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Replacing worn truck cab suspension

Installing new under-cab air bags to avoid an on-road failure

More than a few retired highway tractors find second lives pulling grain trailers on farms. While they may have logged a lot of miles, these trucks are still ideal for short-haul farm duty. But with age comes the need for maintenance.

One of the most common maintenance items on today’s older highway tractors is the need to replace the air bags and shock absorbers the make up the cab suspension. Rubber air bags can deteriorate quickly in our rugged environment. Fortunately, replacing them is a straightforward DIY project.

On this 2007 Freightliner Century, both air bags were deteriorated, with chords showing at the lower corners. So replacement was in order before one blew on the road. And while we were at it, replacing the shocks made sense as well. The combined parts bill for this job came in at just over $200 for two new air bags and two shocks.

Using a blade cut the small airline flush with the bottom of the airbag inlet, as seen here. The fraying rubber on the airbag indicates it would soon fail.
photo: Scott Garvey

To get them installed, we first removed the shock on one side. This is just a matter of removing two bolts, but it if the top bolt is rusted into the shock, getting it out can be a very frustrating process. The lower edge of the rear cab wall comes down just low enough to prevent getting a punch or any tool directly on the bolt to knock it out an awkward process. (Adding some anti-seize to the bolt when replacing it with a new one will make the next mechanic’s job that much easier.)

On the day cab version of this same truck model, access to the upper shock mount bolt is much better, making it easer to remove.
photo: Scott Garvey

Next, we placed a small hydraulic jack on the truck frame and lifted the cab a few inches until the air bag on that side deflated on its own. So, it’s not necessary to drain the truck’s air system to replace these bags.

With the bag deflated it can simply be pulled out of its brackets under the cab and then on the frame cross member. Take a blade and simply cut the small airline off flush with the bottom of the inlet on the old bag and its out. To install the new air bags, just slide the airline into the inlet on the new bag until it clicks into place, then install the bag in the slots on the frame and cab, the reverse of the removal process.

Place a small hydraulic jack on the frame rail between the air bag and shock to lift the corner of the cab and remove the weight from the bag. Lift until the bag deflates on its own. It can then simply be pulled out its mounting brackets and the small air line cut off flush with the inlet at the bottom of the bag.
photo: Scott Garvey

Very slowly lower the jack until the airbag re-inflates. Be careful to stand as far back from the bag as you can during this process. The bag will re-inflate with a pop. You don’t want to be near it if it fails for some reason. Remember not all new parts are perfect!

Install the new shock and then move on to the other side and repeat the process. That’s all there is to it.

About the author

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Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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