Join Date: Sep 2013
Rated Cap: None
[REQUESTED] - Pre-Trip Safety Inspection Checklist
FOREWORD: Remember the old “hip bone's connected to the thigh bone, the thigh bone's connected to the leg bone” song? It sounds corny, but inspecting each component of a vehicle is another piece of the song. "The axle bone's connected to the spring bone, the spring bone's connected to the frame bone". Remember, NO exterior parts are irrelevant to safety, and very little on the interior. And contrary to the old saying, what you don't know CAN kill you. EVERYTHING in this list can potentially keep you and your rig out of SERIOUS trouble.
The list totals 85-125 items - most are repetitive (multiple rims, tires, brake components, shocks, hubs, springs, mounting hardware, lights, glass to check). But it's all there for a reason. An instructor told me, “If you see it, look at it. It's there for a reason. The vehicle will talk to you, IF you know how to listen.” Sounds crazy, but he was right, some common sense in looking at various components and hardware will tell you what you are looking for, and why it's important.
What must be checked on each component varies, based on which system it is part of. Remember, NO part is irrelevant when you get right down to it. As I go down this list, you'll understand why.
CHECK EVERYTHING for proper mounting, cracks or other damage. Anything loose or cracked can create bigger problems if not tended to.
Check METAL for rust, cracks, welds, and missing hardware such as mounting bolts / rivets, anything compromising proper function, secure mounting to the vehicle, potential failure from metal fatigue, or potential to damage other components working in conjunction with it.
Check EXHAUST for leaks, proper mounting, and damage. Leaks will show up as carbon or soot at the source.
Check RUBBER for ABCDFR -- Abrasions, Bulges, Cuts/Chafing, Dryrot, Fraying and Rot (Hereafter referred to as ABCDFR). These can be minor (loose / rattling exhaust), or major (suspension / tires).
Check GLASS OR PLASTIC for cracks / breakage. Check light housings for moisture / condensation, which can blow bulbs.
Check FLUIDS for proper level, condition, leakage or contamination. This can cause other problems. EXAMPLE - Radiator-based automatic transmission coolers can mix coolant with trans fluid, causing premature transmission failure. Coolant-contaminated engine oil can appear milky, causing serious engine damage if uncorrected. High temperature or consistently bubbling coolant can indicate this as well.
If any system needs fluid added, check what the system calls for per application against what is currently in the system. They're NOT necessarily all the same, even comparing against the same engine, depending on model year. Incorrect / incompatible fluids can cause serious damage and expensive repairs that might have been prevented. This applies to all fluids, especially coolant. USE THE RECOMMENDED FLUID! Do NOT overfill as it can cause leaks and serious damage, particularly with engine oil.
Remember, small problems not tended to, quickly become larger, more serious, even dangerous.
For the CDL drivers here (I still have my Class A), I deviate from DOT sequence, for a few reasons. Steering components are normally regarded as part of the engine compartment, but as RE steering components are on the other end, I've split some portions to make things easier to follow for folks with different engine configurations. I've not driven commercially since 2017 due to health issues. So while I can't promise I missed nothing, I've double-checked where possible, noting / adding anything overlooked.
For those without a CDL, doing this checklist right should take about 20-30 minutes – I think CDL drivers will agree. It's a long read, but well worth it. Doing this checklist before travel can prevent breakdowns, unnecessary failures and dangerous situations from taking away from the fun. Not to mention fixing something before travel is a lot cheaper and less trouble than a tow truck. Not everything guarantees a problem that trip, but certain defects will obviously be more urgent than others.
NOTE: I recommend wearing a mask when inspecting brake components, brake dust is carcinogenic.
Are we ready? Good.
Approaching the vehicle, look for tires that appear low on air. If air-ride equipped, also look for any corner of the vehicle sitting low or leaning more than is warranted for where it is parked – this could be low tire pressure, but also a sign of a faulty or leaking suspension air-bag. Look for signs of leakage under the vehicle. Fuel, coolant, engine oil, gear oil, automatic transmission / power steering / hydraulic brake fluid (where applicable).
In the proceeding sections, note any evidence of other leakage on the vehicle's frame or other components.
