Vibration Analysis

Leroy Pershing
October 2023

A 2002 Peterbilt 379 day cab was brought to us with a vibration complaint. This 379 has a Caterpillar C12 engine 13-speed Eaton transmission and is operated at a local landscape company. The customer stated they can feel the vibration in all gears and speeds. This typically means the vibration isn’t coming from the driveline, transmission, rear ends, or wheel ends. Our vibration analyzer sees all of the vibrations simultaneously, so we proceed with the test. I could tell right away that something was wrong with the engine. The entire cab rattled and would have failed the Lexus trick of stacking wine glasses on the hood. I could see on our vibration analyzer an enormous spike at 35 HZ. This spike was 105-110% more than the highest vibration permitted at that frequency. There is actually a specification for maximum vibrations in your truck. I guess they have a specification for everything these days.

The vibration of 35 Hz at engine idle indicates that this is a 3rd order vibration. 3rd order means a vibration is happening 3 times for every engine rotation. So what would be happening 3 times for every one rotation of the crankshaft? Combustion! An inline 6 engine has a firing order of 1-5-3-6-2-4 and is phased 120 degrees apart. This means a cylinder fires, then 120 degrees of crankshaft rotation later. The next cylinder fires, and 120 degrees later, another cylinder fires. One rotation is 360 degrees. If you were to split that 3 ways evenly, you get 120 degrees. This even firing is what makes inline engines so smooth.

So, normal combustion means there are 3 bangs for every rotation, hence a 3rd order vibration. This vibration on the analyzer is normal and will always be there while the engine is running. However, since there is a specification, we can see that the vibration on this Caterpillar is much higher than it should be.

An abnormal 3rd order vibration can suggest a few things that might be wrong. A failed crankshaft damper (replace yours every 500k miles!), a bent crankshaft, some unbalance in the flywheel/clutch assembly, or abnormal combustion. Some of these options are more probable than others. We found that the customer had already replaced the crankshaft damper in attempt to fix the issue. We then inspected the clutch through the inspection panel, and it looked fine albeit you cannot see very much while everything is together. The next most straightforward test is a humble cylinder cutout test through diagnostic software. While the engine idles, we manually turn off and on every cylinder. If everything is working correctly, you can hear a significant difference in the tone of the engine. When we did this test, we heard no change on cylinder 5. Something was wrong with that cylinder. The ECM never threw a check engine light for this, and many people felt this vibration at idle was normal. They would say, “It’s a CAT, of course they shake.” This is the completely wrong answer and comes from people who don’t have a feel for how an engine should feel. This special touch comes from years of experience and why you should be careful where you take your truck. The solution for this vibration was simple. It was a failed injector. Once replaced the injector the vibration dropped by 130%! This was the first of many successful vibration analyses!

Written by: Leroy Pershing, Pittsburgh Power, Inc., 3600 S. Noah Drive. Saxonburg, PA, 16056. Phone (724) 360-4080, Email: [email protected], Website: