believe it or not
 

Story of a w140 m104.
Last week I drove my car to my daughters home.After visiting I put key in to start nothing but a click.Tw0 weeks prior to that the car did the same thing when one battery cell was dry.So again I assume water.Nope battery was fine.
These cars for some reason won't jump start.So I call my son and tell him to bring my 83 300SD battery.Put battery in and she fires.
I get home take the weak battery and put it in the SD ,she fires right up.

What happened? Why w140 won't jump?

Check + and - cables for internal corrosion and solid connections.

cables and connections all look new.I have heard cars with batteries in the trunk won't jump.It was later they added a place in engine area for a jump.Like to know if any w140 has been jump started.Also can you jump a shorted battery.

I guess I will maje a closer positive cable under the hood for jump starting.Maybe the distance of rear cable consumes the amps needed for starting.

I think if you measure the voltage at the starter and compare to your other car you can isolate whether it's the cable. If the starters get the same voltage, then the difference would be at the starter.

I have jump started my grandmothers 98 S500

thank you.I think jumper car must = or be more than the jumpee.

I had the AAA guy not be able to jump start my car. He then noticed that the battery that was in my car was not the correct size battery (w124). He had one in his truck, installed it, and whammo! Started right up. He was dumbfounded.

I found out what was wrong.It was my key.With a chain on it it weighed to much.With weight removed from keys,starts every time

I am doubting thats the issue here....you'd be supporting the weight when turning the key to crank anyways...also, how heavy was your keychain? :eek:

Weight on the chain is not good, but I doubt its the problem. Could be a faulty ignition tumbler/switch if you can really replicate the issue.

It had a bunch of keys and a Big mercedes locket. Now I use one key no problem now for 3 months

You should not be jump starting a Mercedes.

Either remove the battery, charge and reinstall, or use a portable battery to act as a charged battery until the alternator gets your going then disconnect.

Plenty of factory bulletins regarding this.

I had two vehicles jumping mine with combined battery power.No problems,no surges,nothing.