Bottom Line
Maintenance Q&A
Rough rides

By Paul Abelson, senior technical editor

Q. I enjoy reading your articles. I try to make myself believe they make me “mechanically inclined” ... LOL.

I have a 2006 Freightliner Classic XL with a terrible problem that has been costing me way too much money. When I go to the shop, no one can find or figure out what I’m talking about. They say, “I can’t get the truck to duplicate your complaint.”

The truck rides so rough, like a bucking bronco. I find myself holding on extra hard when I see a rough patch of road coming up. It feels like the whole truck is falling apart underneath me. It shakes, rattles and vibrates so bad that it has broken all the seals in the engine and differentials.

I have had the drivelines tested, the yokes replaced, new shocks, new tires, and the air ride height adjusted so many times – I don’t know where normal is anymore. I have had to replace my refrigerator twice because the bouncing made the compressor go out.

Now I have a mechanic telling me the leaf springs have lost their bow on the front end.

Any idea what’s going on?

A. When a shop has been involved as much as your Freightliner dealer has, they usually cover the obvious. With the help of my brain trust, Carl Tapp and Tom Tahaney, we’ll examine other possible causes.

Harmonic vibration starts at the transmission tail shaft bearings and transmits to other components, such as the A/C compressor mounts, causing damage. When it becomes noticeable, it can literally tear a truck apart.

A worn vibration damper allows the severe vibrations. However, dampers are usually good for 10 or more years and are replaced during engine overhauls, so let’s move on.

As many people as have looked at the driveline, one would assume that phasing, angularity, wear, damage, etc. had been checked and repaired or adjusted as needed. That includes the suspension.

Freightliner air ride is simple to check and adjust. One indication that the ride height is set wrong is transmission synchro pins breaking. As for all the seals failing, even though you didn’t mention total truck mileage, most seal warranties are good for about 300,000 miles.

And you mentioned tires. Vibration can be caused by improper concentric mounting to the wheel. Check the “GG” ring near the bead for consistent spacing from the rim. Other possible tire fixes are dynamic balancing on the truck. Make sure all wheels are seated properly on the hubs and check air pressure.

The springs could be worn out, but shouldn’t cause the bouncing. You might also go back and check previous repairs on the drive shafts (yoke alignment and shaft angularity). If misaligned, they can create strong vibrations. Then start looking at three-axle alignment (parallelism).

Last, but not least, check front end alignment. Inspect front-end components for slack, wear, and lack of grease. Check toe, caster and camber. Toe problems should be obvious in tire wear. Caster may affect “road walk” or “lane drift.” Camber usually affects only cornering and is noticed as a feeling of oversteer.

After reviewing all these common possibilities, I got a surprise follow-up note from our reader.

She talked with a mechanic who ran across a truck with the same problem.

“Believe it or not, the air ride was checking out correctly but the air ride leveling valve was the problem. The air bags’ pistons were not getting the same extension at the front axle as they were in the back,” she wrote.

That will make the truck ride as described. Once a properly operating leveling valve was installed, the reader said her “truck rides beautifully.” LL


Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine,
PO Box 1000, Grain Valley, MO 64029; e-mail them to truckwriter@anet.com or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.

July Digital Edition