Road Test

Perform a road test to compare actual vehicle braking performance with the performance standards expected by the customer. The ability of the test driver to make valid comparisons and detect performance deficiencies will depend on experience.

The driver should have a thorough knowledge of brake system operation and accepted general performance guidelines, in order to make good comparisons and detect performance problems.

An experienced brake technician will always establish a route that will be used for all brake diagnosis road tests. The road selected will be reasonably smooth and level. Gravel or bumpy roads are not suitable because the surface does not allow the tires to grip the road equally. Crowned roads should be avoided because of the large amount of weight shifted to the low set of wheels on this type of road. Once the route is established and consistently used, the road surface variable can be eliminated from the test results.

Before a road test, get a complete description of the customer concerns or suspected condition. From the description, your experience will allow you to match possible causes with symptoms. Certain components will be tagged as possible suspects while others will be eliminated by the evidence. More importantly, the customer's description can reveal unsafe conditions which should be checked or corrected before the road test. The description will also help form the basic approach to the road test by narrowing the problem to specific components, vehicle speed, or conditions.

Begin the road test with a general brake performance check. Keeping the description in mind, test the brakes at different vehicle speeds using both light and heavy pedal pressure. To determine if the problem is in the front or rear braking system, first use the brake pedal, then use the parking brake. If the condition (i.e., pull, vibration, pulsation) occurs only with the parking brake, the problem is in the rear system. If the condition occurs when the brake pedal is depressed, the problem is in the front system.

Avoid locking the brakes and sliding the tires. Locked brakes and sliding tires do not indicate brake efficiency. A heavily braked and turning wheel will stop the vehicle in less distance than locked brakes. If the problem becomes evident during this check, make sure it fits the description given before the road test. If the problem is not evident, attempt to duplicate the condition using the information from the description.

If a problem exists, use the Symptom Chart in order to isolate it to a specific sub-system and condition description. From this description, a list of possible causes can be used to further narrow the cause to a specific component or condition. Symptom Related Diagnostic Procedures