn over. I wait approx 30 minutes, then turn key and engine starts and then the same thing happens again. No fuses blown , voltage on the meter when the engine stops is 11.57volts. When the engine starts again the voltage is showing 12.4. Have changed the battery. On the dash the battery light does not come on or go off and the clock has stopped working.

Any ideas please
Dave

It sounds like an electrical problem between the battery and the ignition or starter relay or computer if your model has one. I would start by checking battery terminal connections and where the ground wire attaches to the body/chassis. Also check all related wiring to make sure accessible terminals are tight and that no wires are abraded and shorting to ground.

PS: I own a much earlier 107, so no detailed knowledge of the 300SL

Hello. Thanks for your reply. Engine starting is no problem; its the engine turning over. Its like the battery is flat, and re-energises itself when left. The voltage meter shows that the battery voltage has dropped.

Dave

It could be that the battery is failing. You need to get it load tested. Low cost testers are availble and at least in NA, most parts stores will do the test at no cost.

If the battery maintains acceptable voltage under load, then I would look at the other issues, starting with the battery ground connection. Bad connections will result in low voltages when under load, but not with no load.

Thanks for your quick reply. Have checked the battery and the battery holds a charge, and doesn't lose any voltage. I have a feeling that it may be alternator trouble ie. car starts and ticks over using only the battery voltage, not the alternator, so when the battery voltage is too low for the ignition system, it cuts out. I have removed the 3 cables coming from the alternator and there is no voltage ie. put red lead of voltage meter and black lead on earth with the leads disconnected from the battery, revved the engine and no voltage showing on the meter.

Thanks again
Dave

In order to test a battery, you have to put a load on it. Only real way is to use a tester designed to do that. I have an old style one like this, but there are many available on Amazon and elsewhere.

Measuring voltage doesn't tell you much. It may not be a battery problem, but a simple test is required to eliminate it as the issue. Even without a proper tester you can use the lights and other loads on car. These Youtube vdeos are quite good.

I would be careful running an alternator with the wiring disconnected. You can damage it that way.

I would like to thank all members who replied to my post about my problems with my Mercedes.

I have fitted a new alternator and it has solved the problem which was the car was running on battery power only and when that ran out it stopped, hence why it wouldn't turn over when this happened.

Thanks again, Dave