Air Cab Suspension: One Inch Low and Bottoming Out
2019 conventional cab tractor with air cab suspension: two bags at the rear of the cab fed through a tee from one height control valve, plus two cab shocks. 310,000 miles. The driver complains the cab bottoms out on bridge joints and that the left rear corner of the cab sits low. On level ground at full pressure: left rear cab height 5-1/2 in, right 6-1/2 in, spec 6 in ± 1/8 in. Both bags hold pressure for 24 hours with no leak found by soap testing. The valve link is present but its lower rod is bent about 15°, and it is in the second hole from the end. Cab mounts, pivot bushings, and shocks are all original. The cab carries an APU and added equipment.
1. Can a single valve produce a 1 in side-to-side split? Does the bent link explain it?
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Average the heights: (5.5 + 6.5) ÷ 2 = 6.0 in, exactly spec. With one valve feeding both bags through a tee, the valve is holding the correct average, so do not adjust the link as a height correction.
A single valve cannot create a split by itself - both bags see the same pressure, and the valve closes on average height. The split has to come from:
- One bag not producing full force at that pressure (internally damaged, wrong bag, collapsed internal bumper).
- A restricted or kinked line on one side.
- Mechanical difference: bent or worn bag perch, worn cab pivot or mount bushing, or a cracked mount bracket letting that corner sit low.
The bent link is not the cause of the split. It changes the link ratio and can make the valve hunt or over-travel, so straighten or replace it - but do it after the split is diagnosed, and re-check the average height afterward, because correcting the link will move both sides.
2. Diagnose the bottoming complaint. What do you check and in what order?
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Bottoming is a damping and travel problem, not a height problem. The bags hold pressure and the average height is correct, so the cab is not resting on the stops - it runs out of travel over a bump.
Check in this order:
- Shocks. At 310,000 miles they are the prime suspect. Disconnect one end and stroke it - a good shock has firm, smooth resistance both directions. No rebound lets the cab drop and slam the stops on every joint, and it lets the cab oscillate after the bump.
- Bump and rebound stops - missing, broken, or crushed. With the stop gone, the bag takes the impact and can be damaged internally.
- Cab mounts and pivot bushings - wear allows more cab motion than the suspension was designed for and can feel like bottoming.
- Weight - the added APU and equipment may have exceeded what the cab suspension was rated for. Compare against the rating before condemning parts.
Replace shocks in pairs, then re-measure height and road test.
3. After replacing the shocks and correcting the split, what verification steps close this job out?
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- Re-measure both sides on level ground at full pressure after a dump and refill, and confirm the split is within 1/8 in and the average is within spec.
- Re-check the valve link geometry - with the bent rod corrected, confirm the link is in the specified hole and the valve is not hunting or over-traveling at either end of the stroke.
- Leak-check the repaired side again after the road test; disturbed fittings are a common source of a new, slower leak.
- Road test on the same bridge joints the driver complained about, loaded as the truck normally runs. Bottoming that persists points to weight or the stops, not the shocks.
- Document the measured heights before and after and the shock condition, so the next complaint about cab ride starts from data rather than from scratch.