Plain English
Words ExplainedEvery piece of jargon on this site, roughly in the order you will meet it.
Why this page exists
Most truck documentation is written for people who already know the vocabulary. This is the translation. Nothing here replaces the manual, but it makes the manual readable.
Torque
How hard a bolt is twisted. Measured in ft-lb (foot-pounds) or N·m (newton-metres). One ft-lb is about 1.36 N·m.
Fastener
Any bolt, nut or stud holding two parts together.
DEF
Diesel Exhaust Fluid. Urea solution in its own tank, squirted into the exhaust to clean it up. Called AdBlue in some markets.
DPF
Diesel Particulate Filter. A trap in the exhaust that catches soot, like a furnace filter.
Regen
The process that burns soot out of the DPF. A parked regen happens stopped; a passive regen happens while driving.
SCR
Selective Catalytic Reduction. The box that uses DEF to turn smog-forming gas into nitrogen and water.
NOx
Nitrogen oxides — the smog-forming gas the SCR removes.
Aftertreatment
The whole emissions package on the exhaust: DPF, SCR and their sensors.
SPN
Suspect Parameter Number. First half of a fault code — what is wrong.
FMI
Failure Mode Indicator. Second half — how it is wrong: high, low, erratic, open circuit.
Derate
The computer cutting power to protect itself. Fix the cause, not the derate.
Inducement
An emissions-enforced derate. Ignore a DEF or NOx fault long enough and the truck limits you to walking pace.
CK-4 / FA-4
Two modern diesel oil standards. FA-4 is thinner for economy and is not approved for every engine.
ELC / OAT
Extended Life Coolant / Organic Acid Technology. Long-life antifreeze needing fewer additive top-ups.
VDS / EOS / CES
Manufacturer oil standards for Volvo, Mack and Cummins. An oil can meet CK-4 and still miss these, which matters for warranty.
GVWR
Gross Vehicle Weight Rating. The most the truck itself may weigh, fuel and driver included.
BBC
Bumper to Back of Cab — how long the cab and hood are.
Bobtail
Tractor with no trailer. The steer axle usually scales heavier than you expect.
Line haul / vocational
Long-distance highway work, versus construction, refuse and dump duty. Different service intervals.
PM
Preventive Maintenance — the scheduled service visit.
Hub-piloted / stud-piloted
Two ways a wheel centres on the hub. The torque values differ, so check which you have.
PreSet hub
A ConMet hub with pre-adjusted bearings. Torque the nut and do not back it off, unlike a conventional hub.
Slack adjuster
The lever turning air pressure into a brake application. Its angle and stroke matter at inspection.
U-bolt
The clamp holding the leaf spring to the axle. Under-torqued U-bolts wear springs and tyres.
J560
The seven-pin plug between tractor and trailer. Pin 1 is always white ground.
J1939
The computer network cable running around the truck. Two 120 ohm end resistors keep it alive.
TMC / SAE / API / ISO 22241
Standards bodies: truck maintenance practice, network and connector standards, oil ratings, DEF quality.
B10 / B50 life
The mileage by which 10 percent (or 50 percent) of engines are expected to need overhaul. A design figure, not a guarantee.