Technical information is one of the most important tools for diagnostic technician. I would not be far off even if I say it is THE most important tool now. 50 years ago wiring diagram for the whole car looked like this:
This one is for 1965 Mustang
One of the last wiring diagrams that I’ve used in printed form looked like this:
And that was 10 years ago. That book is for a humble and relatively simple small delivery van Nissan Kubistar
For more complex vehicles wiring diagrams books comes in volumes of two, three or four books of this size.
If I would still have to use books for wiring diagrams whole basement of my house would not be enough to store them. Actually its more like my whole house and garage wouldn’t be enough just to store wiring diagram books. And beyond that I’d still would need books for DTC charts, parameter values, system operation descriptions and so on.
In a beginning of my career I was able to rewire the whole car like old Porsche 924 without wiring diagram at all. Took me two or three days to do that while back then I only had a fraction of my current knowledge. Probably I still can do the same job for modern car but it would take me months, not days.
There is no so much need to use manual when you swap engine or pull the dash out. It helps to save time and provides with useful directions when you doing the job first time, but if manual is not available its not the big deal. Things change when it comes to electrical systems. Now without service manuals and wiring diagrams you are either slowing down big, BIG time or get stuck altogether. Practically, shop manual for this kind of work is as essential tool as diagnostic scan tool, DMM or wrenches and sockets.
Fortunately, access to the technical info is much more convenient now. I don’t even have to go to the book store or library, most of the required technical information could be found on internet. Services like AllData, Mitchell and Identifix are out there, OEM technical information resources are also available. Very few of them come for free (which is sucks), but most of them priced reasonably and definitely worth to pay for. One hour of access to most OEM tech info websites usually is around $5 -10 and it saves literally hours of significantly more expensive diagnostic time. Services like Identifix require monthly subscription but they also extremely helpful, in some ways even better than manufacturer websites in some others – not so good.
By the way, situation on European market in terms of access to technical information was much worse then here while IMHO cars out there are more complex, technologically advanced and less reliable.
The reason why I charge technical information access fees to customers – it actually saves their money. If I would have to keep active subscription to each and every OEM and aftermarket tech info service than I would have to charge way more for labor due to much higher overheads. Specialization on a single make of cars is another way around it but since we live in rather rural community with relatively small population it is not a viable business strategy when you already specialized on diagnostics and electrical repairs.