Hi,

Always try the easy/cheap things first. It may well be a dead battery, but the fact that you couldn't jump start means that there may be a bad earth between the battery negative post and the car body. Follow the lead from the negative post to where it bolts to the car. If it is either rusty or covered in a chalky white deposit then you probably have a bad earth. remove the bolt and clean the terminal and where it attaches with a stiff wire brush before replacing and re-tightening. Also clean the posts on the battery and the terminals that attach to them.

If after doing this you are able to jump start it then you will need to get the battery condition checked. If you can't start it then the next level of fault finding starts. Let us know how you get on. Good Luck

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bobw
1983 380SL
1991 BMW 735iSE