Open engine compartment / hood, checking that it is securely mounted. Tilt-forward hoods have straps on each side securing it to the cowl. Most tilt-forward hoods have a handhold formed dead-center above the grille. The proper, safe method to open these is to use the front bumper as a step, using your hand and your weight to pull on this handhold, using your weight to slowly pull the hood forward. Be careful, most are fiberglass and can crack or break easily at the hinges if not opened evenly. If not handhold-equipped, slowly raise the hood by its fenders near the securement strap area. An assistant can help prevent damage to the hood in this situation, but not COMPLETELY necessary. Leave the hood open as necessary, but be sure the hood is securely latched again on both sides before moving the vehicle.
Check again for leaks under this area.
Inspect all engine accessories (alternator / generator, power steering pump (if applicable), serpentine tensioner, air-conditioning compressor (if applicable), and the water pump. All should be securely mounted, and not leaking.
Check engine oil for proper level (generally regarded as between ADD and FULL), and contamination from metal or other fluids. Diesel engine oil is usually dark. Deep black oil is likely overdue for maintenance. Milky appearance indicates cooling system problems (cracked engine block, cylinder head, weak head gasket). Oil level being too high can indicate this as well, coolant usually mixes with the oil but if it doesn't, the oil will sit on top of the coolant and appear overfilled.
Check oil filter for damage or leaks. Check engine oil cooler (if equipped) for damage or leaks on lines and fittings. It is important to know engine oil lubricates AND COOLS the engine's rotating assembly. Yes, coolant and oil both cool the engine, but should never mix. Low oil level can kick engine temperatures up a bit, just as being low on coolant will. Hence, an oil cooler can reduce engine temps.
Some automatic transmissions can be checked for proper fluid level when cold. Refer to manufacturer's instructions to check for proper level and contamination. Check for leaks in the front seal / torque converter and fluid pan / tailshaft seal areas as well.
Check fuel filter and water separator (if equipped) for leakage or signs of damage.
Check engine coolant level. Inspect recovery tank for cracks, other damage or leakage. Some buses (mostly RE pushers) have a sight glass in the overflow tank for verifying engine coolant level and condition. The type of coolant dictates proper color. Ethylene glycol should be a reasonable shade of green – orange or brown indicates corrosion / contamination in the system. Extended-Life Coolant is often orange, sometimes purple or pink.
Check radiator cooling surface area for damage, leaks and debris (leaves / dirt) that can block airflow and cause overheating, especially on RE buses.
Check hoses and clamps for secure mounting, splits, soft/weak sections, leakage. Check hoses for ABCDFR. Any of these warrant replacement. Though the type of defect determines if immediate action needs to be taken, most should be fixed before travel for peace of mind.
Check alternator wiring for proper mounting, chafing, burns or melting. Any of these are trouble brewing.
Check drivebelt(s) for ABCDFR or other wear. A shiny, “glazed” appearance indicates slippage / improper adjustment. Push in on any top surface of any drivebelt. Any give is called “deflection” – more than 1/4” requires adjustment or tensioner replacement.
Check power steering fluid (if applicable) for proper level and that it is free of contamination.
Check washer fluid reservoir for proper mounting / fluid / level (water and standard washer fluid can freeze in cold weather).
Check hydraulic brake master cylinder (if equipped) for proper mounting / fluid level, leakage, and contamination. Diesel-powered vehicles with assisted hydraulic brakes either have a hydro-boost unit or a vacuum pump for standard vacuum-assist brakes. Hydro-boost uses power steering pressure to reduce pedal effort, similar to vacuum boosters on gasoline engines. This system can sometimes push power steering fluid into the brake system, rendering it inoperative. Either of these assist systems should be checked for proper mounting and leaks (vacuum or fluid).
Check air brake system compressor (if equipped) for proper mounting, and drivebelt condition if applicable. Some are gear driven and part of the engine, others are belt driven.
Check exhaust manifold / pipes / turbocharger for proper mounting and free of leaks or damage. Leaks will show dark spots of carbon in the immediate area. Turbochargers are generally oil-lubricated by the engine. Inspect for oil leaks in this area. Check water-to-air intercoolers for leaks and damage.
Check engine and transmission mounts for cracks, wear or other defects.
Check entire engine for fluid leakage.
Check frame for cracks, non-factory welds or signs of fluid leaks.
FOR CONVENTIONAL (DOG-NOSE) BUSES AND TRUCKS, the STEERING / SUSPENSION / HUBS / RIMS / TIRES / BRAKES portion of the pre-trip inspection applies here. Follow it accordingly.
