4L80E 4WD Highway Speed Vibration on Smooth Surfaces - keywords 246EAU 261 263 263EAU axle case GT4 GT5 high noise prop propshaft rattle resonance shaft transfer

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Highway speed vibration on K2500HD 4WD vehicles equipped with a 4L80E automatic transmission and a one piece propshaft. EVA readings will show a first order driveline vibration.

The cause of this vibration can be several. Listed here are some of the known causes and likely improvements. Different vehicles may respond to different corrections.

1. Compare the customers concern with an identical vehicle with similar mileage. Also inspect for any aftermarket running boards or step bars that could ground out transferring vibrations into the cab. Some vibration is inherent to this type of vehicle. If the customer's vehicle has a more intense vibration (as compared with the EVA), continue with the following.
2. Note during the road test if the amplitude level can be changed by cycling the transmission to neutral, uncoupling the transmission and TCC, re-engaging in D4 and resuming. Remember to obey all local speed limits. This may indicate another part of the driveline other then the propshaft may be causing the vibration.
3. Propshaft balance and runout. Operate the vehicle at the concern speed with the front propshaft removed. If the vibration is unchanged, check the rear propshaft runout and correct if needed. If the runout is within specifications, system balance the front of the rear propshaft using the EVA.
4. Transfer case output shaft, slip yoke, bushing area. Inspect the fit of the slip yoke to the output shaft and transfer case output bushing. Compare the fit to a known good vehicle. Correct as necessary. If there seems to be excessive clearance in this area, it may contribute to the vibration. Also, indexing the slip yoke in different positions in relation to the transfer case output shaft looking for the lowest possible runout will help this concern.
5. An improvement can sometimes be made by installing a dampened rear axle propshaft flange from a truck equipped with an 11.5" axle. Part number for this dampened flange is 12471501 (use any future number that this number supersedes to).

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.