September, 2010

Vehicle accessories

How to spot a problem

Mark Lemay

In this article we will look at troubleshooting some of the more popular accessories such as Windshield Washers and Power Windows.

How you troubleshoot these systems will depend on how the accessory is controlled. Like most other vehicle systems, many of the functions that used to be handled by contact type switches have been replaced with computers and vehicle networks. In this article we are going to take a generic approach to computer controlled accessory systems.

With any accessory troubleshooting, you should start by pulling a wiring diagram to determine how the accessory operates. Next, if the accessory is computer controlled, you should scan the module and check for any trouble codes that may be present. 

Problems can be broken down into three major failure categories.

First

A feature of the system fails to operate, such as a power tailgate will not lift or a sliding door will not open and a trouble code is lodged in the module. By following the published diagnostic procedure location of the fault should be straight forward.

Secondly

The module is completely inoperative and will not respond to the scan tool or communicate. To test this type of failure, verify the module power and ground circuits and then test the communication circuit between the DLC connector and the module. If the communication circuit and powers and grounds test good, then the module will have to be replaced. Be sure to check the module installation procedure before quoting the customer, as many modules now require programming and configuration to the vehicle before they will work properly.

Third

The most common type of failure is an output device such as a power window or door lock that will not operate. For this example, we are going to use a passenger window that will not operate from a GM vehicle. A quick check for codes revealed no faults so a wiring diagram will have to be consulted to determine how the system works. The wiring diagram shows the powers and grounds, motor wiring and the communication circuit. The wiring diagram also shows that the switch is in fact a module. With a scan tool, observe the window switch reading and move the switch to the down position. Scan data should show the switch in the down position. (Fig 1) If the data does not match the switch position, then the switch/module is defective and will have to be replaced. If the switch shows the correct input, then the module should respond and drive the window down. To check the window circuit, access the wiring to the window motor and using a voltmeter as shown in figure 2, measure the voltage to the motor while operating the switch. If you measure battery voltage, then the window motor is defective and will have to be replaced. If battery voltage is not achieved, then the module is defective and will require replacement.

Following these simple troubleshooting steps should allow you to diagnose most accessory failures.