by George Lyle
The reason that I solder all connections is that it is impossible to produce high quality crimp connections with commonly available tools and hardware. If you compare the factory-made crimp connections with the common "crimping pliers" connections you will see that the factory connections are much better tighter. When disassembled, the wire inside factory connections is usually bright and shiny, just as it went together 30 years ago! Insulated crimp-on connectors are really horrible, and shouldn't be used at all. I won't even mention Scotchlok connectors except to say that they should be properly labeled as "corrosion promotion devices."
I use the commonly available _uninsulated_ crimp-on connectors, but then flow solder into the crimped region to complete the job. I then use two pieces of heat shrink tubing (one over the crimp, one over the entire connector) to strenthen the joint and prevent flexing.
Use a good soldering gun, the little soldering pencils are good for electronics work, but you need more heat for automotive wiring. Weller makes a good dual-heat 140/100W gun. For shrinking the tubing, I use an electric paint-stripping heat gun, which is quite a bit cheaper than the electonic heat guns. Kester makes good solder (and there is quite a bit of difference in ease of application here!)
Most bad solder connections are due to too little heat. To properly solder a connection, you should apply heat to the connector until the _connector_ is hot enough to melt solder. You may apply a bit of solder to the tip to aid heat transfer to the connector, but you should never apply solder to the tip while it is touching the connector. Melt solder into the crimped joint until you see solder flow out the opposite side. Hold the joint stationary while the solder solidifies. The result is a nice shiny solder fillet.
If solder refuses to flow, the joint is probably contaminated with oil or corrosion. I've found that spray carb cleaner is effective in getting rid of oil, but make sure that you air out the wire before soldering.
Don't work too hard to save a contaminated connecion. Simply cut it back a few inches and splice in a new section of wire of the same type. I have a couple of old wiring harnesses that I rob wire from so as to keep color codes intact.