Springs
Springs set the ride height and store frightening amounts of energy doing it.
- Ride height is set by the spring. A broken coil, sagged spring, or (on trucks) a broken leaf reduces ride height at that corner.
- Spring compressors contain the enormous stored energy in a coil spring during strut disassembly and reassembly. Skipping them can be fatal.
- An electric air compressor (usually underbody or in a wheel well) supplies pressurized air to the air springs, controlled by ride height sensors.
- Air suspensions can activate and lift/lower the vehicle unexpectedly. Disable via the service switch or fuse before working underneath.
Whether you're diagnosing a saggy corner, disassembling a strut, or sliding under an air-suspension vehicle, spring work is where careless techs get hurt — so this is as much a safety lesson as a diagnostic one.
Ride Height Lives in the Spring
When a vehicle sits low on one corner, think spring first. Ride height is set by the spring — a broken coil, a sagged and fatigued spring, or on trucks a broken leaf drops that corner. Shocks and struts don't hold the car up (the spring does), so a worn damper won't cause a lean, and neither will a soft bushing or a worn tie rod. Measure ride height at all four corners against spec; a corner that's low tells you where to look, and a broken coil often hides in the lower perch under the rubber isolator where you have to look deliberately to spot it. Remember the downstream effect too: wrong ride height changes every alignment angle, so fix the spring before you align.
Working Safely: Coil Compressors and Air Suspension
A compressed coil spring on an assembled strut stores enormous energy — enough to maim or kill if it releases uncontrolled. Coil spring compressors exist to contain that energy during strut disassembly and reassembly. You clamp the compressor onto the coils, take the spring tension off the upper mount, and only then remove the strut shaft nut. Skipping the compressor, using a damaged one, or seating the jaws carelessly can be fatal. This is one of the few jobs in the shop where the failure mode is a projectile.
Air suspension adds its own hazards and hardware. These systems replace steel springs with air springs fed by an electric compressor, typically mounted under the vehicle near the air reservoir (or in a wheel well), with ride height sensors telling the control module when to add or release air. The critical safety step: before working under an air-suspension vehicle, disable the system using the service switch or by pulling its fuse. The system is happy to self-level with the ignition off in some designs — the compressor can kick on or air can vent, and the vehicle can lift or drop unexpectedly while you're under it. Disable first, then lift, then work.
| Item | Standard | Notes |
|---|---|---|
| Side-to-side ride height difference | Typically 0.5 in. max | Measured at manufacturer-specified points — verify |
| Low corner cause | Spring — broken, sagged, or fatigued | Dampers do not set ride height |
| Broken coil location | Commonly the bottom coil, hidden under the isolator | Look deliberately; it hides |
| Replacement policy | Axle pairs | Keeps rate and height matched across the axle |
| Air suspension service prep | Disable via service switch or fuse before lifting | System can self-level with the key off |
| Corrosion pitting on coil wire | Replace | Pits are stress risers that grow fatigue cracks |
Ride height specs and measuring points are vehicle-specific — verify against service data.
- On a level floor, measure ride height at all four corners at the manufacturer's specified points. The numbers tell you which corner is low and by how much — eyeballing lies on sloped floors and low tires.
- Inspect the low corner's spring deliberately: pull back the lower isolator to check the bottom coil for a break, and on leaf springs look for a cracked or flattened leaf and a sheared center bolt.
- Rule out the impostors — a collapsed spring isolator, a shifted spring seat, or prior collision damage can drop a corner with the spring itself intact.
- On air suspension, scan for codes, watch the ride height sensor data, and soap-spray the air springs and lines. A compressor that runs constantly is chasing a leak; a corner down overnight has one.
- Replace springs in pairs, then send the car to the alignment rack. Ride height sets every alignment angle, so the spring gets fixed first and the alignment is measured after.
- Quoting shocks or struts for a sagging corner — dampers control motion, springs hold the car up. The sag survives the new dampers and so does the complaint.
- Disassembling a strut with a worn or carelessly seated spring compressor. A compressed coil stores enough energy to maim, and a compressor that slips at full tension is a shrapnel event.
- Sliding under an air-suspension vehicle without disabling the system — some designs will self-level with the ignition off, and the vehicle can drop while you are under it.
- Replacing one spring on an axle. Mismatched rate and height across the axle changes handling and cocks the alignment.
The left rear sits 3/4 in. lower than the right, and the damper on that corner is dry and passes a bounce test. Where do you look?
The spring. Ride height is the spring's job — check the bottom coil under the isolator for a hidden break, and compare against the other side. The healthy damper is irrelevant to height; it never held the car up in the first place.
An air-suspension SUV is level at night and down on one corner by morning. What is your diagnostic approach?
A leak hunt. Overnight sag means air is escaping with the system asleep — soap-spray that corner's air spring, its line, and the valve block connections and watch for bubbles. Check compressor runtime too; a compressor working overtime to mask a leak is on its way to burning out.
Why does a spring replacement always end with an alignment?
Ride height defines the suspension's working position, and camber, caster, and toe all change as the suspension moves through its arc. New springs restore the height the geometry was designed around, which shifts every angle away from where the worn springs had settled — so the angles must be re-measured and set at the corrected height.
The left rear sits 3/4 in. lower than the right, and the damper on that corner is dry and passes a bounce test. Where do you look?
Missed one? The reasoning above comes straight from the SUS exam bank, so this is the standard you will be held to.