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You Can't Always Go With the FlowPosted 10/7/2002By Mark Owens
Flow Charts: Most flow charts check system operation and give some diagnostic aids but often are of no help with trouble codes. If the system works, you are left at a dead end. I suggest viewing the same inputs the computers see. Review conditions and inputs that are needed for the test to pass or fail. Examples of inputs: feedback voltages from pressure and temperature sensors, position sensors, on/off switches, etc. If not understood, evaporative systems are difficult to diagnose. Most flow charts advise installing special tools and checking to see if the system holds pressure. Don't worry about pressure - focus on vacuum! During operation, the system does not hold pressure, but instead pulls a small amount of vacuum on the tank, canister and hoses. Vacuum is pulled using a purge solenoid (normally closed) connected to engine vacuum and a vent solenoid (normally open). The evaporative system is a good example of viewing the same data as the computer. The computer turns on both solenoids at the same time, allowing the engine to pull a vacuum with the purge solenoid open and the vent solenoid closed. The computer monitors vacuum with a sensor located in the top of the tank. Vacuum is displayed on the scan tool in inches of water vacuum. Since approximately 7 inches of water vacuum equals 1 inch of mercury, there is very little vacuum in the system. Some scan tools can control both the purge and vent solenoids at the same time, and monitor pressure status all on the same screen. If your scan tool cannot do this (most can't), you are limited to energizing the solenoids manually and/or checking sensor voltage with a digital volt ohm meter (DVOM). To test vacuum on the system: With the gas cap off, the sensor should read approximately 1.5v, indicating no vacuum or pressure. Vacuum will increase voltage and pressure will decrease voltage. Pull a vacuum on the system by using the solenoids and monitor the amount of decay on the pressure sensor. If sensor values are changing, there is a leak. If these testing devices aren't available, there are smoke machines on the market that are compatible with evaporative systems to aid in leak diagnosis. Computers are fast and smart. If a component is inoperative and controlled by a computer, look for trouble codes. Some codes will inhibit operation of a component to protect itself, and/or for safety reasons. Do not assume the problem is solved if the malfunction indicator lamp (MIL) does not come on. OBD-II codes are labeled A, B, C and D. Type B codes require the failure to occur twice on two different drive cycles before turning on the MIL. Some codes have strict criteria for performing a self-test, and will not be performed unless the intake air temp and engine coolant temp are close to the same value at startup. This means a cold engine. It also means the engine completes a warm-up cycle (engine coolant temp raises 40 degrees Fahrenheit above startup temp and reaches a minimum of 160 degrees Fahrenheit). When dealing with modules and solid-state components, sometimes it is a good idea to disconnect the battery, remove power to the component, or clear codes before and after diagnostics or repairs since internal circuit protection or reset logic is incorporated into the module. With more modules being used, most aftermarket scan tools cannot communicate with systems other than the basic powertrain. Use flash codes and on-board diagnostics when available to confirm correct information from the scan tool and to obtain codes not available through the use of a scan tool. Examples: A computer-controlled cruise control can be inhibited if a trouble code is stored. An injector may be inoperative if the computer is detecting a cylinder misfire. The compressor clutch will be inoperative with a low refrigerant code. The low fuel lamp can be illuminated with a body trouble code. Not all codes will illuminate the MIL (for example, Codes C and D type will not). Notes about testing: If a system has a feedback circuit, testing should be backprobed at the component being tested, with all the components plugged in. An example would be temperature or mode doors in a heating and ventilation system equipped with actuator position sensors.
Remember: Up to 33 percent of all trouble code problems can be diagnosed by doing a visual inspection of related components.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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