ASE Practice Test (A3: Manual Drive Train & Axles) 2 — Questions and Answers

Question 1: A manual transmission grinds when shifting into 3rd gear but shifts smoothly into all other gears. The synchronizer ring for 3rd gear is suspected. What does the synchronizer do?

Prevents the gear from engaging until the driver fully depresses the clutch
Matches the speed of the gear to the main shaft before engagement (Correct answer)
Reduces gear noise by dampening gear mesh vibration
Locks the gear to the main shaft permanently when selected

Correct answer: Matches the speed of the gear to the main shaft before engagement

A synchronizer ring matches (synchronizes) the rotational speed of the gear being selected to the speed of the main shaft through friction. If the synchronizer is worn and cannot equalize speeds quickly enough, the dog teeth grind during engagement.

In a manual transmission, most gears are in constant mesh (they are always turning) but are not connected to the main shaft until selected. To shift into a gear, the transmission must engage dog clutch teeth on the gear with corresponding teeth on the synchronizer hub, which is splined to the shaft. If this engagement occurs while the gear and shaft are spinning at different speeds, the dog teeth collide and grind. The synchronizer ring prevents this by using a conical friction surface to contact the gear cone, slowing or speeding the gear to match shaft speed before the dog teeth engage. When a synchronizer ring wears, its friction surface loses effectiveness, and it can no longer equalize speeds quickly enough. The driver feels this as resistance and grinding when shifting into the affected gear. The grinding occurs because the dog teeth are trying to engage at mismatched speeds. Worn synchronizers require transmission disassembly and replacement of the synchronizer assembly (hub, sleeve, ring(s), and struts/keys). Contributing factors include insufficient clutch disengagement (clutch drag), incorrect fluid type, low fluid level, and shifting technique (not waiting for full clutch disengagement before engaging the shifter).

Question 2: A technician is checking a four-wheel-drive transfer case and finds the chain stretched beyond specification. What symptoms would the driver most likely report?

Difficulty shifting between 2WD and 4WD
A slapping or rattling noise from the transfer case during acceleration and deceleration (Correct answer)
Transfer case fluid leaking from the front output seal
Binding during low-speed turns in 4WD

Correct answer: A slapping or rattling noise from the transfer case during acceleration and deceleration

A stretched drive chain in a transfer case creates excessive slack, which causes a slapping or rattling noise as the chain alternately tightens under load and goes slack during deceleration. This noise is often confused with engine valve train noise but is located under the vehicle.

Chain-drive transfer cases use a roller chain to transfer power between the front and rear output shafts. Like any drive chain, the transfer case chain can stretch with age and mileage, increasing the amount of slack in the chain. As the chain stretches, it develops noticeable slack on the non-tension side. During acceleration, chain tension on one side keeps the slack side quiet, but during the transition from acceleration to deceleration (engine braking), the chain briefly goes slack on both sides and then snaps tight on the previously-slack side. This alternating tension/slack creates a characteristic slapping or rattling sound from the transfer case area. The noise is typically most noticeable at low speeds during on/off throttle transitions. Under sustained load (steady acceleration or steady deceleration), the noise may be less prominent as the chain is consistently loaded on one side. Additionally, a stretched chain will cause vibration at certain speeds as the chain oscillates between tight and loose tension. If left uncorrected, the chain can wear the sprocket teeth, potentially jump teeth and cause 4WD engagement issues, or ultimately break. Transfer case chain replacement requires transfer case removal and disassembly.

Question 3: During a CV axle boot inspection, a technician finds the outer boot is cracked and has thrown grease onto the inside of the wheel well. What is the MOST important next step?

Replace only the boot and repack with fresh grease
Clean the grease from the wheel well and monitor for leaks
Replace the entire CV axle shaft assembly
Inspect the CV joint for wear and damage before deciding on repair (Correct answer)

Correct answer: Inspect the CV joint for wear and damage before deciding on repair

Before deciding on repair, the CV joint must be inspected for contamination and wear. If the boot has been cracked and throwing grease, dirt and water may have entered the joint. A contaminated or damaged joint requires axle replacement, while an undamaged joint may be repaired with a boot kit.

CV (constant velocity) joint boots serve a critical protective function: they retain the special CV joint grease and exclude contaminants (water, dirt, debris) that would rapidly destroy the joint. When a boot is torn or cracked, the joint's protection is compromised. The severity of damage to the underlying joint depends on how long the boot has been damaged and operating conditions. A boot torn recently that hasn't thrown much grease may have a joint that is still serviceable with a reboot and fresh grease. A boot that has been cracked for thousands of miles in rainy or muddy conditions likely has contamination throughout the joint. Inspection involves disassembling the outer CV joint from the axle shaft (or removing the entire axle for bench inspection), cleaning the joint, and examining the ball bearings, races, and cage for pitting, spalling, corrosion, or flat spots. A clicking noise during turns would indicate worn balls or races. If the joint is in good condition, a quality boot kit (including boot, clamps, and fresh grease) provides an economical repair. If any wear or contamination is found, replacing the complete axle shaft is the recommended practice because a worn joint will fail prematurely even with a new boot. Many technicians prefer to replace the complete axle shaft regardless of joint condition to avoid a repeat repair.

Question 4: A rear-wheel-drive vehicle with a solid rear axle exhibits a clunking noise only on initial acceleration from a stop and not during driving. What is the MOST likely cause?

