Fretting, Brinelling, and CAN Diagnostics HOME Diagnosing CAN
(Controller Adaptable Networking) is a lot
different from the way most techs have
been testing electrical circuits for the past
several years. The old way of running a
circuit consisted of sending a voltage or a
ground from a switch to the load.
Now with the CAN systems involved the
whole process is different, and different
tools have to be used in order find the
source of the problems as well as new terms
to learn and understand.
Tools you’ll need to be comfortable with. One of the key tools is the scanner. The more you spend on a scanner the more information you’re likely to get for your money. A simple code reader may not be adequate enough for some of the applications you’ll be encountering. There will be numerous times that you’ll need to either recalibrate or synchronize a component, such as a cam synchronization or recalibrating a BPP (Brake Pedal Position switch), and many other possibilities. These over the counter cheap code readers are most likely not going to have these capabilities. So check out what you’re getting before you actually buy it. Another main tool to get use to using is the Oscilloscope. There will be a lot of times where you’ll have to look at different screen patterns to determine if the proper amount of voltage, current, or resistance is present. If you’re not familiar with the use of a scope sites like AVI offer some great training videos on O-scope reading, as well as many other sites that offer on line information on the proper use of a scope as well. Here’s a great little website that offers on on-line test of your O-scope skills, http://people.sinclair.edu/nickreeder/Flash/oscilloscope.htm try it out when you get the chance.
There are several great sites on the internet that have full libraries of scope patterns you can look at to compare your oscilloscope pattern with a known good one. Identifix, Autonerdz, Pico, are just a few I use on a regular basis.
Don’t leave out your Digital voltmeter and test light. You’ll still need these as well. They are going to be just as important as the scanner once you start getting involved with the testing procedures.
Keep in mind, almost all the service codes are written in a way that you don’t need to use a scope to find the solution. This has been true for years and is probably the leading factor why so many techs don’t reach for an oscilloscope when testing different systems in today’s cars. But as with everything else in the industry… things change, and this is one time a scope is going to make a big difference in your testing abilities.
What to look for As a rule, CAN systems run on very small amounts of voltage and current. In order to work under the various conditions a car is subjected to extra care has to be taken to insure those values don’t vary by very much. Using Ohms law as a basis of this discussion (V=I R) or (I=V/R) or (R=V/I) the amount of current flow in a circuit is greatly affected by the resistance, and one of the main things to be concerned with is fretting. Fretting
Fretting refers to wear and sometimes corrosion damage at the contact surfaces of a connector or terminal. Fretting is more pronounced under a load and in the presence of repeated surface motion, and can vary with vibration. Fretting is defined as: "A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force." (The load refers to the voltage in the circuit.)
One side effect of fretting is known as false brinelling. False brinelling is damage caused by fretting. Brinell damage is characterized by permanent material deformation (without loss of material) and occurs during a load event, whereas false brinelling is characterized by material wear or removal and occurs over an extended time from vibration and light loads.
What does this mean to you as a technician? This is your no#1 place to look for those pesky intermittent or even no communication problems with a CAN system. With these small current levels and low voltage requirements any form of corrosion, movement, lack of a solid connection, or all three can be the reason for such communication loses. This is where I see a lot of components that are replaced that didn’t need replaced. A lot of times the actual problem was the connector to the component and not the component itself. In time, if there was any fretting or false brinelling involved the exact same problem will come back again. Example problem;
2005 Chevy Cobalt loss of communication
The CAN bus lines are on the data link connector pins 6 (+) and 14 (-). These signal leads should show around a 60 ohm resistance between the two terminals which is the normal reading for this pair of wires. (Two 120 ohm resistors in parallel). Using the scope hooked up to these two pins on the ALDL you should see a steady resistance level. But, if any fretting is involved a simple wiggle test will show a fluctuation on the meter. (A good ohm meter can work in place of the scope as well.) Narrowing down where this fluctuating resistance is coming from by wiggling small sections of the terminal or leads should give you the answer the source of the loss of communication. Basically what is happening is a very small amount of corrosion on the terminals is causing a very small amount of resistance to be picked up by the meter. This is the same resistance that the computer is seeing and is more than likely what is causing the intermittent no communication. The CAN operates on milliamps of current so even a very small amount of resistance effects the signal. (Sometimes a quick shot of connector cleaner solves the entire problem and no parts need to be replaced.) With luck this test should help give you one more way of narrowing down and testing the causes of some of those intermittent CAN problems we face today. Good luck!