Probable Causes
The probable root causes are listed in order below:
• Mounting Brackets
• Unbalance of the Clutch Disc
• Alignment of the Clutch Disc
• Misfiring Cylinder
• Resonance of the Vehicle
Recommended Actions
Perform the following diagnostic procedures in order:
1. Check for loose mounting brackets.
a. Start the engine.
b. Run the engine through the entire Idle Speed range. Check all vibrating components for the following problems:
• Loose mounts
• Broken mounts
• Loose brackets
• Broken brackets
• Loose fasteners
• Broken fasteners
2. Check for unbalance of the Clutch Disc and/or misalignment of the Clutch Disc.
a. Start the engine.
b. Run the engine through the entire Idle Speed range while the clutch is disengaged.
If the vibrations are reduced, there is a balance problem with the clutch disc or the
clutch disc is out of alignment.
c. Repair the clutch disc or replace the clutch disc.
3. Check for a misfiring cylinder.
Further analysis requires the use of a vibration instrument. Any instrument that can accurately measure the displacement of the vibration and the frequency of the vibration will be sufficient. The 4C-3030 Vibration Analyzer can be used to provide a quick identification and complete identification of all vibration frequencies that are present in a constant speed measurement. Refer to Special Instruction , SEHS7914, “Troubleshooting Vibration In Vehicular Equipment” for additional information.
If the frequency of the vibration is half of the engine rpm (half order), the vibration is caused by a misfiring cylinder.
If the frequency of the vibration is 3 times the engine rpm (third order), no corrective action
can be taken on the engine because this is the firing frequency of the engine. The problem is in
the resonance of the cab or the problem is in the resonance of the chassis.
4. Check for a problem regarding the resonance of the vehicle.
a. Measure the vibration of the engine through the entire speed range.
b. Record the speed of the vibrations when the filter is out.
c. Record the amplitudes of the vibrations when the filter is out.
If all of the amplitudes are less than 4.0 mils, the engine performance is within the specification.
d. If the amplitudes are greater than 4.0 mils, run the engine at high idle when the filter is installed.
e. Check all vibrations over 4.0 mils for each of the following orders of frequency:
• Vibrations of the first order
• Vibrations of the second order
• Vibrations of the third order
• Vibrations of any other order
Perform the following tests in order to check the vibrations:
1. Check the vibrations of the first order.
a. Remove the vibration damper.
b. Recheck the vibration.
If the amplitude of the vibration is less than 4.0 mils, check the damper pilot.
If the runout of the damper pilot is less than 0.10 mm (0.004 inch), install a new damper.
If the runout of the damper pilot is greater than 0.10 mm (0.004 inch), correct the runout.
c. If the amplitude of the vibration was greater than 4.0 mils after the removal of the damper, remove the transmission’s pressure plate and clutch disc.
d. Recheck the vibration.
e. If the amplitude of the vibration was greater than 4.0 mils after the removal of the transmission’s pressure plate and clutch disc, an engine component is out-of-balance.
The following components could be out-of-balance:
• BrakeSaver
• Connecting rod
• Crankshaft
f. If the amplitude of the vibration was less than 4.0 mils after the removal of the transmission’s
pressure plate and clutch disc, check the flywheel runout and the flywheel housing runout.
If the flywheel runout and the flywheel housing runout are within the specification, there is
a problem with the clutch and/or a problem with the transmission.
2. Check the vibrations of the second order.
There is a heavy piston and connecting rod or there is a light piston or connecting rod.
3. Check the vibrations of the third order. The vibration is being caused by the frequency
of the firing order. A problem with resonance exists within the vehicle.
4. Check any other significant orders of vibration.
a. Check all driven accessories.
b. Check all fan blades.
c. Check for excessive gear backlash.
