Beginning in the 1970s, GM started using a specialized weather-resistant terminal design called “Weather-Pack”. This modular terminal system was designed to seal the wires and terminals from corrosion. These are very common in vehicles built from the 1970s up to the mid 90s. In the 1990s, weather-packs gave way to the upgraded version called “Metri-Pack”, which is a similar modular terminal design with some key improvements. If you are working on the electrical system on any modern vehicle, you will encounter these types of terminals.
The Weather Pack terminal is always inline, meaning that the connectors are in a flat line and are a uniform shape, so they only connect to each other (male and female halves), and not directly to component terminals. All WP terminals are round, with a male pin and a female barrel. Another limitation of WP terminals is that the small terminal size limits the amp rating to 20 amps, anything more runs the risk of melting the pack itself. WP terminal blocks come in sizes from 1 up to 7 wires. Components for Weather Packs are ridiculously cheap, so repairs or custom connections are cheap and easy.
Metri Pack is the next generation of modular terminals. These terminals use flat blades and rectangular female terminals in a metric size, which is where the “Metri” name comes from. Most of the MP terminals are sealed, but not all of them are, as interior connectors don’t need to be sealed. The biggest improvement in modular termination with the MP design is the connector block itself. There are tons of shapes and styles, with inline, double stack, and more variations in different series. 150-series uses 1.5mm terminals and is rated at 14 amps, the 280-series uses 2.8mm terminals and services up to 30 amps, 480-series has a (are you ready for this) 4.8mm terminal rated at 42 amps, and the 630-series services 46 amps with, drum roll please, 6.3mm terminals.
Assembling these types of terminals requires a couple of special tools. The main tool you need is a pair of barrel-style crimping tools, preferably with ratcheting action that does not unlock the tool until the crimp is completed. The ratcheting tools are fairly expensive, but there are non-ratcheting tools available. The second tool is nothing special, just a small diameter pick used to disassemble the terminals from the block.
The key to crimping WP and MP terminals is to use the right size terminal for the wire and not to under/over-crimp the terminal. Under crimping will leave you with a loose fitting connection and over crimping can actually cut the terminal in half.
Removing terminal pins is fairly simple. You can buy a special release tool for WP and MP terminals, but a small flat-blade screwdriver, or pick, works just fine. Slip the tool into the front side of the terminal to release the pin and pull of the wire from the back side. Some terminals release from the backside of the block. You should be able to see the pin inside the terminal block.
Start the crimp by stripping a small length of wire. How much depends on the terminal and size of the wire, typically 3/16-1/4-inch. Place the wire inside the crimping lugs on the terminal. If the terminal is insulated, be sure to slide the silicone plug over the wire BEFORE the crimp terminal.
Here is where it gets tricky- the tool has a special shape that forms an “M” shape on the tangs, pushing them into the wire, clamping it to the terminals. Sometimes it is easier to make this happen by gently squeezing the tangs inward a tad before loading the terminal into the tool.
Once set, squeeze the crimp tool until the ratchet releases.
For insulated terminals, slide the silicone plug up to the terminal and into the insulator lugs (the rear-most large lugs, and crimp these with the corresponding round die on your crimping tool.
With all of your terminations complete, the terminals are slid into the terminal block until the pin clicks into the place. You should feel it click and you might hear it too. Give the wire a tug to make sure it is set.
That is just about all there is to assembling Weather Pack and Metri Pack terminals. They are very good terminals, much better than the typical individual insulated terminals you buy in packs at the store. Repairing a bad wire in a WP or MP terminal block is the best solution over bypassing it.