Drivetrain review

Theo admitted at the start of the drivetrain session that planetary gears had always looked like magic to him. Marty took the lid off a coffee tin full of washers and showed him: one big ring, one small sun in the middle, three washers between them on a cardboard carrier. Hold one, drive one, take power from the third, and you get a reduction, an overdrive, a reverse or a straight-through lock. "That's all a planetary set ever does. Everything else in an automatic is deciding which one to hold." Kiran had used the same picture to explain Ines's power-split hybrid in her high-voltage term, and still found the questions on clutch packs and shift timing the hardest in her practice sets.

Drivetrains are split across two tests. A2, Automatic Transmission/Transaxle, has 50 scored questions on general diagnosis, in-vehicle service, and off-vehicle repair of the transmission and its electronic controls. A3, Manual Drive Train and Axles, has 40 scored questions on clutches, manual transmissions and transaxles, driveshafts and half shafts, differentials and axles, and four-wheel-drive and all-wheel-drive components.

The review follows AUTO-310 (Drivetrains and Transmissions).

Gears, ratios and torque

A gear ratio is the number of teeth on the driven gear divided by the number on the driving gear. A ratio greater than 1 is a reduction: the output turns slower than the input, with torque multiplied by the same ratio, less friction losses. A ratio less than 1 is an overdrive. The overall ratio in a gear is the transmission ratio times the final drive ratio. Vehicle speed follows from engine speed, the overall ratio and the tire's distance per revolution, and that relationship appears in exam questions about speedometer error after a tire size change, about engine speed at highway cruise, and about why a vehicle feels sluggish with taller tires.

# Overall ratio and road speed in each gear
import math
gears = {1: 4.696, 2: 2.985, 3: 2.146, 6: 1.000, 10: 0.636}
final = 3.42
tire_d = 31.5                 # tire diameter
rpm = 2500
for n, g in gears.items():
    overall = g * final
    wheel_rpm = rpm / overall
    speed = wheel_rpm * math.pi * tire_d * 60 / 63360
    print(f"gear {n}: ratio {g:.3f}, overall {overall:5.2f}, at {rpm} rpm = {speed:5.1f} mph")

gear 1: ratio 4.696, overall 16.06, at 2500 rpm =  14.6 mph
gear 2: ratio 2.985, overall 10.21, at 2500 rpm =  22.9 mph
gear 3: ratio 2.146, overall  7.34, at 2500 rpm =  31.9 mph
gear 6: ratio 1.000, overall  3.42, at 2500 rpm =  68.5 mph
gear 10: ratio 0.636, overall  2.18, at 2500 rpm = 107.7 mph

Clutches and manual transmissions

AUTO-310 lessons 3 and 4 covered these. A clutch that slips under load points to a worn disc, oil on the friction surface, a weak pressure plate or no free play in the release system holding it partly released. A clutch that drags, so gears grind and the vehicle creeps with the pedal down, points to too much free play, air in a hydraulic release system, a warped disc or a seized pilot bearing. Chatter on engagement often comes from oil or glazing on the disc or worn mounts. Dual-mass flywheels absorb engine pulses; excessive free rotation or rattle at idle shows wear.

In a manual transmission, synchronizers match the speed of the gear and the shaft before the collar engages. Grinding into one gear points to that gear's synchronizer or blocker ring; jumping out of gear points to worn engagement teeth, a weak detent or worn mounts letting the case move. Gear oil must be the specified type, because some synchronizers need a fluid that is not too slippery: GL-5 oils can harm some brass synchronizers that need GL-4.

Automatic transmissions

The torque converter couples the engine to the transmission through fluid. At stall, with the output held and the engine at full throttle, it multiplies torque, typically by about two; as speed rises it approaches a one-to-one coupling, and a converter clutch then locks it mechanically for efficiency. A stall test that reads above specification points to slipping clutches or bands; below specification, to an engine lacking power or a stator one-way clutch that freewheels. Converter clutch shudder, often at light throttle around 60 to 50 mph, is commonly a fluid problem first.

Planetary gearsets give the ratios, with clutches and bands holding or driving members. In a simple set, holding the sun and driving the ring gives a reduction out of the carrier; holding the ring and driving the sun gives a larger reduction; driving the carrier gives overdrive; holding the carrier gives reverse; locking any two members gives direct drive. The exam often gives a clutch application chart and asks which element is faulty when one gear is missing: the element used only in the missing gear is the most likely cause.

