Maybe I missed it somewhere in the previous posts, but
have you really verified that it's the battery and not another gradually failing Bosch starter? I've not worked on a 190, but on the older German cars (especially air-cooleds) these often develop a "stickiness" inside the solenoid so that sometimes it engages (usually when cold) and sometimes it won't (usually when hot). I've fixed a lot of them for myself and for friends by disassembling everything, polishing the big contacts inside the solenoid, and then relubing it all carefully with high-temperature stuff, but if you've got to pay a real professional, it's probably better nowadays to just have a rebuilt put in.
Often a starter with these sorts of problems will kick in if someone gives it a whack with a wooden or rubber mallet while someone else holds the key in start position, but they often also deceive you buy working when jumper cables are attached, for some reason (perhaps the higher voltage from a running alternator, but I've seen it happen with an extra battery, too).
Last edited by Fimum Fit; 11-04-2002 at 01:38 PM.