Checking and re-wiring the coil pack harness wires
On many Focuses, the heat under the hood makes the coilpack harness brittle. This leads to cracking insulation and eventually corroded, frayed, and broken wiring. This can cause the car to run very rough, and might even cause a small electrical fire!
Time Required: 10-45 minutes, depending on if you need to actually re-wire the harness
Pin extractor or small flat-head screwdrivers
Metra 71-1784 radio harness (can be purchased at Best Buy) to dissect for wires
RTV Black sealant
heat shrink tubing
To check the coilpack harness, pop the hood open and find the coilpack. It's the black brick next to the driver's side of the engine where the 4 spark plug wires terminate. Examine the 3 small wires going into the coilpack:
1) Draw a diagram of the wire colors and what hole they go into just in case the wires break
2) Press up on the little metal clip under the plug, and pull the plug off the coilpack.
3) Look for broken, stretched, discolored, burned or damaged insulation and/or wiring.
If the wiring is damaged:
1) Pop out the plastic insert on the harness plug
2) use a pin removal tool to pop out the pins. If I recall correctly, you go at it from the inside, but pull them out the back (inside = part plugging into the coilpack, back = where the wires go in)
3) cut the pins off past the damaged wire
4) Use the same procedure in step 2 to get the pins/wires out of the radio harness you bought. You only need 3 pins intact.
5) strip the harness wires
6) Solder the new wires and pins onto the old harness wires
7) Shrink tube the connections
8) Take note of which wire goes into which hole, snap into place
9) seal the outer side of the plug with RTV Sealant
10) put the plug back onto the coil pack and let the RTV set for a while