Diagnostic Information for Cab or Body Vibration/Noise at Speeds above 72 km/h (45 mph)

Some customers may comment on a vehicle vibration and/or noise while driving at speeds of 72 km/h (45 mph) or greater. This condition may be difficult to isolate or localize to a specific component. Symptoms may mimic a driveline, engine, suspension, transmission or tire and wheel assembly concern.

Airflow or wind may cause a cab or roof-mounted service platform (catwalk), installed by a refrigeration upfitter, to vibrate. The platform is intended as a surface for refrigeration technicians to stand on to avoid damaging the cab roof panel while servicing the rear body mounted condenser and auxiliary engine. Certain platform designs or installations may contribute from mild to severe cab and chassis vibration or noise. It is possible in some cases to install certain components of these platforms in a reversed position or to improperly locate components fore or aft of ideal.

Caution: You MUST remove the platform from the cab roof to achieve a reliable diagnosis. Do not attempt to add weight (ballast) or fabricate any type of airfoil or deflector for testing. These items may not isolate the critical area of the platform and may create an unsafe condition for surrounding motorists.

1. Remove the platform from the cab roof to confirm the condition. This will usually involve drilling out driprail attaching rivets.
2. Test drive the vehicle. If possible, have the owner/operator present to confirm that the intended vibration has been eliminated.

Any modification and/or permanent removal of the platform is to be performed at the owner's expense.

The above concern may have many possible sources. This bulletin is not intended as a substitute for diagnostic troubleshooting. The service platform should instead be considered as a possible and quickly identifiable known vibration source, whenever a vehicle so equipped is presented with a wind noise or vibration concern. Additional information about this concern and how to avoid it will be published in the Isuzu N-Series/Chevrolet, GMC W Series Body Builder's Guides.