Crimp and Heat-Shrink Connectors the Right Way

Why this happens

A good crimp cold-forms the terminal barrel around the copper strands so tightly that they become one solid, gas-tight mass. That joint is mechanically strong and resists corrosion. Pliers, the wrong die or the wrong terminal size leave air gaps and loose strands, which raise resistance, heat up under current and eventually fail. Adhesive heat-shrink then seals the joint against water.

What you need

Step by step

1. 1

   Make the circuit safe

   This guide is for low-voltage wiring such as lights, horn, accessories, signal wires or your own bench projects. Switch off, remove the key and unplug the battery connector where the manual allows. Check with a multimeter that the circuit is dead before cutting anything. Never work on the main battery cables or any high-voltage car wiring; opening sealed harnesses may void warranty.

2. 2

   Match the terminal to the wire

   Check the wire size printed on the insulation or measure it, then choose terminals whose barrel is rated for that size. Use the crimp die marked for that terminal type, because insulated, non-insulated and open-barrel pins all need different dies. Practise on a spare length of the same wire before working on the vehicle.

3. 3

   Slide on heat-shrink first

   Cut a piece of adhesive-lined heat-shrink long enough to cover the terminal barrel and about a centimetre of insulation, and slide it onto the wire before you crimp. It is easy to forget, and you cannot fit it afterwards over a ring terminal or a connector housing.

4. 4

   Strip to the right length

   Strip just enough insulation for the strands to fill the barrel, with a tiny gap between the insulation and the barrel end. Use a stripper sized for the wire so no strands are nicked or cut. Twist the strands lightly in their natural direction. Do not tin the strands with solder before crimping, as solder creeps and the crimp loosens over time.

5. 5

   Crimp in one full stroke

   Insert the wire so the strands are visible at the front of the barrel but not sticking out beyond it. Place the barrel in the correct die, seam facing the right way for open-barrel pins, and squeeze the ratchet tool through its full cycle until it releases. A ratchet tool will not let go until the crimp is complete, which gives consistent results.

6. 6

   Inspect and pull-test

   Look at the crimp: the barrel should be evenly compressed with no strands outside it, and insulation grips should hold the insulation, not the copper. Hold the terminal and pull firmly on the wire. It must not move. If it slides out or strands break, cut it off and redo it. A failed pull test on the bench is far better than on the road.

7. 7

   Shrink the tubing

   Slide the heat-shrink over the barrel so it overlaps the insulation. Heat it evenly with a heat gun, moving constantly, until it shrinks tight and a little adhesive appears at the ends. Keep the heat away from nearby plastic parts and other wires. Let it cool before handling. Never use a lighter or open flame near a vehicle.

8. 8

   Test the circuit

   Check continuity through the new joint with a multimeter, then reconnect the battery and test the circuit works. Recheck the joint after a few rides to make sure it is still secure. Label the wire if it is one you may need to find again.

How to tell it worked

Each crimp passes a firm pull test, the heat-shrink is tight with adhesive visible at the ends, continuity is good and the circuit works reliably after a few rides.

Questions