FOR REAR-ENGINE BUSES, this portion of the pre-trip inspection is finished. Close engine bay door, ensuring it is securely mounted and latched.
STEERING / SUSPENSION / HUBS / RIMS / TIRES / BRAKES
Check steering gear box and hydraulic hoses for ABCDFR, proper mounting, cuts, pinching, leaks or other signs of damage or wear.
Check steering shaft and rag joint (if equipped) for damage and deterioration.
Check steering linkage for bends, cracks, or other damage. Cotter pins should be in place and securely mounted. All joints should be fairly tight. Some minimal play is normal, but if one hand can wiggle it freely, problems are likely or imminent.
Check leaf springs / coil springs for cracks, rust, breakage or abnormal wear. Leaf springs should in line with each other, not shifted or asymmetrical. Pins in the axle beam keep leaf springs aligned properly with each other and with the axle beam. Such breakage or misalignment / cracks, etc. can indicate chassis problems... I know this from experience. It can mean a bent frame, but usually occur from rough roads / hard bumps at speed.
Check spring-to-frame-mount bolts for signs of cracks, rust or missing hardware.
On smaller cutaway van-based skoolies with independent front suspension, the upper control arms, bushings and mounting bolts serve the same function as spring-to-frame-mounts – check them for damage, bends, corrosion, dry rot and abnormal wear or play – if you see cracks or daylight in the rubber in the bushings and the control arm, they are worn out and need replacement)
Check leaf-spring-to-axle-mount bolts for signs of cracks, rust, breakage, missing hardware or abnormal wear. These will be U / C-shaped bolts, one fore and aft of each side of the axle, wrapping around the axle beam, passing through a clamp and securing the springs to the axle beam.
On smaller cutaway van-based skoolies with independent front suspension, bushings in the upper / lower control arms, and ball joints serve the same function as leaf-spring-to-axle mounts – check them for cracks, dryrot or abnormal wear. Check ball joints and wheel bearings by raising the corner of the vehicle in question, holding the wheel / tire top and bottom and rocking it back and forth toward you. If the hub center wobbles, wheel bearings need attention. If the wheel / tire rocks top and bottom, the ball joints are bad.
Check shock absorbers and bushings for proper mounting, free of leaks, bends, damage. They're generally filled with oil and leaks will be obvious.
Check brake hoses/lines/fittings (hydraulic or air) for ABCDFR, and ensure they are not kinked, bent, leaking, broken or severely rusted / damaged.
Check air brake chamber (if so equipped) for proper mounting and damage such as rust, cracks, dents, leakage. A loose dust cap can allow water / dirt to enter the chamber and damage it – one of the little things that becomes big quickly. Check the clamp to ensure it is secure. Drum brakes will have slack adjusters.
Check slack adjusters (one per chamber / hub). Ensure linkage pins are in place and that the adjuster is not bent or otherwise damaged. The push rod should also be free of damage and not bent. Tugging on the slack adjuster should show no more than 1” of play.
Disc brakes are not common on air systems, but have no slack adjuster to my knowledge. Air-brake skoolies built before 2010 will likely have drum brakes anyway. Smaller cutaway van-based models will likely have hydraulic brakes with discs on the front, possibly all four corners.
Check brake drums and linings for proper securement, no signs of grease contamination, cracks or other damage.
Check drum brake linings for proper mounting, cracks or portions of the lining missing. Linings should be 1/4-inch thickness minimum. Some vehicles may have backing plates that make it difficult to check this.
Check tire for uneven wear, flat spots. Also check for ABCDFR, proper mounting and pressure (you will need a high-pressure range gauge – these tires typically hold 80-120 psi), damage, and foreign objects in tread or between dual wheel assemblies. Steer tires (front) should be 4/32” tread depth minimum. Retreads are okay on drive tires only, they are ILLEGAL for a steer axle, ESPECIALLY for a bus. For drive tires, 2/32” tread depth is acceptable.
Check wheel / tire for ABCDFR, proper mounting, loose / missing nuts / bolts, rust, cracks, welds, bends or other signs of damage. Check valve stem for ABCDFR, twisting, damage or leaks. Valve stem cap is a common fail item for a DOT or other safety inspection.
Check differential pinion and gear cover seals for proper installation, mounting and free of leaks.
