Manual M.D.: Quick Guide to Diagnosing Manual Transmission Troubles

The manual transmission is not dead — especially not in the hot rodding universe.

More importantly, your manual transmission is not dead. It may sound sick or move sluggishly, but chances are your ailing manual transmission can be saved. The first step is pinpointing the problem, so we’ve assembled this quick guide to diagnosing common manual complaints. Keep in mind, this guide is intended as a starting point to put you on the right track toward treating your sickly stick shift. Each individual transmission make and type is different, so the specific causes and cures vary depending on what you have (refer to your vehicle service manual). We’d also recommend you check out our post on troubleshooting clutches, since some transmission troubles can be traced to that clutch.

Many common manual transmission problems fall under one of these areas:

Problem: Hard Shifting Into Gear

Possible Causes: This is commonly caused by improperly adjusted linkage between the gear shift lever and the transmission, which increases the force required to shift gears. Other common causes are bent, jammed, rusted, or otherwise damaged linkage. A bent shift fork, worn shift shaft, worn or defective synchronizer, or twisted mainshaft could also be the culprit. If any of these components are worn, you’ll need to replace them. Be sure to fill and maintain your transmission lubricant to keep everything operating smoothly.

Problem: Transmission Sticks In Gear

Possible Causes: This condition may be caused by many of the same issues as hard shifting. For example, improper shift-linkage adjustment, or the linkage failing to move freely, may be the source of the problem. This would prevent the clutch from disengaging. Also, if a synchronizer sleeve does not slide freely on the hub splines, the transmission would stick. Lack of lubricant (or the wrong lubricant) can also cause a sticky tranny.

Problem: Transmission Jumps Out of Gear

Possible Causes: Believe it or not, this problem could be the result of a stiff shifter boot, which could pull the shift back to neutral from any position. Make sure the boot is not too rigid or stiff and replace if necessary. From there, you’ll need to turn your attention to the inside of the transmission and look closely at the synchronizer sleeve. If it has worn splines or worn teeth on the gear, this would allow the transmission to jump out of gear.

If you began noticing the transmission jump out of gear after the clutch or transmission had been serviced, check for proper alignment between the engine and transmission. Misalignment can cause the transmission to jump out of gear. If you also notice a pulsating clutch pedal, there may be a clutch housing misalignment.

Other causes may include a weak detent spring, worn pilot bearing, or broken input shaft retainer.

Problem: Gear Clash When Shifting

Possible Causes: The most common cause is a worn or defective synchronizer. Gear clash can also occur if the clutch does not fully engage and if a gear sticks on the mainshaft. A worn or dry pilot bearing or bushing can cause clutch drag, which prevents the clutch from disengaging easily in a certain gear. This is typically accompanied by a loud noise and rough shifting. Finally, low lubricant levels can lead to gear clash.

Problem: Noisy Transmission While In Gear

Possible Causes: A noisy transmission can often be the result of a transmission oil leak, which will eventually lead to insufficient oil levels. It could also be the sign of a defective friction disc in the clutch or defective engine damper, although these noises are typically only heard at certain engine speeds. Other causes include excessive clearance between the gears and mainshaft, broken gear teeth, worn bearings, worn synchronizer, worn pilot bearing, or low lubricant levels. You’ll need to inspect each of these areas and replace as necessary.

You may be able to narrow down the potential problem area by identifying the type of noise your transmission is producing:

• Whining or growling: Worn, chipped, or cracked gears (will be loudest in the gear that puts the greatest load on the worn gears).
• Hissing or bumping: Worn bearings.
• Metallic rattles: Worn or loose parts in the shift linkage or gears loose on the shaft splines.

Again, listen closely to determine at which gear the noise is loudest. This will help you pinpoint the troubled components.

Problem: Noisy Transmission in Neutral

Possible Causes: This is usually caused by misalignment between the transmission and engine, or a defect in any of the rotating parts. This could include a dry or worn bearing, a broken or worn gear, a bent countergear, or excessive countergear endplay.

Problem: Noisy Transmission in Reverse

Possible Causes: Sources of this problem may be traced to wear and tear on any number of components. Check your reverse-idler gear or bushing, reverse gear on the mainshaft, or countergear. You should also inspect your shift mechanism for any damage that could be the source of the problem.

Problem: No Power Through Transmission

Possible Causes: If power doesn’t flow through the transmission when it’s in gear with the clutch engaged, you may be looking at pretty big issues. It could simply be that your clutch is slipping, or it may be the sign of a broken gear, busted mainshaft, stripped gears, or sheared splines. It could also be a broken shift fork or linkage part. Inspect closely and keep your fingers crossed.

Problem: Transmission Oil Leaking

Possible Causes: Take a close look at your transmission fluid first. If it’s not the correct type — or if your lubricant is filled too high — it may foam excessively. This foam will completely fill the case and leak out.

If your lubricant checks out, turn your attention to the gaskets and seals associated with your transmission. If a transmission gasket, oil seal, oil slinger, speedometer pinion seal, interlock plate seal, or extension housing seal is the source of the leak, replace as necessary. Also, inspect the detent plug and fill plug, and replace those as necessary. If you still haven’t found the source of the problem, inspect the transmission case or extension housing for cracks.

SOURCE: Automotive Mechanics, Tenth Edition