Out of all of these, I found the stethoscope one to be the best. I played with that idea long time ago, so even though its a nice way to relate the two, the initial design (#8) is not quite creative enough for us to be taken seriously.
We are not against using it, but definetly has to be incorporated with more creativity
Our maintenance practices are a matter of life or death for the motorist, and they need to know we are serious about it before we even touch their vehicle.
I am very pleased with the changes you made to get to #18. Please remember to take into consideration the children companies that I refer to in the general comments. Akronix showed an example of how the logo can be slightly modified to incorporate the various themes of the company's divisions. As much as I really like this one, I have not been able to see an easy transition to Fleet Doc Racing, Or Fleet Doc Marine, etc.
So although I am looking for one logo right now, in the near future I will be looking for a small modification of that logo to represent the other companies and would rather keep as much of the original in place as possible.