Check axle / hub seal for proper mounting, loose/ missing nuts / bolts, signs of leakage and proper oil level if applicable. Steer axle hubs generally have a grommet which can be removed to check the hub oil level. Drive axles have a pinion seal that must be checked for leaks, and a plug in the third member (center) of the drive axle where the gears are housed, which can be removed to check the gear oil level. Checking differential oil level generally isn't required unless a leak is obvious. Some hubs have a sight glass for viewing the oil level, similar to the one for checking coolant on some buses.
Check lug studs for signs of shiny metal or stripped threads, indicating loose lug nuts. The rim may also show damage around lug holes, in the form of rust or shiny metal where the rim has been wobbling.
Check frame for cracks and non-factory welds.
This procedure includes steering linkage, but everything else should be repeated for EACH AXLE INDIVIDUALLY, and each HUB/WHEEL/TIRE OF EACH AXLE.
This portion of the pre-trip inspection is finished. Proceed to IN-CAB.
Check entry doors and windows for proper mounting and operation, signs of broken glass or other damage. Seals must be intact.
Check steps / footwell for foreign objects.
Check handrails for proper mounting and anything that can snag clothing or hair.
Check floor and cabin for loose or foreign objects on the floor, and all seats / other interior are securely mounted.
Check all doors windows / emergency exits for proper mounting, operation and latching.
Check dashboard for foreign objects blocking defrost airflow to windshield.
Check emergency kit – Fire extinguisher (properly charged and mounted), spare fuses and/or circuit breakers (whichever the vehicle uses), three red reflective triangles for breakdown. A first aid kit is a good idea, as are self-powered caution beacons for electrical system failures to increase visibility.
Check steering wheel lash (freeplay). There should be no more than 10% freeplay in the steering (2 inches on a 20-inch diameter steering wheel, for example). Any more indicates problems in the steering gear box, steering linkage or other connected parts.
Check parking brake to ensure it is engaged properly. Air brake hand valve should be pulled / popped completely out, any hydraulic / mechanical parking brake should be properly set.
Deploy wheel chocks aft and rear of one or more drive wheels.
Turn key switch ON, but DO NOT START. Check system voltage gauge, if equipped – it should be at or just above 12.0-12.5 not running. Less can indicate a battery or charging problem.
Lodge something against brake pedal to turn the brake lights on. This allows checking them with reverse lights for function.
Place transmission in R (reverse), and check reverse lights for proper mounting and function with key switch on / engine off for safety. Do NOT do this with the engine running.
This portion of the pre-trip inspection is finished.
NOTE: Some hydraulic systems may have an application-pressure gauge to ensure brake system pressure is satisfactory and applies the brakes properly. Some systems are air-over-hydraulic, using an air brake chamber to actuate a hydraulic parking brake. NEITHER of these mean you have true air brakes. For air-over-hydraulic systems, check parking brake chamber as described in the STEERING / SUSPENSION / WHEEL HUBS / RIMS / TIRES / BRAKES and air system operation as described in the AIR BRAKE SYSTEM PRESSURE WARNING TEST section.
Proceed to AIR BRAKE SYSTEM PRESSURE WARNING or HYDRAULIC BRAKE SYSTEM PRESSURE WARNING test, as equipped.
AIR BRAKE SYSTEM PRESSURE WARNING TEST
Ensure parking brake is set and drive wheels are chocked.
For manual transmissions, ensure transmission is in neutral by depressing clutch pedal. Shifter should move freely side-to-side in neutral, returning to a central position. Place automatics in N (Neutral) or P (Park) with parking brakes set / wheels chocked.
Check Supply pressure gauges (some vehicles have two tanks and two gauges – marked 1 and 2, or PRI and SEC in many cases). Residual pressure should be 90-120 psi or better – any less indicates a leak that needs attention.
If air pressure is satisfactory, WITH DRIVE WHEELS CHOCKED, release the parking brake by pushing in the hand valve knob. Watch the supply pressure gauge. It should not drop more than 1 psi. If it does not, depress the brake pedal and watch your supply gauge. It should drop slightly on application, but hold steady while you hold the brake – no more than 3 psi loss in one minute for a single vehicle, 4 psi loss in one minute with an air-brake trailer attached. If it drops more, there is a leak somewhere that needs attention.
Next, pulse (don't pump) brake pedal slightly and watch application pressure gauge. The harder you press it, the higher the pressure. Some will have different standards for this, but normal application pressure modulation at a stop should be anywhere from 50-90 psi or more.
