MANUAL TRANSMISSION PROBLEM DIAGNOSIS

Fig. 18-13. External linkage and floor shifter is commonly adjusted by installing pin through holes in shifter levers. Transmission levers and shifter must be in neutral. Adjust rod length until rod passes easily through holes in each shift lever.

Normally, a manual transmission will provide many thousands of kilometers of trouble-free service. Quite often, it will last the life of the car without major repairs. However, driver abuse and normal wear after prolonged service can cause transmission failure.

As a mechanic, the first step toward fixing a transmission is to determine why the problem developed. Was it because of driver abuse (speed shifting, drag racing, lack of maintenance), normal wear (extremely high odometer reading) or another trouble?

After proper diagnosis, the mechanic can decide whether the transmission must be removed from the car for major repairs. Or, the mechanic may find that a simple linkage or clutch adjustment will correct the problem.

Gather information on problem

To begin diagnosis, gather information on the transmission trouble. Then, test-drive the car to verify the complaint.

Find out in which gears the transmission acts up: first, second, third, high, in all forward gears, when shifting. Does it happen at specific speeds? This information will help you decide which parts are at fault.

Fig. 18-14 shows typical transmission troubles. Study them carefully.

Fig. 18-14. Study types of problems found in manual transmissions

 

Gears grind when shifting

A grinding sound or gear clashing noise when shifting is frequently caused by incorrect trans- mission linkage adjustment. If the transmission linkage is badly worn, the gears inside the trans- mission may not engage properly. If the clutch is dragging, the synchronizer teeth may grind trying to equalize gear and output shaft speed, especially when shifting out of neutral.

Problems inside the transmission may also cause gear grinding upon shifts. Worn or damaged synchronizers, shift forks or rails, and excessive wear in bearings and shafts may all prevent the gears from engaging smoothly.

Manual transmission noise

When a manual transmission is noisy (roaring, humming, or whirring sound), first check the transmission lubricant. It may be low or contaminated with metal particles.

Transmission noises will usually tell the good mechanic where the problem is.

For example, if the transmission is NOISY IN ALL GEARS, something common to all of the gears is at fault. Transmission bearings may be worn, shaft endplay spacers worn, or a shaft may be damaged.

On the other hand, if there is only a NOISE IN ONE GEAR (first, second, third), then the problem is due to components related to that gear. Look at Fig. 18-14.

Transmission hard to shift

When a manual transmission is hard to shift into gear, first check the linkage, Fig. 18-14. Make sure the linkage is lubricated and moving freely. A bent or misaligned shift rod will cause hard shifting. Also inspect the operation of the clutch linkage. If the clutch is not releasing completely, the transmission can be hard to shift.

Transmission jumps out of gear

When a transmission jumps out of gear, the driver’s shift lever “pops” into neutral while driving. First, check the transmission linkage and shift lever arms. If the shifter assembly is badly worn, it should be rebuilt or replaced.

A worn clutch pilot bearing may also cause the transmission to jump out of gear. Severe vibration set up by the wobbling transmission input shaft can wiggle and move the shift forks and synchronizers.

Other parts inside the transmission can cause jumping out of gear. They include: worn synchronizer inserts and springs, worn shift fork assembly or shift rails, wear and excessive play in the counter- shaft and mainshaft assembly.

Manual transmission leaks lubricant

Lubricant leaks in a manual transmission are caused by ruptured gaskets, worn seals, loose fasteners, or damage to the case, housings, or covers. When this problem occurs, check the lubricant level in the transmission. If too full, oil can leak out.

Also, check that all housing or cover bolts are tight.

When a seal leaks, always check for wear in the shaft bearing or bushing. If the bearing or bushing is worn, the wobbling action of the shaft can make the NEW SEAL LEAK.

Some of the gaskets and seals in a transmission can be replaced without removing the transmission from the car. For example, the rear housing seal and gasket can normally be installed in-car.

Manual transmission locked in gear

When the shifter is locked in one gear, check the transmission shifter assembly and linkage. Look for bent shift rods, worn linkage, bushings, or shifter arms. Also check linkage adjustment. With a shift rail type mechanism, worn or damaged rails, detents, or forks could be the cause.

A transmission can also become locked in gear when drive gear teeth are broken. The teeth can jam together and be locked by bits of metal from chipped gear teeth.

Manual transmission diagnosis chart

Refer to a diagnosis chart in a service manual when a problem is difficult to locate. It will be written for the exact type of transmission.

KNOW THESE TERMS

Transmission ID tag, Transmission jack, Dummy shaft, Shaft run out, Shift linkage alignment pin, Gear clash, Hard to shift, Jumps out of gear, Locked in gear, Transmission diagnosis chart

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