TOYOTA - Mk2 MR2 - Harness

Wiring Harness

Repair Only Full Harness
Difficulty(0-5): 3 5
Time: 2 hours 20-40 hours
Tool Specialty(0-5): Repair: 3 Full: 4
Cost: $5-150 $300-1,200

Tools Required:

  • Sockets: 10mm, 12mm
  • Wrenches: 12mm, 14mm
  • Needle Nose Pliers
  • Specialty: Multimeter (AC Votls, DC Volts)
  • Specialty: Terminal Extractor
  • Specialty: Open Barrel Crimper 22-10 AWG
  • Specialty: Wire Stripper
  • Specialty: Wire Cutter
  • Here are some Amazon links hand selected by me for the exact parts I used in this project. Your purchases through these links help fund this site and keep new info coming:

PARTS REQUIRED (Repair Only):

  • Wire: PVC or Tefzel insualted 14-20 AWG
  • Connectors: As needed from Toyota or other suppliers
  • Terminals: 090 sealed is popular in many 90's Toyota engine connectors
  • Splices: Open Barrel
  • Heatshrink: 1/8"- 1/4"+
  • Wire Loom/Sheath: Various lengths/diameters
  • Wire Seals/Plugs: 070, 090
  • Here are some Amazon links hand selected by me for the exact parts I used in this project. Your purchases through these links help fund this site and keep new info coming:

Over time the wiring harness on many older cars becomes brittle and weak. Electrical problems can be a real pain to sort out and having a failing harness can troubleshooting very difficult. If you find a few wires breaking or cracking itís a good idea to repair the harness or flat-out replace it. Wire harnesses are expensive from the factory ($500-1500) so many people wonít want to replace it. Thankfully, they are generally cheap to repair once you have access to the bad section of wire. All you need is a few select tools and small sections of wire and heatshrink.

Below you can see a hole worn through the wire loom (the protective plastic shell over the harness). Once the protective loom was compromised the wire insulation started to go as well. The wires underneath were then bare and exposed. They were all rubbing against the intake manifold and causing shorts to ground. This causes all sorts of confusion in the electrical system and can cause all sorts of problems.

01 Testing

  • Step 1: Determine if you have a wiring problem
  • This can be difficult. The easiest and fastest method may not even require you to remove any other parts. The fuse box has many circuits readily available and the ECU is easy to access in the trunk. For the fuse box, all you need is a probe that can get down to the exposed wiring terminal in the connectors. Use a stray wire, pin or other thin, conductive object to check resistance between various circuits in the fuse box and ground. Only certain circuits should have any reading when measuring resistance to ground. Most circuit problems can be detected by checking for open circuits and shorts to ground. Clip one end of the multi-meter to ground (virtually any bolt on the engine) and check various circuits. Expected values can be found on this website HERE or in the Toyota Wiring Manual.

  • Step 2: Diagnose
  • Once youíve found the problem circuit you can search for obvious signs of damage. Some fixes can be done with the harness in car. If so, consider yourself lucky and skip over the next step(s) to remove the harness. If you canít see anything obvious youíll need to take out the harness. This isnít too bad on the 5SFE, but I canít comment on the ease for the 3SGTE.

  • Step 3: Remove the harness
  • I prefer to start at the furthest section of harness. The order isnít crucial, but most everything needs to be disconnected before you can remove any given section of the harness. The harness is semi-flexible, but donít force it all over the place or you may create new problems.

    Iíd recommend the following order:

    Ground connection that runs over the timing belt; Alternator 12V and plug connector; VSV under intake manifold; Intake manifold ground; Knock sensor under intake manifold; Manifold Pressure sensor; Diagnostic block; Fuel Injector connectors; Cold Start injector; Starter 12V and single wire plug; O2 Sensor; Water temp sensor; Oil Pressure Sensor; HVAC Water temp; Igniter; Coil Pack; A/C VSV; Trunk connections

  • Step 4: Inspect the harness
  • Often times the problem will be obvious if you take a few seconds to look at every wire termination of the harness and inspect the entire loom for signs of wear and tear. Common failures are brittle wires at termination points and worn loom/wire in areas where the loom can rub against an engine part. Installing a harness incorrectly can easily cause sections to wear through.

  • Step 5: Repair the Wire
  • Assuming you donít need to make an entirely new harness, you can repair the section(s) that need some love. The first step is to clean the wound. Remove the loom surrounding the affected area. If your problem is at a termination in/near a connector, you should remove the connector in question. You may need a terminal extractor tool or eyeglass screwdriver. These are detailed above in the ďTools RequiredĒ section.

    Cut out the bad wire/terminal. Strip approximately ľĒ - 1/2Ē of the insulation of the good wire. Youíll need this bare wire to crimp/solder onto your new piece of wire. If youíre replacing a terminal, crimp the new terminal onto the new section of wire before joining the new and old wire. Place a piece of heat shrink onto the wires being joined. Special weatherproof heat shrink is available and helps a ton in keeping your new joint away from moisture, dirt and oil. Normal heat shrink or even electrical tape can also work, though electrical tape alone isnít ideal.

    Itís important to keep the length of wires similar to avoid excess strain on short wires and kinks on longer wires. Once the new wire is stripped and ready to go you can crimp or solder the two wires together. For solder joints, make sure you use flux and clean it away if the flux calls for it once youíre done. I prefer to crimp and would recommend the same for virtually anyone. I think a good crimp is easier than a good solder joint for amateurs. Both options require the right tools.

  • Step 6: Protect the Wire
  • Heatshrink should be used on every joint. If you forget this protective layer electrical tape can be used if youíre careful. After the joint has some protection you want to protect the entire harness of wire. Typically this is done with plastic loom, but it can be accomplished with tape, expandable sheathing or heatshrink.

  • Step 7: Reinstall the Harness