Electronic control uses shift solenoids and pressure control solenoids commanded by the transmission control module, with inputs from turbine and output speed sensors, throttle position, fluid temperature and the range sensor. Modern units learn adaptive values for each shift, which is why AUTO-310 lesson 6 relearned a van after a battery disconnect. Diagnosis starts with the fluid: level checked by the maker's procedure, often at a specified temperature, and condition. Then codes, data, a road test with the scan tool, pressure tests and, last, removal.

CVTs and dual-clutch transmissions

A belt or chain CVT varies its ratio continuously by moving the halves of two variable pulleys. It needs its own fluid, with friction properties that let the belt grip; the wrong fluid causes slipping and judder. A dual-clutch transmission is two manual gearboxes in one case, each with its own clutch, one for odd gears and one for even; it preselects the next gear and shifts by swapping clutches. Dry-clutch versions can shudder when creeping in traffic, and both types need clutch adaptation relearns after repairs. A power-split hybrid's electronic CVT, reviewed in lesson 11, is a planetary set with motor-generators, not a belt.

Driveshafts, axles and differentials

Driveline vibration questions follow AUTO-310 lesson 9 and AUTO-210 lesson 9. A U-joint operating at an angle speeds up and slows down twice per revolution; a two-joint driveshaft cancels this when the working angles at the two ends are nearly equal and the yokes are in phase. Unequal angles, often from a pinion angle error after spring or axle work, give a vibration at twice driveshaft speed. That was the cause of the courier van's buzz in AUTO-310's capstone, traced back to shims left out during the suspension course work. Worn CV joints click on tight turns, outer joint, or clunk and vibrate under acceleration, inner joint; a torn boot is the usual start.

Differentials let the wheels on one axle turn at different speeds in turns. Ring and pinion noise that changes between drive and coast points to gear contact pattern or bearing preload; noise only in turns points to the differential's side or pinion gears. A limited-slip differential needs a friction modifier in its oil, or it chatters in tight turns. Setup questions ask about backlash, measured with a dial indicator on a ring gear tooth, and the gear contact pattern, read with marking compound.

Transfer cases and all-wheel drive

Part-time four-wheel drive locks the front and rear driveshafts together, so it must not be used on dry pavement, where the axles' different paths in turns cause wind-up, binding and tire scrub. Full-time and all-wheel-drive systems have a center differential or a coupling that allows speed differences. Electronically controlled couplings vary the torque sent to the secondary axle and depend on matched tires: a tire of different circumference makes the system see constant slip, which can overheat the coupling. That was Harold's all-wheel-drive case. Gord's transfer case would not engage because of a corroded shift motor connector, which is why the exam's first answer for an electronic shift fault is usually the electrical supply and connector, not the internal parts.

Traps in this area

Drivetrain questions often give a symptom that two components could cause and a condition that separates them: in all gears or only one, on drive or on coast, straight or turning, hot or cold. The most common trap is skipping the fluid check on automatic transmissions; the exam's first step is almost always the fluid level and condition. The second is driveline vibration: decide its order, once or twice per revolution of what, before choosing a cause.

More practice in this area

Differential and transfer case questions often test fluids. A limited-slip axle needs its friction modifier; a transfer case may need automatic transmission fluid or a specific transfer case fluid, not gear oil; electronically controlled couplings use their own fluid, and the wrong one causes binding or noise. When the stem mentions recent service, the fluid used is often the clue.

Kiran's drivetrain errors were split between knowledge gaps on clutch pack and band application, which she fixed by drawing the application chart for a common six-speed from memory every Sunday until she could do it without looking, and reasoning errors on vibration, which she fixed by always working out the order of a vibration before choosing a cause. Marty's shortcut was to ask what turns at the speed of the vibration: the tires and wheels once per wheel revolution, the driveshaft once or twice per driveshaft revolution, and the engine with its firing pulses. Match the frequency to the part that turns at it, and most vibration questions answer themselves.

Manual transmission questions on both exams also ask about shift linkage and cables, which can cause hard shifting or a lever that will not reach a gear, and about the clutch interlock switch that stops the engine cranking unless the pedal is down. A no-crank complaint on a manual vehicle that is fixed by pressing the pedal harder points to that switch or its adjustment, not to the starter.