If application pressure varies as it should, pump the brake pedal. The supply gauges will drop with each pump. At 60 psi, (85 on some buses), a warning light should come on and a buzzer / chime should sound to warn of dropping air pressure. The yellow hand valve knob should pop out when pressure drops into the 20-40 psi range. This engages your parking brake automatically if not already set, which is why it is important to know what your low-air warning sounds like, you have maybe 5-20 seconds to get off the road if it goes off, depending on the severity of the problem. NOTE: Some buses will sound the air horn if equipped, so I don't recommend doing this at campgrounds, state parks, etc.
This portion of the pre-trip inspection is finished. Proceed to HYDRAULIC BRAKE SYSTEM PRESSURE WARNING TEST, if applicable, or ENGINE START.
HYDRAULIC BRAKE SYSTEM PRESSURE WARNING TEST
Step on brake pedal firmly. Check that it feels firm and does not travel too close to the floor. The “BRAKE” warning light should not be on. Set parking brake to ensure pressure warning light functions.
Start engine. Monitor oil pressure and voltage gauges.
Oil pressure should rise immediately within 3-5 seconds of start. If it does not, turn engine off immediately to prevent damage until the problem is found. Oil pressure varies across different engine applications, but should be 35-45 psi on most when idling.
Voltage gauge should read 13.4 – 13.8 volts with the engine running. Less indicates a faulty alternator / generator or slipping drive belt. More indicates a faulty voltage regulator.
Air brakes: The low-air warning will sound until system pressure reaches 60 psi or better. The pressure should rise to 150, then fall back to 90-120 – you'll hear a hiss of pressure blowing off, this is a safety valve on the supply tank to regulate high pressure. If pressure doesn't build steadily within a minute or two, does not reach governor cut-out of 90-120 psi, blow-off valve pressure of 150, or continues to build past 150, do not drive the vehicle, there is a problem.
Hydraulic brakes: The “BRAKE” warning light should come on as a test, and only stay on if the parking brake is engaged. If it stays on when the parking brake is released, do not drive the vehicle, there is a problem.
Check wipers / windshield washer for proper operation.
To test parking brake function, engage parking brake, then remove / store wheel chocks. Place transmission in low gear and gently try to move vehicle with slight, gentle acceleration. It should not move. If it does, stop vehicle and do not drive it, there is a problem.
With parking brake still engaged, turn on lights, activate hazard flashers (four-ways) and walk around vehicle to confirm all lights and signals work as prescribed. Some Navistar vehicles may have a bulb-test feature that can cycle all the bulbs and signals (high/low-beam / flashers / left / right signals / reverse lights, etc).
Engine temperature should slowly rise over several minutes to normal operating temperature (approximately 185-210), depending on engine and application. If it rises fast, or rises above 210-220, shut engine off and do not drive vehicle, there is a problem.
Check heater / defroster for proper operation.
For automatic transmissions requiring a hot fluid level check, check for proper level and contamination. Check for leaks in the front seal / torque converter and fluid pan / tailshaft seal areas as well.
To test brake function, release parking brake, place transmission in low gear, gently accelerate to 5 mph or less, then depress brake pedal. Vehicle should move smoothly and stop abruptly and evenly. If it does not, do not drive vehicle, there is a problem.
EXTERIOR / UNDERNEATH
As you walk around the vehicle, listen and look for any signs of an air leak if equipped with air-ride or air brakes. This is easier with engine off, but system must be at nominal pressure.
Check air lines underneath the bus for leaks and damage.
Check all visible sections of frame for cracks and non-factory welds.
Check driveshafts, universal joints and carrier bearings for secure mounting, damage and wear.
Exhaust should be securely mounted, with no loose, missing or dangling parts / hardware.
Check transmission for proper mounting, leaks or other damage.
Check exterior glass / mirrors for proper mounting, cracks, other damage.
Check all bulbs / lights for proper mounting, function, damage and proper color for usage:
Forward marker / signal lights: AMBER – NO EXCEPTIONS
Forward clearance lights: AMBER – NO EXCEPTIONS
Headlights: CLEAR, WHITE light or as close as possible to white.
Rear marker lights: RED – NO EXCEPTIONS
Rear clearance lights: RED – NO EXCEPTIONS
Reverse lights: WHITE – NO EXCEPTIONS – DO NOT INSTALL AN AMBER BULB IN PLACE OF WHITE IN THESE!
