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Auto Truck Wire Harness Repair

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Wiring Harness Repair

Wire diagrams are getting bigger!

In case you haven't noticed, cars and trucks have a lot of wires running through them nowadays! I have files of wiring diagrams for cars I've fixed over the years. The 1970's cars had diagrams that fit on 2 pages. By 2000 a wire diagram on a Cadillac or high end import was twelve pages or more. The diagrams are mostly online now, but they're either incomplete, or huge.

Often wires get damaged in cars, since they run almost everywhere.

Common causes of wire failures

Collisions and bad body repairs

Since wiring harnesses run inside and through body panels they can be damaged in a collision. Sometimes this isn't obvious because the damaged wire is inside a body panel. A wire can be dislocated in a collision so that it rubs against a sharp piece of sheet metal. It will work for awhile after the collision, but eventually the insulation will wear through and short or ground out.

Even when wires aren't damaged by the collision, body repairmen aren't always expert electrical repair people. Often body shop people will cut a wire harness when there's a connector a few inches away that they could have just unplugged! I've seen taillight wires cut and then just twisted back together and wrapped with masking tape after just a regular overall paint job!

Stereo and accessory installations

Many installers get paid about $25 to put in a complete system. The very best free stereo installations only meet my bare minimum wiring standards, and the worst ones are my number one source of under dash repairs.

Trailer light harnesses

Besides tending to dangle down and get ripped off by road hazards, trailer wiring harnesses once again are most often done at best to my minimum standard. At worst: the number one cause of tail light and brake light failure (on vehicles that have trailer wiring, that is)

Water in connectors

Electricity and water don't mix! If wiring connectors get wet they can corrode and fail. This can happen from rusted out body panels, or body panels whose water drain holes have been plugged by leaves and other debris. Spilling a drink or coffee on your console, or leaving the windows down in the rain can also soak some wiring. Convertables and "T" tops, along with "Sun or Moon" roofs are notorious for leaking, and destroying electric wiring in the process. Trunk seals can leak and cause tail light problems.

Improper wire routing

Improperly routed wires can be from a careless mechanical or body repair, but quite often the improper routing is from the factory! When ever I've been baffled by an intermittent electric problem on a vehicle it's been because of a wiring harness rubbing against a sharp edge on a body panel or motor part and grounding out sometimes.

Bad Connector, ground, splice, or junction

Unless the wiring harness is damaged, most of the time a wire failure is not inside the wiring harness. Usually the failure is in one of the connectors that connect the harness to components or another harness. Sometimes the electric terminal will not latch into the plastic connector tightly and shove back a bit when connected at the factory. In this case the wire will work for awhile, then fail, or start making intermittent contact. Sometimes it can get hot, melting the plastic connector. Ground wires are crucial, especially for computer components. Many ground wires bolt to the engine, and sometimes mechanics will forget to re-connect the ground wire after doing a repair. Some ground wires go to a sheet metal screw on the body, which can loosen up or corrode, causing a bad ground.
A VERY common ground fault
A very common ground fault occurs in tail lights. The symptom: both tail lights work, but one is slightly dimmer than the other. When the brake is applied, one brake light comes on, but the dim tail light side goes out completely: no light at all! When the brakes are released the tail lights return to "normal", but one is slightly dimmer than the other.
This is caused by a bad ground to the tail light bulb socket. The tail light / brake light bulb has 2 filaments in one bulb: a small filament (tail light) and a larger and brighter filament for the brake light. On some domestic vehicles and trailers the larger, brighter element does the turn signal also. When the bulb has a bad ground the smaller filament will get a ground through the larger filament until the large filament is powered up. Then the small filament will go out, because it has positive current to both sides. This is not always a wire problem: sometimes the bulb base or bulb socket itself can be corroded and not making a good connection. A rolled up piece of sandpaper can clean the socket, and sandpaper can clean the bulb base too.

Wire insulation failure or short

Bad insulation on a wire is an electric problem is waiting to happen. When the insulation is damaged, a wire can short to ground or to another wire, causing all sorts of problems.
Common causes of insulation damage
The plastic insulation on auto and truck wiiring can be damaged in many ways. Heat and age by themselves can eventually destroy plastic wire insulation. Oil, brake fluid, and transmisision fluid exposure can accelerate insulation damage. In the mid 1980's and again in the mid 1990's Ford bought some bad wire that failed a lot sooner than it should, especially if exposed to some oil leaks and heat. I've seen some Ford engine wiring harnesses where all the insulation has crumbled off: just bare wires hanging all over the place.

I've also seen rough welds and frame areas with wiring harnesses improperly routed over them. After a few years the rough edge eats through the wire and grounds it out. In these cases the wire may not ground out all the time. This can lead to some weird problems, like "I hit a bump and the engine shuts off"

Wire Repair

There are several ways to repair car and truck wiring harnesses. These are listed in order of quality: the first few are just for emergencies, the last is as good or better than the factory wiring.

