Bi-directional tests improve the productivity and, ultimately, the profitability of a scan tool in any automotive service facility. Although this feature increases the tool's price, its diagnostic value more than offsets the extra cost.
During the 29 years I have authored this column, I have observed traditional tire dealers delving deeper and deeper into general automotive repair.
This endeavor has required greater investments in diagnostic equipment — especially the ubiquitous scan tool. Various owners and managers have quizzed me about the features, benefits and related costs on these troubleshooting tools.
On one hand, I do not and would not expect tire dealer executives to be as informed as their technicians are on diagnostic issues. But on the other hand, a topic such as bi-directional controls is easier to grasp than most bosses realize.
Nonetheless, I will keep this discussion as simple as practically possible.
For openers, a scan tool connects to an industry-standard data port under the dashboard. This port is commonly called a DLC connector.
A scan tool communicates with the vehicle's computer(s) through the DLC.
As the cost and complexity of a scanner increases, the tool does much more than, perhaps, read and erase trouble codes. The more-expensive scanners display line upon line of computer test data in either alphanumeric and/or line graph-formats.
Arguably, the next level of scanner sophistication is a tool that offers bi-directional control, also known as "active" testing.
An active test enables a user to command an on-board computer to perform a task for diagnostic purposes consciously. (Tire dealers who service TPMS are using a version of an active test.)
But here, I'll use the example of a horn.
On older vehicles, touching the horn button energized an electrical relay called the horn relay. When activated, the horn relay routed electrical current to the horn. Then the horn honked.
But today, honking a horn may follow this format or a similar one. An on-board computer sends a voltage signal to the horn button on the steering wheel.
Pressing the horn button closes a switch, grounding the voltage signal. ("Grounding" means pulling the signal down to zero volt.)
When the appropriate on-board computer measures zero voltage at the horn "input" terminal, it concludes that the driver wants to blow the horn. Therefore, that computer energizes the horn relay, honking the horn.
Now suppose that your technician has a scan tool equipped with a wide selection of active tests.
The work order for the vehicle at hand states that its horn doesn't work. The tech connects this scanner to the DLC under the dashboard and then finds an active test for the horn on the scan tool menu.
Next, he uses it to command the horn to operate.
Honk. The horn operates as commanded.
If issuing the scan tool command made the horn honk, we can draw several time-saving conclusions.
First, the horn relay, horn and related wiring are all working. Second, the computer also should be working normally because it performed the task that the tech commanded.
Now the tech can focus on potential problems on the input side of the computer system such as the horn button switch and its related wiring.
A scanner's active tests enable techs to command various components (for example, cooling fans, fuel injectors, camshaft solenoids, etc.) to operate during diagnosis. It's difficult to tally how much troubleshooting time active tests save in the course of a year.
Note that bi-directional controls have become much more common on aftermarket scanners.
But typically, OEM scan tools or diagnostic software offer the most-robust menus of active tests for a given make and model of vehicle.