I thought I read at some point that there is an exemption possible for mechanics, but I could be misremembering.
It is very normal in your industry for you to get paid by "billable hours". If you ran your own shop and were billing customers directly, you'd be getting paid the same way. If the guidelines say that a specific repair should take 5 hours, then you can only bill the customer for 5 hours of labor, even if it took you 8 hours. On the other hand, if it took you 3 hours, you still get to bill for 5. So the faster you are able to work, the more money you make...provided you have enough work to keep you busy most of the time you are in the shop.
If you owned your own shop, you'd technically get to keep the full amount you billed the customer, plus the markup on the parts. HOWEVER, you'd also have to pay the overhead, which is a lot more then you think....rent, utilities, tools, INSURANCE, service manuals, etc. Plus you'd be doing EVERYTHING yourself until you could afford to hire someone to help out...which is more overhead. This is why your employer pays you less then they bill out.