Road Test
ROAD TESTNoise, vibration and harshness (NVH) diagnosis must start with the customer interview and be followed by a road test.
NVH usually occurs in five areas:
- tires
- engine accessories
- suspension
- drivetrain
NOTE: A gear-driven unit will produce a certain amount of noise. Some noise audible at certain speeds or under various driving conditions is acceptable. Slight noise is not detrimental and must be considered normal.
- body and interior trim panels
It is important that an NVH concern be isolated into its specific area as soon as possible. The easiest and quickest way to do this is to perform a road test.
1. Use the Vehicle Road Test Evaluation form. Make notes on the backside of the form throughout the diagnostic routine.
2. Make a visual inspection as part of the preliminary diagnostic routine, writing down anything that does not look right. Note the tire pressures, but do not adjust them yet. Note leaking fluids, loose nuts/bolts, or bright spots where components are rubbing against each other. Check the cargo area for unusual loads.
NOTE: Do not make adjustments until a road test and a thorough visual inspection of the vehicle is performed. Do not change the tire pressure or the vehicle load. This may reduce the condition's intensity to a point at which it cannot be identified clearly. It may also inject a new condition into the concern, preventing proper diagnosis.
3. Perform the Neutral Engine Run-Up (NERU) test. It identifies engine or exhaust-related vibrations and helps in sorting out vibrations that are found in the road test.
a. If the vehicle is equipped with a tachometer, it can be used. Otherwise, a tach must be connected.
b. Locate the vehicle away from other vehicles and walls which may reflect sound differently than a road.
c. Put the vehicle in N (neutral) or P (park), and do not set the parking brake or press on the service brake since these are not engaged in a road test.
d. Run the engine rpm up from an idle to approximately 4000 rpm and note any moans, vibrations, noises, etc., and the rpm at which they occur. Sometimes it is possible to "tune in" on these conditions by running the rpm up and down to determine a precise rpm at which they occur; in other cases they will fall over a broad range of rpm. This establishes a baseline against which driving vibrations can be measured.
e. If you suspect that the exhaust system is vibrating, hang a ring of keys or something similar from the tailpipe and listen for the rattling of the keys as the engine is run up and down.
f. When conducting the road test, refer to the results of the NERU test to distinguish vibrations and noises caused by the engine from those caused by the transmission, the driveshaft , or the axle. If a vibration or noise occurs at a particular speed, try operating in another gear at the same speed. This changes the engine rpm and helps to distinguish engine induced vibrations and noises. If it occurs at a particular rpm, use different gears to test at the same rpm at different road speeds.
4. Perform the drive engine run-up test.
WARNING: SET BOTH THE PARKING BRAKE AND SERVICE BRAKE AND MAKE SURE TO PERFORM THE TEST WITH ENOUGH SPACE AHEAD OF THE VEHICLE TO ELIMINATE THE POSSIBILITY OF AN ACCIDENT IF THE VEHICLE UNINTENTIONALLY LURCHES FORWARD.
CAUTION: Do not conduct this test for over 30 seconds, or without periodic driving or shifting to neutral to circulate transmission lubricant; otherwise the transmission will overheat causing severe damage to the automatic transmission.
Place the transmission selector lever in D (DRIVE) and run the engine rpm up and down between an idle and approximately 2000 rpm. Note the nature of any vibrations and noises and when these concerns occur in relation to engine rpm.
5. Road test the vehicle and define the condition by reproducing it several times during the road test.
NOTE: The type of road and its surface condition are important factors in the road test. A smooth asphalt road that allows driving over a range of speeds is best. The brushed concrete road surface found on many expressways and the coarse aggregate sometimes found in concrete can mask many vehicle noises and make NVH diagnosis difficult.
a. A tachometer must be used.
b. Note the fuel level. Some vehicles change in their response to various excitations when the fuel level changes.
c. Try to duplicate the conditions with the customer present, particularly the speed and throttle operation.
d. Find the speed where the concern is most severe.
e. Accelerate gently through this speed to a few mph above it and then coast back down a few mph below it and note if the concern changes.
f. Repeat this procedure, if necessary, to get a feeling for the behavior. Then drive about five mph above the speed, put the transmission in NEUTRAL, and coast down. Note any change in behavior.
g. Try "floating" the driveline by backing off slightly on the throttle at the speed at which the condition occurs. The idea is to unload the axle gears and the universal joints as much as possible. If the concern does not change in all these modes of operation, the cause may be driveline imbalance since the imbalance is not changed by the throttle position.
6. Perform the road test quick checks as soon as the condition is reproduced. This will identify the proper method of diagnosis. Run through the quick checks more than once to make sure you are getting a usable result. Road Test Quick Checks