Rear signal lights: Depending on application. Some are clear (white light) bulbs in red housings, others are amber bulbs in amber housings. If rear signal lights are clear, DO NOT INSTALL WHITE BULBS – reason being that this will blind drivers behind you when you signal. WHITE LIGHT TO THE REAR OF A VEHICLE IS ILLEGAL – REVERSE LIGHTS EXCEPTED. USE ONLY WHITE BULBS FOR RED HOUSINGS, AMBER FOR CLEAR.
Check fuel door, engine bay door / hood and all entrances / exits for proper securement and latching.
Check fuel lines and fuel cap at tank for leakage and proper mounting.
Check fuel tank exterior for smell of raw fuel / other signs of leakage.
Check fuel tank mounting straps for proper mounting. Metal straps should have a rubber isolator between them and the tank to prevent friction breaching the tank.
Check exhaust system to ensure it is properly secured with no signs of damage or leaks.
Check wiper blades and arms for proper mounting, dryrot or other damage.
Check body panels for proper mounting to vehicle, missing rivets or other damage that can cause separation from vehicle.
AIR SUPPLY TANK (AIR / AIR-OVER-HYDRAULIC BRAKES ONLY)
Air brake system supply tanks have an air drain valve that should be opened periodically to remove moisture that builds up over time. DOT regs say once a day for everyday driving. Twice a week should suffice for a skoolie when traveling, mainly because a skoolie is not likely to run 10 / 11 hrs or 600 miles a day as a commercial truck or bus might. However, as this prevents moisture damage to brake chambers and icing of air lines and other parts of the air brake system, I recommend you be mindful of this and do it more often in colder weather and climates, especially if idling the engine for extended periods of time.
PLACEMENT OF REFLECTIVE TRIANGLES
Should triangle placement become necessary, it should be done as soon as possible (within 10 minutes max) as follows:
Undivided highway (2-lane road) – #1 should be 100 feet AHEAD of the vehicle, #2 should be 10 feet BEHIND the vehicle, #3 should be 100 feet BEHIND the vehicle.
Divided highway (median / island between opposing lanes) – #1 should be 10 feet BEHIND the vehicle, #2 should be 100 feet BEHIND the vehicle, #3 should be 200 feet BEHIND of the vehicle.
Exception as follows when disabled within 500 feet of a hill crest or curve that obscures vision of the stopped vehicle, and applies on divided or undivided roadways, both to give more warning to traffic approaching from behind, and that of oncoming traffic of potential traffic in their lane to clear the disabled vehicle.
#1 should be 500 feet AHEAD of the vehicle, #2 should be 200 feet BEHIND the vehicle, #3 should be 500 feet BEHIND the vehicle.
I recommend some items on this list be repeated en route, and again when you stop for the day.
In these applications, most diesel engines shouldn't exceed about 1500-1800 rpm, and most gas engines shouldn't exceed 4000 rpm. There are a few exceptions to this rule. Consult your manual or an appropriate dealer / repair shop for what applies to your engine.
If you experience overheating with no safe place to stop, turning the heaters on wide-open can help to cool the engine. It may buy you enough time to reach a safe place to stop.
DESCENDING STEEP HILLS
Downshift transmission to control speed and save brakes descending steep hills. Usually one gear down from the one you climbed in, but don't assume. Hydraulic brakes are more fade-prone descending hills. Overheated brake components can boil the fluid. Without proper braking / engine braking to control speed, you can lose control.
IF you have an engine brake, do NOT use it in wet / slick weather. The engine brake can cause a skid and result in a loss of control or a rollover.
Some pointers to prevent brake fade... DO NOT RIDE THE BRAKES! Two techniques for this. Watch your RPMs (tach) with either.
Snub braking – On-off technique. Let speed go to the hill's maximum rating, then brake to 5 mph less. Release pedal. Rinse, lather, repeat.
Stab braking – A 'coast and burn' technique. Essentially, jam brakes hard for a moment, releasing just before lock-up. You get maximum braking in that instant with minimum heat buildup. Not recommended for most situations, but it's saved my bacon a time or two.
A smell of burning or extreme heat descending steep hills means your brakes are overheating and will soon stop working. Stop as soon as possible, secure vehicle and let brakes cool. Overheating brakes CAN / WILL catch fire, and damage drums / rotors.