There's a lot to be said for helping less-than-able bodied people into the professional world through specialised education and training. You don't have to have vision to be a telephone operator, you don't need legs to be a wicked computer programmer, and even if you are paralyzed from the neck down, you can be an amazing film critic.
However, when someone suggested to me that they were involved in a project to open a school which was aiming to train people who were deaf to become automotive mechanics, I had my doubts. Can someone who is unable to hear anything work on a car? Sure. But probably not to the same level as a mechanic with fully-functioning hearing can.
Hobby mechanics rely heavily on hearing when diagnosing problems in the engine. A running car is, in fact, a symphony of sounds playing together. Professionals have access to more equipment, but hearing still needs to be a weapon in the arsenal.
Diagnosing problems by sound.
A thumping sound that subsides over time can be a flat-spotted tyre. An intermittent hissing sound from under the bonnet can be a leaking exhaust manifold. A constant hissing sound can be a leaking intake manifold, a problem with the air filter, or a leaking / malfunctioning vacuum hose, or the perfectly normal sound from a turbocharger. A ticking sound from the rear of the car can be a failing U-joint. A whining sound from the wheels when braking can be a brake disc failure. A thumping / thunking sound from your wheels as you are turning through a corner can be a failing CV joint or driveshaft. A whining sound can be a slipping auxiliary or ventilator belt. A rapid clicking sound from under the bonnet can mean a problem with the valves, tappets or cams in the engine. A rumbling sound could be a problem with the engine mounts, a gearbox problem, or a sign of something more serious amiss...
The list goes on, and a good mechanic can take a car for a quick spin around the block, and diagnose a long series of problems just by the sound.
The other issue is that people will frequently bring in cars with complaints along the lines of 'the car makes a funny noise when I...', or 'lately, the car is making more of sound X when I drive normally'. The problem is that people don't know the difference between a 'clicking' sound, a 'knocking' sound or a 'tapping' sound, while to a mechanic, there is a massive difference. If the mechanic is unable to replicate the problem (due to not being able to hear the sound), the fact that there is 'a sound coming from the engine' doesn't help diagnosis any.
Working around sound-diagnosing problems
Many of the problems identified by sound are also present as vibrations: A vibration felt through the steering wheel could be a bearing or hydraulic problem. A vibration felt throughout the whole car could be a wheel or suspension problem, etc. But many problems are simply 'drowned out' with the regular sounds and movements of an automobile, making diagnosis extremely difficult.
With some other problems, the problem just might be more time consuming: If a car refuses to start, a mechanic gets in the car, and tries to start it. If it fires briefly, you know you might have a fuel delivery problem or ECU problem on your hands. If it does nothing at all, you know it might be an electrical power delivery failure, or some sort of safety device. All of this can be done without any sort of hearing being involved, but there are some things a good mechanic will listen for.
A fuel pump, for example, normally difficult to access, it doesn't heat up, and it doesn't vibrate. The quickest way to find out if it is working is to listen for it. If it makes a sound, it works. If it doesn't, it is malfunctioning. A deaf mechanic could easily waste an afternoon trying to find a problem that a mechanic with full hearing would have picked up within seconds.
The problem, then, is in the parts of the engine that aren't visible, which means 'most of the internals'.
Turning it into team-work
Of course, if an able-hearing mechanic were to do the diagnosis, a deaf engineer could do the work to rectify the problem – the fuel pump problem described above, for example, would be a time-consuming job, but it is pretty straightforward to replace the pump, and no hearing is required.
Some jobs, like a full engine rebuild, are a nerve-wracking activity, especially in the first couple of seconds after re-starting the engine for the first time. Thing is, if something is amiss, spotting the problem is often sound-based before any vibrations or visual clues come into it. Furthermore, if you were to fail to react to a sound in this situation, you could easily do permanent damage to an engine.
For engine work, this means that a deaf person can at best be an assistant to an able-hearing mechanic. They would no doubt be a very useful assistant, but I believe it would be impossible to fully be able to diagnose, maintain, and service all aspects of a car.
One issue that I believe is too easy to ignore in this question as well, is safety. Cars and garages are inherently dangerous places: Explosive fluids, heavy machinery, sharp edges, quickly-rotating parts, and high-temperature items are all around. A lot of accidents are avoided narrowly by one mechanic spotting something about to go wrong, and shouting.
If you are working underneath a car on a ramp, and the ramp starts creaking, you'd get the hell out of there. If you are deaf, you'd never get the warning signal in time.
By working to a rigorous health-and-safety regime, a lot of these problems can be avoided, but many mechanics would be hesitant to having to live with losing the extra layer of safety offered by a 'WATCH OUT', when necessary.
What deaf people can do
I think there are many tasks within automotive repairs where you don't need a sense of hearing. Body work, paint work, suspension set-up, interior re-trimming, cooling systems, heating systems, air conditioning, tyres, wheels, brake systems and detailing are all tasks that don't require hearing – as long as adequate measures are in place so safety isn't jeopardised.