I'll recommend what I can since nobody else has commented. I'm not really familiar with Volvo so I'll keep it generic.
Get yourself a factory service manual and get acquainted with it. Do your next oil change. Then do a tune-up, you'll familiarize yourself with some basics that'll come in handy later on. Plus you'll get a taste of wrenching and see how you like it. If you can't already pop the hood and name everything that you see you should spend some time with the manual to learn what you're dealing with.
Look at the different models of the 940 that came out. What engine and transmission options were there? Would your car benefit from some upgrades off a donor car?
Find some forums. Chances are that somebody has done a write-up on how to modify your car. Maybe they dropped a V8 in it, maybe they threw a turbo on the stock motor. See what information is out there and see if you have the means to make something work.
Also important: Tools are expensive. If you decide to start modifying your car yourself you'll need tools, a place to store them, and a place to work on your car. People talk down on Harbor Freight tools but they're cheap and you can get the job done with them. I'd recommend starting with cheap tools and If you stick with the hobby start upgrading whatever breaks(ratchets) or gets lost(sockets) with better quality tools. Maybe you have a friend who is into cars and has tools, it's cheaper to buy that guy beer than to buy tools.
Hopefully that'll get you started. If you find a good online resource research the fuck out of it.
Our podcast has recently discussed some of the issues with techs currently in the field. The most opinionated episode is coming up soon!
https://www.buzzsprout.com/1182755
I'm going to share your question with a group of shop owners, and a group of techs if that's okay - we may do a podcast on the topic or post some responses here.
You could try Automotive Technology: A Systems Approach by James Halderman. One book covers 90% of the material. I know you can get some editions in pdf format, not sure about ebooks. That was my main info source durine my block training. And during your search, stay the hell away from anything by Thomas Birch. He's knowledgeable, but the text is painfully mind numbing.
Edit: found a link, the 5th edition is available in ebook format on amazon. https://www.amazon.ca/Automotive-Technology-Principles-Diagnosis-Service-ebook/dp/B00XIH45HG
Also this book.
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Are those cleaners and sealants being used by the automobile manufacturers? Then yes. The approach works well for other types of work also, things that move from idea and development through to manufacturing can make use of Advanced Product Quality Planning and the Production Part Approval Process. I understand the aerospace (aviation particularly) are moving toward this as well.
That sure is super strange. It comes up in my search right at the top. I'll have to look into why that is. This should be a direct link to it (hopefully), if you want to try that. https://play.google.com/store/apps/details?id=com.diagnosticapp.app&hl=en&gl=US
You can definitely fuck em up. My Milwaukee can put that shit on a 1400 ft/lbs, so you gotta be careful with the electric guns. Air guns can be calibrated for speed and impact strength, so there is less possibility, but they also spin much faster than electric guns so you can fuck em up that way.
My unofficial rule of thumb is to only let it impact for a split second after it gets tight. You'll know by the sound.
You can also get a Torque Limiter, and those are made so that you cannot over torque, as they are set to a specific value.
Personally, I like having two guides. I've found the Motorage guides ( https://smile.amazon.com/ASE-Certification-Test-Prep-Training/dp/1934855472/ref=sr_1_8?ie=UTF8&qid=1504457558&sr=8-8&keywords=ase+study+guide ) to be way more informative. I like Delmar cause they have way more ASE test questions.
I have A1, A2, A4 - A6 and my C1. I haven't failed one yet with the use of both of those guides.
Sometimes if I'm in study mode, I'll also use http://www.freeasestudyguides.com/ and poke around on my phone a bit with it.
The trick, in my opinion, is to give yourself about two weeks per test and study at least one hour a night. Do that, and you should be golden :)