When you need to write about vendor-supplied equipment, you may find that the source material provided by your company is outdated or incomplete. In that case, you will need to dig further to get what you need an accurate procedure. Twenty years ago that meant searching through microfilm, microfiche, corporate libraries, and writing to or calling manufacturers and then waiting for them to mail what you needed.
The Internet eliminates the need for most of those headaches. Every once in awhile a situation will arise where no information is available online, usually because the company went out of business years ago, but there is almost always a way to find what you need.
The obvious place to start is the vendor's web site. Look for a link to something like a "Literature and Document Library" or "Manuals and Drawings" (it may be under "Support" or "Downloads"). You may need to look up the specific product and look for a link on that page.
Some companies will have an extensive amount of literature on line. For a company that is a major supplier to an industry, or several industries, it is in their best interest to make this information readily available to their clients to cut down on requests for the manuals, brochures, data sheets, catalogs, and drawings. John Deere, for instance, has online versions of all their manuals, including complete sets of engineering drawings. I have occasionally found information on the manufacturer's international sites that is not available on the American site.
Other companies have a great deal of information, but you need to register on the web site to prove you are working for a client before you can get to it. Permission will be given immediately or in a couple days, depending on the level of verification required.
If you cannot find anything on the manufacturer's site, you can search for the equipment in a using Google or another search engine. You can sometimes find manuals, drawings, and marketing materials on distributors' sites. Trade magazines, industry forums, and even other clients may have information on their sites.
As a last resort, if the piece of equipment is used for a very specific purpose, you can search for similar equipment by other manufacturers. If you collect enough data on competitor's equipment you can deduce that your equipment will have common characteristics that will require the same maintenance. Then, by examining the equipment itself and consulting subject matter experts, you can produce a procedure that can be further reviewed and verified in the field.