Repairing old harness and connectors

After thirty-plus years, E30 wiring often has brittle insulation, crumbling connectors and corroded joints. A full new harness is expensive and hard to find, so most cars are kept going by repairing the damaged parts properly. This guide covers how to find the damage, what materials to use, and what to avoid.

Finding the damage

Harness faults are often intermittent: a fuse that blows now and then, a light that flickers, a motor that works only after a thump. Look in the places where wiring flexes or heats up.

• Door and boot hinge boots. The rubber boots that carry wires between the body and the doors or boot lid. The wires flex on every opening and break inside the boot.
• Near the engine and exhaust. Heat bakes the insulation and loom, so it cracks and flakes.
• Under the dash and behind the cluster. Old connectors and joints get disturbed when you work there.
• The boot harness. Repeatedly blown parking light or dash light fuses often trace back to damage in the rear harness.
• Ground connections. Corrosion here causes strange problems all over the car. See the ground points guide.

Flex the wires gently and watch for a symptom to come and go. Use a test light or a meter at both ends of a suspect wire. The wiring pages on this site show which wire goes where, so you can check the right one.

Crumbling loom and tape

The outer wrap on older harnesses dries out and flakes away. The wires underneath are often still fine.

• Re-wrap with proper harness tape, a non-adhesive or fabric tape made for wiring looms. It does not leave a sticky mess, and it stays put in heat.
• Avoid plain PVC electrical tape as a long-term wrap. Its adhesive creeps and goes gummy, and it unwinds in heat.
• Adhesive-lined heat-shrink (often 3:1 or 4:1) works well to sleeve a section, or the boot where the harness enters a connector.
• Corrugated split loom can protect a section that needs to be easy to open again.

Check each wire in the damaged section before you re-wrap it. If the insulation itself is cracked, repair it as below, not just cover it up.

Repairing a broken wire

1. Cut back to clean, bright copper on both sides of the break.
2. Slide a piece of adhesive-lined heat-shrink over the wire first, so it is ready to go over the joint.
3. Join the wires. Either crimp them with a proper butt connector and a ratchet crimper, or solder them. A solder joint should be twisted or lapped neatly first, and heated until the solder flows through it.
4. Slide the heat-shrink over the joint and shrink it until the adhesive seeps out at each end.
5. If the wire is under strain, support it with a loop or clip so the joint does not take the load.

Use wire of the same gauge or larger. The wiring pages show each wire's size, so you can match it. Use a different colour only if you mark the change, and keep a note, so the next person can follow it.

Crumbling connectors

Old plastic connectors go brittle and crack, and the terminals corrode. You do not have to replace the whole harness to fix one.

• If a connector housing has cracked but the terminals are good, a replacement housing and new terminals can be fitted to the wires.
• If a terminal is burnt or corroded, cut it off and crimp on a new one of the same type, using a crimper made for that terminal.
• If a whole connector has crumbled, cut it out and fit a new one, or splice in a short pigtail with the new connector already on it.
• Clean corroded contacts with a contact cleaner and a small brush, and use dielectric grease on connectors that see moisture.

When a plug is damaged and you cannot get the exact part, a similar sealed connector from an automotive supplier will do. Match the pin count and the terminal width, and check that it keeps the same pin positions.

Engine swaps and harness work

When you move an engine between cars, the engine harness and the engine management have to match. As a rule, use the donor car's harness and engine computer together, and do not try to adapt the old one.

• Label every plug as you disconnect it, with tape and a pen or a photo, before you unplug the harness from the engine.
• Use the wiring pages on this site to confirm where each wire goes, so you can check the new connections.
• Check the grounds as you go. An engine that ran perfectly in the donor can misbehave with a poor ground in the new car.

When to replace it

A harness is worth replacing, or sourcing from a donor car, when the damage is widespread: crumbling insulation along most of its length, many connectors gone, or repeated faults after repair. For a harness with a few bad spots, repair is cheaper and just as reliable. Whichever you choose, check the grounds and fuses, because those often cause the faults that look like a bad harness.

Written from the factory wiring diagrams on this site and general diagnostic practice. Voltages, resistances and pressures are typical figures, so confirm them against your car's manual. Take care with fuel, high voltage and the battery.