Coast Test

NOTICE: When performing a "Coast Test" on a vehicle equipped with an automatic transmission, caution must be taken when shifting transmission range selector lever from "NEUTRAL" to "DRIVE" to avoid transmission damage.

When shifting from "NEUTRAL" to "DRIVE", remember to:
1. Maintain engine speed at 1500 RPM.
2. Brake to 40 km/h (25 mph).
3. Return transmission range selector lever to "DRIVE."

Drive the vehicle past the vibration speed, shift into "NEUTRAL" and coast back through the vibration speed. During a coast test, two kinds of vibrations may occur: shaking or a buzzing. A shaking vibration is usually a problem with the tires, wheels, brake drum and/or brake rotor. A buzzing vibration is usually a driveline problem.

To determine whether a propeller shaft is causing the problem, drive the vehicle through the speed range and note at which speed (vehicle and/or engine) the problem is most pronounced. A tachometer may be used. Shift the transmission into a lower gear range and drive the vehicle at the same engine speed as when the problem was most pronounced in direct drive. Note the effect on the problem.

To determine the engine speed if a tachometer is not used, divide vehicle speed by the transmission gear ratio in which the problem occurs.

Example: With the HYDRA-MATIC 4L60-E automatic transmission in 2nd gear range, divide by 1.63. If the problem is most pronounced in direct drive at 88 km/h (55 mph) the same engine speed would be produced in 2 range at 55 km/h (55/1.63 =34 mph).

If the problem is still present at the same engine speed whether in direct drive or a lower gear range, a propeller shaft cannot be at fault since a propeller shaft speed varies. If the problem decreases or is eliminated in a different gear range at the same engine speed, a propeller shaft may be at fault. Refer to "PROPELLER SHAFT RUNOUT MEASUREMENT" and also "PROPELLER SHAFT AND UNIVERSAL JOINT DIAGNOSIS."