The good practice to observe when re-assembling electrical wires and connectors in a car

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What not to do when working with wires on a car

I bought a second hand van and discovered that someone had intervened on the electrical harness that runs on the engine. Although everything is operational and the check engine light is off, the way the wires were cut and wrapped with tape, with the harness left open, can only lead to some trouble at some time.
This indeed gives an example of what not to do when working a car's wires, since simple and cheap measures can ensure wires integrity and long-serving life as shown in this pictorial.

Attaching wire harnesses around the engine

Dismantling wire harnesses for accessing or removing mechanical parts is much common when intervening on a vehicle.
It is essential to re-attach the electrical harness of wires to its exact fixing points as it was before disassembling.
Many car cables attaching clips can be simply refitted despite being deformed when removed but you might have encountered harnesses that are attached with cable ties and had to cut them.
You need then to use new cable ties and re-attach the wire harness as it was before your intervention.
Observe the following rules:

Cable tie material to be nylon
Cable tie to be black
Cable tie to be of the maximum width that the holder can accommodate.
Do not over-tighten a cable tie that fixes a wire harness, avoid elongating it not to weaken it.

If the wire harness insulator in a car was covered by tape at its ends, you probably encountered an undone tape.
Cut it and replace it with new adhesive electrical insulation tape of good quality.

Special tape, identical to the original one found in new cars is widely available online.
It's a cloth tape (fabric tape) that withstands high temperatures and provides an excellent mechanical protection:

Electrical connectors in a car's engine compartment

Many connectors simply break when being removed due to the ageing of their holding mechanism.
Inserting the connector without its holding device is not recommended at all.
You need to make sure you find a way to hold it in place like the use of a cable tie in this case.

Do not reconnect an electrical connector in a car without cleaning it first with a contact cleaner.
Use WD40 or equivalent oil if you have no contact cleaner but make sure in both cases to let it dry prior to reconnecting.
You can use both contact cleaner to clean the contacts and WD40 to lubricate the connector holding mechanism.

Protecting new wires in a car

If you fitted a new device to your car like a gauge, horn, rpm, or any accessory that comes with wires, you need to protect the wires prior to routing them and attaching them.
The best protection remains the one of old O2 sensors wires which is "OEM" quality and made with the right materials.
Insert the wires into the O2 sensor protection sleeve for building a wire harness.

Do not hesitate to add a heat shrinkable sleeve prior to or after using the old O2 sensor protection in a manner to cover the wires or the harness at the exact location where they enter into the device's cable gland.