Just replaced the rpm sensor on my 240.It was easier than expected.
1. Get some surgical gloves - makes clean-up easier.
2. spray some pb blaster on the and around the bolt[should be done the day before].
3. the bolt is 10mm. use a 12 point wrench to slowly turn it. A 6 point wrench will limit your ablitiy to make small turns.
4. once you get the bolt turning a little remove it with your fingers,much faster
5. installation is the reverse.
note i think a gear wrench may fit in that tight spot and be easier that a regular wrench
Oh,and you will be working from the top,engine compartment,passenger side. No need or room for sockets. Good luck.
Also if you are changing the temp sensor [for the computer] removing the intake manifold will make this go a lot easier.
Nelson