Worn differential pinion bearings
Worn universal joint (U-joint)
Excessive ring and pinion backlash in the differential (Correct answer)
Loose or missing axle retaining bolts

Correct answer: Excessive ring and pinion backlash in the differential

A single clunk on initial acceleration from rest indicates excessive backlash in the ring and pinion gear set. The free play in the gear mesh allows the pinion to snap against the ring gear when torque is first applied, producing a single clunk rather than a continuous noise.

Backlash is the free movement between mating gear teeth — the amount the ring gear can move without moving the pinion. A small amount of backlash (typically 0.004-0.010 inch for most rear axles) is required for proper gear lubrication and thermal expansion. Excessive backlash occurs when gear teeth wear, gear bearings wear and allow shaft movement, or when the ring and pinion are incorrectly set up. When the vehicle is stationary, the drivetrain has no load. When the accelerator is applied, torque attempts to rotate the driveshaft and pinion. If there is excessive backlash, the pinion rotates freely through the backlash clearance before suddenly contacting the ring gear teeth with full torque force. This creates a single, sharp clunk felt and heard from the rear axle area. Once the vehicle is moving, the gear mesh is consistently loaded in one direction, and the backlash space is taken up — hence no noise during steady driving. The clunk may also occur when lifting off the throttle (as the gear mesh reverses loading) or when reversing. U-joint wear typically produces a clunk during direction changes (forward to reverse) with some vibration during driving. Pinion bearing wear produces a whine or growl under load. Loose axle bolts would cause a different type of noise pattern.

Question 5: When performing a clutch replacement, a technician notices the flywheel friction surface has slight heat cracks (heat checks) but is within the resurfacing specification. Should the flywheel be resurfaced?

No, heat cracks within specification are acceptable and resurfacing is not needed
Yes, heat cracks indicate metal stress and the surface should be resurfaced or replaced (Correct answer)
No, resurfacing will make the cracks worse by exposing softer metal
Yes, but only if the customer is also replacing the clutch disc

Correct answer: Yes, heat cracks indicate metal stress and the surface should be resurfaced or replaced

Heat cracks in a flywheel indicate surface stress from overheating. While they may be within thickness specification for machining, the cracked surface will cause uneven clutch engagement, accelerated disc wear, and potential future cracking. Resurfacing is the correct action to restore a smooth, flat friction surface.

The flywheel friction surface must be smooth, flat, and within dimensional tolerances to provide proper clutch disc engagement. Heat cracks (also called heat checks) appear as a network of fine cracks in the surface, caused by repeated thermal stress from clutch slippage. These cracks form when the surface is repeatedly heated and cooled, causing surface metal to alternately expand and contract. While heat cracks may be within the thickness specification that allows resurfacing (flywheel minimum thickness is stamped on many flywheels), they represent a compromised surface that will cause problems. The cracked surface is uneven and can cause the clutch disc to grab inconsistently, resulting in shudder during engagement. The cracks concentrate stress and may propagate over time, potentially causing sections of the flywheel surface to break free — a potentially dangerous failure. Machining (resurfacing) removes the cracked surface layer, restoring a smooth, flat contact area. The machinist must verify sufficient material remains after resurfacing. Deep cracks, hard spots (blue discoloration areas indicating severe overheating), or cracks that are too deep to machine out require flywheel replacement. A clutch replacement without flywheel attention is a false economy — a new clutch disc against a damaged flywheel will wear prematurely and perform poorly. Always machine or replace the flywheel as part of any clutch replacement.

Question 6: A limited-slip differential (LSD) equipped vehicle exhibits chattering during low-speed turns. What is the MOST likely cause and correct fix?

Worn spider gears requiring differential rebuild
Incorrect gear oil — standard hypoid gear oil used instead of friction modifier additive (Correct answer)
Worn axle bearings causing excess movement in the differential
Incorrect ring and pinion backlash setting

Correct answer: Incorrect gear oil — standard hypoid gear oil used instead of friction modifier additive

LSD chatter during low-speed turns is a classic symptom of incorrect gear oil. LSDs require friction modifier additive in the gear oil to properly lubricate the clutch packs. Standard hypoid gear oil without friction modifier causes the clutch discs to grab and release rapidly, producing the chattering noise.

Limited-slip differentials (clutch-pack type) use a stack of alternating friction discs and steel plates to provide torque-biasing capability. Under normal conditions, the clutch packs slip slightly to allow differentiation during turns. Under wheel spin, the clutch packs apply more force, limiting the speed difference between the two axle shafts. The friction coefficient of the clutch pack material is specifically engineered for limited-slip operation. This requires gear oil with appropriate friction modifier additives (often called limited-slip additive or positraction additive). These additives condition the friction material to achieve consistent, smooth engagement with the correct slip characteristics. When standard hypoid gear oil (GL-5 rated) without friction modifier is used, the coefficient of friction is too high and inconsistent for the clutch pack material. The clutches grab rather than slip smoothly during turns, resulting in a rapid grab-release cycle that produces the characteristic chattering noise, often felt through the floor and heard from the rear axle area during parking lot turns. The fix is to drain the differential, flush it with clean solvent or fresh gear oil, refill with the manufacturer-specified gear oil, and add the specified amount of friction modifier additive. Some manufacturers specify a dedicated limited-slip gear oil that already contains the proper additives. Chatter typically stops immediately after the correct lubricant is installed.

ASE Practice Test (A3: Manual Drive Train & Axles) 2

A manual transmission grinds when shifting into 3rd gear but shifts smoothly into all other gears.
The synchronizer ring for 3rd gear is suspected.
What does the synchronizer do?