Why this happens
Crimping and soldering both make good electrical joints, but they behave differently on a vehicle. A good crimp is flexible at its edge and handles vibration well, which is why vehicle harnesses use it. Solder wicks up stranded wire and makes a stiff section, and the joint can crack where flexible wire meets that stiff section under constant vibration. Some connectors are designed for solder, others only for crimping.
What you need
Tools
- Ratcheting crimp tool with correct dies
- Temperature-controlled soldering iron
- Wire strippers
- Heat gun
- Digital multimeter
Materials
- Matching terminals and connector pins
- Rosin-core electronics solder
- Adhesive-lined heat-shrink
- Cable ties
Before you start
- Work only on de-energised low-voltage circuits; disconnect the battery first.
- Soldering irons and heat guns cause burns and fires; use a stand and a fire-safe surface.
- Solder in a ventilated area and wash your hands after handling solder.
- Do not modify main battery cables, motor phases or high-voltage car wiring.
Step by step
- 1
Check what the connector was designed for
Look at the terminal. Open or closed barrels with no solder cup are designed for crimping. Solder cups, like those on XT60 connectors, and PCB pads are designed for soldering. The connector maker's datasheet says which method and tool to use. Following the design is the first and most important rule.
- 2
Use crimps for wire-to-wire and harness work
For joining wires, ring terminals, spade terminals and most waterproof automotive connectors, use the correct crimp with a ratcheting tool and adhesive heat-shrink. Crimps are quick, repeatable, and handle vibration well, which is why they dominate in vehicle wiring. Always pull-test each crimp before fitting it into a housing.
- 3
Use solder for PCBs and solder-cup connectors
Solder components to circuit boards, solder-cup power connectors and fine wires to sensor boards in maker projects. Use a temperature-controlled iron, clean tip and rosin-core solder. Heat the joint, not the solder, so it flows smoothly and looks shiny and concave.
- 4
Do not mix the two on one terminal
Do not tin wire before crimping it, as solder creeps under pressure and the crimp loosens. Do not flood a finished crimp with solder either, because it wicks up the wire and makes a stiff, brittle zone just behind the terminal. Pick one method per joint.
- 5
Support soldered wires against vibration
Where you have soldered a flexible wire, prevent it from flexing at the joint. Slide heat-shrink over the joint and a short length of wire beyond it as strain relief, and secure the wire with a cable tie or clip a few centimetres away so vibration does not bend it at the solder edge.
- 6
Inspect both kinds of joint
A good crimp has no stray strands, holds firm under a pull test and has the insulation grip on insulation. A good solder joint is smooth, shiny and fully wets the pad or cup, with no blobs, cracks or dull grainy surface. Check continuity with a meter in both cases.
- 7
Think about future repairs
Crimped connectors can usually be unplugged and replaced in the field with a crimp tool. Soldered joints need an iron and power. On anything you will service often, use connectors that are crimped and unpluggable so a repair does not need cutting and resoldering.
How to tell it worked
Each joint uses the method the connector was designed for, passes a pull test and continuity check, and stays reliable after weeks of riding vibration.
Stop and call a technician if
- The joint you need to make is on the main battery lead or inside the battery or controller.
- You find cracked solder joints on a controller board; that is a workshop repair.
- A repaired joint gets hot under normal load.
Questions
Is soldered always better because it conducts better?
Not on vehicles. A proper crimp has very low resistance and better vibration resistance; solder is best where the part is designed for it.
Can I solder a wire joint and cover it in heat-shrink?
Yes, for low-current signal wires, if you add strain relief so the wire cannot flex at the joint. For power and harness work, a crimped splice is usually better.