The stored-energy hazards in a vehicle are compressed springs, pyrotechnic airbag and seat-belt pretensioner units, and pressurised systems such as fuel injection, cooling, air conditioning and gas struts, and each is made safe by an order of steps that starts with removing the battery negative terminal. Never cut, drill, heat or open an airbag, pretensioner, gas strut or air-conditioning line, never loosen a seated coil spring without a compressor, and never go under a vehicle held only by a jack.
A parked vehicle still holds energy in springs, explosive charges and trapped pressure, none of which announce themselves. The sign to look for and the safe order are different for each.
Each Stored Energy Needs Its Own Sign To Look For And Its Own Order To Release It
Coil springs. Hazard: the spring is held compressed between seats, and releasing the retaining nut frees it violently. The parent article says suspension springs store enough energy to kill. Sign: a coil spring seated on a strut or shock, held by a top nut or a cap. Safe order: support the vehicle, then either leave the strut assembly intact and remove it whole, or compress the spring with a proper spring compressor before releasing the nut. Keep your face and body out of the line of the spring.
Leaf springs. Hazard: tension in a pack of steel leaves held by a centre bolt and clamps. They are safer to remove than coil springs but still hold tension (parent article). Safe order: support the axle and the frame, clamp the pack, then undo the shackles and U-bolts.
Gas struts (bonnet, tailgate and boot lifts). Hazard: a sealed cylinder holding pressurised gas; heating, cutting or drilling it can throw the rod or burst the cylinder. Sign: a cylinder with a rod that resists being pushed in and holds a lid open. Safe order: prop the lid with a rigid stick, then lift the retaining clips from the end fittings and take the strut off whole. Do not cut, drill, grind or heat it.
Airbags. Hazard: undeployed airbags contain explosive charges and can deploy, breaking bones, if disturbed (parent article). Sign: trim marked SRS or AIRBAG in the steering wheel hub, dashboard, seats, pillars and roof edges. Safe order: disconnect the battery negative terminal first, then wait 10 minutes, the figure in the parent article, before working near airbag locations, so the backup capacitor can discharge. Waiting times differ between makes, so use a longer one if you know it. Do not probe airbag connectors with metal tools (parent article). Do not cut wire bundles to the unit, drill, strike, heat or burn it. If you must remove a module, unbolt it, unplug it, carry it with the cover facing away from your body and set it down cover side up.
Seat-belt pretensioners. Hazard: a small pyrotechnic charge in the belt retractor that fires in a crash and can fire if the unit is disturbed or heated. Sign: the retractor assembly at the pillar or buckle, often marked. Safe order: treat as a live charge. Do not open, drop, strike, heat or cut the retractor. Take webbing from the buckle or anchor end if you need straps, and leave the retractor in the vehicle.
Pressurised systems.
- Fuel injection holds 40-60 PSI of fuel pressure with the engine off (parent article). Remove the fuel pump relay and cycle the ignition to relieve it before disconnecting a line, and keep a rag round the fitting.
- Cooling system coolant is under pressure of 15+ PSI and above 100 °C (parent article). Never open a radiator cap on a warm engine; severe burns result.
- Air conditioning holds refrigerant under pressure. It frostbites skin, displaces air in low enclosed spaces, and some refrigerants are flammable. Leave lines closed, or open a fitting slowly outdoors, with gloves, eye protection and your face turned away. Never cut or heat a line.
The vehicle itself. Never work under a vehicle supported only by a jack. Use jack stands, ramps or cribbing, stacked wood or concrete blocks in a stable pyramid. A vehicle falling off a jack weighs 1,500-3,000 kg and is invariably fatal (parent article).
What You Need
- Jack stands, cribbing, wheel chocks.
- A spring compressor, clamps and ratchet straps.
- Eye protection, face shield, gloves and boots.
- Rags, containers and sand.
- A partner.
Method: The Safe Order
- Park on level ground, chock the wheels, set the brake.
- Remove the battery negative terminal first, every time; see Battery & Electrical Safety.
- Wait 10 minutes before working near airbags.
- Relieve fuel pressure; let the engine cool.
- Support the vehicle on stands or cribbing.
- Leave airbags, pretensioners, gas struts and air-conditioning lines closed unless you need them, and do not heat or cut any of them.
- Release springs only with a compressor or clamps.
Common Mistakes
- Taking a torch to a spring, strut or A/C line. Heat turns stored energy into shrapnel.
- Probing airbag connectors with a meter probe or screwdriver.
- Sitting in front of a steering wheel while removing its bolts. Stand to the side and carry the unit away from you.
- Opening a warm radiator cap.
Safety
Energy that was stored for years can release in a fraction of a second
Compressed springs, airbags, pretensioners, gas struts and pressurised lines can maim or kill when released unexpectedly. Stay out of the line of any spring, wear eye and face protection, and do not cut, drill, heat or open a part whose stored energy you cannot see. If you are not sure whether a unit is live, treat it as live and leave it in place. Work with a partner and stop if anything hisses, ticks or moves unexpectedly.
What’s Next
- Learn next: Safe Extraction Procedures — the whole order of operations
- Combine with: Springs, Belts, Bearings & Gears — using the springs once they are safely out