Regardless of the type of crimp terminal, they all share one thing in common: the need to be crimped properly. Though a proper crimp may be relatively simple to obtain in the controlled environment of a well equipped vehicle manufacturer, that objective often proves to be elusive to the shade tree mechanic trying either to repair an OEM electrical circuit or to install an after market electrical accessory. In frustration, the tendency is to avoid crimp terminals, and to return to the familiar land of solder. Sometimes, we strike a compromise that combines the worst features of both: we attempt to reinforce a poor crimp by adding solder.
Most crimp terminals are designed to be crimped, not soldered. If the crimp was done poorly, solder won't save it. And if the crimp was done properly, solder is unnecessary. In fact, soldering a crimped terminal may weaken the mechanical connection, may reduce electrical conductivity, and may damage the terminal. As a general rule, you should not solder a crimp terminal.
In the case of insulated wires, a proper crimp actually consists of two crimps: one crimp to cold weld the wire strands to the connector barrel; and a second crimp to secure the insulation to the connector. The first crimp establishes electrical continuity; the second crimp provides stress relief to prevent physical separation. Both crimps require the correct tool (“crimper”) and the correct crimp terminal. Without either, a proper crimp cannot be obtained.
There is no such thing as a universal crimper suitable for use on all crimp connectors. A crimper must be matched to the connector according to various factors, including the presence or absence of insulation, the wire gauge, and the design of the surface to be crimped (e.g., open or closed barrel). This matching is accomplished by “dies” that either are integral to the crimper or are replaceable elements. But even with the correct tool, a proper crimp cannot be obtained if the crimp terminal is not suited for the job.
Suitability again depends upon a variety of factors. On the most basic level, the crimp terminal and the wire to which it is crimped must physically must “fit” each other. Obviously, a crimp terminal designed for 14 – 16 AWG wire should not be used to connect 18 – 20 AWG wire. But suitability also means that the crimp terminal be able to carry the circuit's current, and that it be able to operate within its intended environment. For example, regardless of the AWG wire “fit,” a crimp terminal whose current carrying capacity is only 10 amps should not be used if the current is likely to exceed that amount. And a crimp terminal that will be exposed to the elements or to harsh engine conditions must be protected within a weatherproof connector.