Solving wiring diagrams -- a technique

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In understanding Vanagon (or any other) wiring diagrams there is a not-so-little trick that will make it vastly easier to figure out what's really going on in a circuit. Before you can do this of course you have to be familiar with Bentley 97.3-97.6. so the diagrams make any sense at all.

The trick is this: First copy the page in question (an enlarged copy can be helpful because the wires are quite close together in the original pictures) so you won't wreck the original if you mess up somewhere. Get yourself an assortment of colored pencils/markers, at least five or six including brown and red. Then take a look at http://picasaweb.google.com/dbeierl/FiguringOutVanagonWiringDiagrams# for a working example of how it's done. You may want to keep it open to refer to or make a color print of it to use the first time or two you do this.

Start by locating the grounds. Since this is a physical schematic some of them may originate far away from where you're looking even if it would be easier to understand if they simply drew all of them straight down to line at the bottom line of the page, which is chassis ground. Every terminal labeled as 31 is a ground. Every ground you find, draw a brown line next to the wiring for its whole length. For switched grounds I recommend a dashed line instead of solid upstream of the switch.

Then look for the +12 supply wires and color them red. You may well want to use different colors for unswitched (terminal 30), switched by ignition (terminal 15), and switched by the load reduction relay (X-terminal on ignition switch operates it; power upstream is 30 at the relay, downstream (switched) is 87 like all relay outputs. Again, power that's switched downstream should use a dashed line downstream of the switch.

Now pick a wire and figure out what it does. Follow it off the page if necessary and label where it goes on your drawing. If you can't figure it out, pick another wire. As soon as you know what it does, label it as such then pick a color for it and draw solid or dashed line as appropriate. If a single wire does multiple things, as in the circuit from 53e at the motor up to the switch, down to the relay and thence to the motor 53 terminal, use multiple solid or dashed lines as appropriate (Note: the pink wire should be dotted for its entire length and there should also be a brown dotted line for the whole length of that circuit. Can you discover why? Answers on request. Hint: I missed one of the functions of the auto-park switch).

Keep on doing this for wire after wire until you understand them all (or are certain you understand enough of them, which may or may not be true). Each wire you figure out makes the rest easier, and the colored lines take away the visual anonymity of Joe Random Black-line-on-the-page. Now you can follow the circuit around and find out where a particular problem might (or must) lie as well as where it cannot.

This is the second time I figured out this particular wiper circuit. The first time I forgot this trick I learned thirty years ago, and it took me over an hour. This time I remembered, and it took about ten minutes after I found the colored pencils and photographed the page. And I was easily (but not in ten minutes) able to establish that the relay cannot possibly be correct as drawn (since at some point it would hook +12 directly to ground) and how it would actually have to be (which happens to coincide with the diagram molded into the side of the actual 19 relay, but I didn't know that at the time. I definitely did *not* figure that part out the other time.

Adds Roger: "Once you learn these diagrams, you can troubleshoot any circuit easily, or do a conversion. Many of the color codes for Vanagon are the same for Jetta , Golf etc."

Originally posted on the vanagon mailing list by David Beierl with additions from response by Robert Keezer