Feature October 1, 2011 by
Mike Farrell, Curriculum Developer CARS
Fretting Over Intermittent Faults?
Today's modern vehicles have an ever increasing number of electronic systems and electrical options that must meet the driver's high performance demands as well as provide reliability. The components, along with the associated wiring and...
Today’s modern vehicles have an ever increasing number of electronic systems and electrical options that must meet the driver’s high performance demands as well as provide reliability. The components, along with the associated wiring and connectors, are also being packaged into smaller and smaller spaces. Finding customer concerns such as intermittent no-starts, service engine soon or other warning lights becomes increasingly difficult, especially when the condition cannot be duplicated. Some of these intermittent conditions may be caused by what is described as “fretting.” Fretting is the build-up of non-conductive oxidized debris between two electrical contact surfaces inside an electrical connector. These connections are most commonly used to join components and harnesses. Fretting corrosion is caused by vibration, thermal cycling and small amplitude motion between the electrical contacts. On high current power circuits fretting will cause a permanent increase in the circuit’s resistance. On low current signal circuits fretting may cause high resistance which can result in intermittent connections. On critical sensor circuits the loss of the signal to a control module may cause the vehicle to function abnormally for a short periods of time. This condition may or may not result in diagnostic trouble codes being set. Fretting is most commonly found on tin plated terminals. A tin plated contact is constructed with a thin layer of tin oxide (A) that is used to protect the malleable tin body. The illustration shows the thin tin oxide layer cracking (B) due to the pressure required for good terminal contact (Step 1). Step 2 shows the microscopic amount of fretting caused by corroded material that forms due to micro motion, thermal cycling or vibration. As the thin tin oxide layer wears, the tin portion of the terminal is exposed to air and will form a layer of insulating tin oxide (C). As the terminal fretting motion continues the amount of tin oxide debris will also continue to build (Step 3). This scenario will continue until enough debris has built up and the high resistance that results will cause an intermittent condition (Step 4). In the example, tin terminals were used as they are the most common type on a vehicle. Fretting corrosion can also occur on terminals made of copper, iron, nickel and zinc, all of which are non-noble metals. Non-noble metals are susceptible to forming oxides. Noble metals such as gold and silver are not susceptible to fretting corrosion but the cost of these metals makes their use in automotive applications very limited. Fretting corrosion does not have the same appearance as water corroded terminals. In severe cases, fretting appears as small dark smudges or streaks located where one terminal contacts another electrically. On non-severe cases the corrosion will not be evident to the naked eye. In the case of an intermittent condition, replacing a component or control module may temporarily repair the customer’s concern, but after a short period of time the same concern will re-occur. While most terminals manufactured for today’s vehicles are designed to resist fretting, the harsh conditions that are present can still result in the condition. Diagnosing a customer concern caused by fretting can be difficult. If the condition caused a diagnostic trouble code to set and the wiring and component(s) were tested with no fault evident, use service information to identify any connectors that are in the circuit. If no trouble codes set, review the symptoms of the condition and determine which components and modules may be causing the fault. Again, using service information, locate the wiring circuits and connectors. In order to reduce fretting, try to minimize the movement between the electrical connector terminals by installing all wiring and connectors into the OEM clamps and brackets. Also, be sure to check the terminal contact between the male and female electrical terminals. Terminals with high contact force resist fretting better than loose terminals. Tin contacts require at least 100 grams of normal contact force and should be lubricated. The use of dielectric lubricant can help prevent a reoccurrence of fretting. A thin film of dielectric lubricant will reduce the friction and help prevent wear due to motion and protect the terminal surface area from atmospheric oxidation. The lubrication should be applied lightly to both sides of the connector or component. Be sure not to apply too much lubricant as it may cause the connection to hydro-lock when refastening. After reconnecting the components wipe away any excess dielectric lubricant and attempt to duplicate the customer’s concern. For more information on automotive technology visit CARS OnDemand training at: www.carsondemand.com If you have completed any of the Advanced Technology CARS OnDemand courses, CARS would appreciate you taking the time to complete a short 3 minute survey. To access the survey please go to the CARS website, log-in and under “Account Details” choose “Participants Surveys” – “Advanced Technologies Student Survey.” Your feedback is important to us.