Twist and tape

If you get a free stereo install you'll probably get a twist and tape wiring job. The wires are stripped, twisted together, and taped with electric tape. This can work for low voltage connections, but it also can come loose and fail quite easily. The only time I'll use electric tape on a wiring harness is when the harness has rubbed against something sharp and grounded out a wire, but the wire isn't badly damaged, just "skinned" a bit. Since I will be relocating the wire away from the sharp surface I will often tape up the harness if all of the wires are still intact. Sometimes I'll wrap an old section of radiator hose around the harness to furher protect it from damage. I NEVER MAKE A SPLICE OR REPAIR USING THE TWIST AND TAPE METHOD! Of course if you're out in the woods or on the side of the road a twist and tape repair can get you home where you can fix it properly!

Scotchlock connectors

These are often used to "tap in" to existing wires. They require just a pair of pliers to install, and are often used on stereo installations and trailer wiring. A "blade" with 2 notches cuts through the insulation on the wire and makes contact with the conductor. A plastic latch clicks over the blade and holds it in place. These connectors are easy to use and don't damage the wire being "tapped into" much. They aren't good for carrying much current, though, and can loosen up and make a bad connection.

Crimp Connectors

Crimp connectors are made out of a small metal tube with a plastic covering. They come in different sizes for different wire gages. They often are color coded: commonly pink is the smallest, blue the next size up, and yellow the largest. A special pair of pliers is used to crimp the connector onto the wire. Often wire strippers have crimp pliers as part of the handle opposite the stripping end. There are 2 types of crimps for crimp connectors: one uses 2 semi-circular notches to crush the metal tube in the connector into an oval shape. The second and more secure is the "W" style crimp, which actually puts a small dent deep into the tube as well as collapsing it into an oval. Some "high end" crimp connectors have sealable ends to prevent water entry and corrosion. A dab of RTV silicone can be squirted into the connector ends after crimping to prevent water intrusion.
The good and the bad side of crimp connectors
Crimp connectors are for sure better than Scotchlocks or twist and tape. Some manufacturers use crimp connections from the factory, however their crimps are hard to duplicate, especially with the crimp terminals you get at most parts stores. The advantage of a crimp connection over a solder joint is that the solder joint can break just past the solder area if the wire is subject to vibration or movement. In practice I avoid this by reinforcing my solder joints on either side so any flex will be well past the solder joint.

Solder joint with heat shrink tube

This is how I do most all of my electric wire repair. I'll put one or two sections of heat shrink tube over the wire ends, then twist them together and solder them. I then slide the heat shrink tube over the solder joint, and heat it with a cigarette lighter or heat gun. Often I'll "double heat shrink" it, putting another slightly larger piece of tube over the first. This stiffens the repair around the solder joint and prevents the wire from breaking from vibration or movement.
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Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.

George G. Scott, Jr.


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Auto, Car, and Truck Article List

A
ABS: Anti-Lock Brake Systems
ADVANCE: Car ignition timing
ALTERNATORS and Car Battery
AUTOMATIC TRANSMISSIONS
B
BAD CAR DESIGNS
BATTERIES: Auto, Car or Truck
BELTS AND HOSES
BEARINGS
BODY AND BUMPER REPAIRS
BRAKE REPAIRS: Car or Truck
C
Car Washing and Care
CARBURETORS:Car & Truck
CHECK ENGINE LIGHT
CLEANING: Engine Cleaning
CLUTCH REPAIRS: Car & Truck
COMPRESSION: Car Engine
COMPUTER CAR CONTROLS
CV JOINT OR CV AXLES
D
DISTRIBUTORS (IGNITION)
E
ELECTRIC WIRING REPAIR
ENGINES: Car & Truck
ENGINE CLEANING
EXPANSION PLUGS
F
FREEZE PLUGS
FUEL AIR MIXTURE
FUEL INJECTION: Car & Truck
FUEL PUMPS: Car & Truck
G
GAGES AND "IDIOT LIGHTS"
GASKETS AND SEALS
GLASS: WINDOWS AND WINDSHIELDS
H
HEADS & HEAD GASKET
HOSES AND BELTS
I
"IDIOT LIGHTS" AND GAGES
IGNITION TIMING: Car & Truck
J
AUTO JACKS: lifting cars safely
K
L
LEAN "Car runs lean"
LIGHTS: WARNING OR "IDIOT LIGHTS"
Limp Home Mode
M
MIL Light
MANUAL TRANSMISSIONS
N
NO START: Car Won't Start
O
OIL CHANGES
OIL: What's right for your car?
OIL LIGHT ON OR GAGE LOW
P
PCV Valve
Q
R
RADIATORS: Car and Truck
RICH: Car runs rich
S
SEALS AND GASKETS
SERVICE ENGINE SOON LIGHT
SPARK PLUGS
STARTERS: Auto, Truck
T
THERMOSTATS
TIMING: IGNITION TIMING
TIMING BELT & TIMING CHAIN
TIRE REPAIR
TRANSMISSIONS: AUTOMATIC
TRANSMISSIONS: MANUAL
U
V
VACUUM ADVANCE
WARNING LIGHTS OR "IDIOT LIGHTS"
Car Washing and Care
W
WATER PUMP REPAIR
WINDOWS AND WINDSHIELDS
WIRING REPAIR
X
Y
Z

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