Hello, I have an 1980 w126 500sel that I having trouble to get started. At first I thought it was the battery because when we hook it up to jump it, it takes a while but finally will turn over and start (after turning the key numerous times with it sometimes turning over and clicking but sometimes not), but as soon as you stop the engine and try to turn the key, it will click once and not turn over. Tested the battery it is at right under 12V. Any other suggestions on what I can check? Much appreciated....