I've purchased (actual) shop manuals for my cars a couple times. The first time was for a mustang. They are better than the chilton style manuals but barely. I used to joke that the mustang manual was the most detailed choose your own adventure guide I'd seen. Because that is what they are for, they are a guide to diagnosing and replacing whatever part/module happens to be malfunctioning. Which is why the first page was something like "Customer is complaining of" followed by a long list of generic things, and the page to turn to to start diagnosing the problem. The sections on interpreting/diagnosing MIL codes are really where they shine, but the sections on repairing things are frequently "use special service tool $TOOLNUMBER" to remove part A from part B where as these days 10 mins on youtube frequently will have a shade tree mechanic way to get around the problem without spending $100 on some tool you will only use once.
So, you shouldn't imagine that the shop manuals detail the protocol between differing components or whatever. While they frequently will have complete wiring diagrams (with pinouts) between modules, I have yet to see a schematics for any of the modules, even when disassembling them shows that they are little more than a few passive components and a relay, motor, etc. That is where the real savings/knowledge come in anyway, its rebuilding the $300 dealer only door lock with a $.50 motor from the electronics supply house that has brushes that won't wear out in 5 years. Or swaping a good film cap for an electrolytic in some part that costs hundreds of dollars.
Basically generic problem solving skills make using those manuals needless. I have them for my late 90's car I keep around but I haven't opened them in probably 10 years. I don't own any manuals for the more recent ones. I drive toyotas now and they simply don't break regularly enough to need purchasing manuals. I can usually quickly diagnose if I'm going to be able to repair the problem myself in a couple hours at which point I proceed to do it, or just take it to the dealer/wherever.
I had obtained shop manuals for every car I ever owned. Almost always I only used them to find out how to disassemble something. Somewhat interesting observation that before model year 1995 or something like that the shop manuals generally included detailed mechanical drawings of various custom tools and jigs (detailed enough that you could conceivably manufacture the thing with dremel and lathe), since then you only get order numbers.
Probably earlier that that. I purchased the mustang set in the early 90's for a late 80's mustang. It definitely had a lot of special service tools usage.
(fox body, had no idea when I sold it for $500 in the late 90's they would be considered so cool today).
So, you shouldn't imagine that the shop manuals detail the protocol between differing components or whatever. While they frequently will have complete wiring diagrams (with pinouts) between modules, I have yet to see a schematics for any of the modules, even when disassembling them shows that they are little more than a few passive components and a relay, motor, etc. That is where the real savings/knowledge come in anyway, its rebuilding the $300 dealer only door lock with a $.50 motor from the electronics supply house that has brushes that won't wear out in 5 years. Or swaping a good film cap for an electrolytic in some part that costs hundreds of dollars.
Basically generic problem solving skills make using those manuals needless. I have them for my late 90's car I keep around but I haven't opened them in probably 10 years. I don't own any manuals for the more recent ones. I drive toyotas now and they simply don't break regularly enough to need purchasing manuals. I can usually quickly diagnose if I'm going to be able to repair the problem myself in a couple hours at which point I proceed to do it, or just take it to the dealer/wherever.