My Manual Transmission Makes a Gear-Grinding Sound

Updated on April 25, 2020
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Gears in a manual transmission grind or clash because of:

  • The use of the wrong gear lubricant
  • A clutch in need of adjustment
  • A binding or unadjusted gearshift linkage
  • A binding clutch
  • A damaged or worn synchronizer

The grinding you hear during shifting comes from a gearset that remains partially engaged while another gearset is trying to engage the output shaft or mainshaft.

The following sections describe the common problems that can lead to a manual transmission gear or gears to grind. They may help you diagnose the problem with your transmission.

It's a good idea to have the repair manual for your particular vehicle make and model when trying to diagnose manual transmission issues. The manual can give you some pointers for your particular application.

If you don't have this manual yet, you can buy a relatively inexpensive, aftermarket manual through Amazon. Haynes manuals provide step-by-step procedures for many maintenance, troubleshooting, and component replacement jobs you can do at home.

Index
1. Checking for Low Fluid Level or Improper Gear Lubricant
2. Clutch Linkage Adjustment
3. Binding or Unadjusted Gearshift Linkage
4. Drag or Binding Clutch
5. Damaged or Worn Synchronizer (Blocking Ring)
VIDEO: Worn Synchronizer Cuasing Gear to Grind
6. My Reverse Gear Grinds
7. Dealing with a Grinding Manual Transmission

A Gearbox and Its Parts

Parts wear or abuse can cause a manual transmission to grind.
Parts wear or abuse can cause a manual transmission to grind. | Source

Legend: Gearbox ZF 16S181

I. input shaft
II. main shaft
1. input shaft
2. splitter high gear wheel,
3. bearing
4. splitter synchronizer
5. splitter sliding sleeve
6. gear wheel (broken)
7. gears sliding sleeve
8. gear driven wheel
9. rear gear shift rail
10. gears shift rail

Low transmission fluid level can cause gears to grind.
Low transmission fluid level can cause gears to grind. | Source

1. Checking for Low Fluid Level or Improper Gear Lubricant

Low fluid level can cause gears to clash. Check the transmission fluid level and top it off, if necessary, with the proper fluid, and check for potential leaks.

Car manufacturers recommend specific lubricants to use with specific transmissions, depending on the particular vehicle model. If you recently replaced the transmission fluid, make sure you used the correct fluid for your application.

For the most part, newer vehicle models don't require a change of transmission fluid. But a transmission leak will cause fluid level to drop. On a late model vehicle, make sure to check for potential leaks, if fluid level is low.

Older transmissions required a change of fluid at regular intervals, usually every 20,000 miles (32,186.88 km). Consult your car owner's manual or vehicle repair manual for the recommended lubricant for your manual transmission and, if necessary, the fluid change interval.

You may replace the transmission fluid yourself at home, if necessary. Consult your vehicle manual for the proper procedure.

A leaking clutch hydraulic system can also cause trouble.
A leaking clutch hydraulic system can also cause trouble. | Source

2. Clutch Linkage Adjustment

The clutch linkage connects the clutch pedal to the clutch fork. This linkage lets you engage and disengage the clutch assembly. In general, you can find three types of linkages:

  • On older vehicle models, the shaft and lever linkage was common. This system includes a release lever and rod, overcenter spring, pedal-to-equalizer rod, equalizer or cross shaft, and pedal assembly.

    • With this type of systems, adjustments may be necessary as the clutch disc or other part in the assembly wears, which may cause the gears to grind. Consult your vehicle repair manual to make the necessary adjustments or inspect the linkage assembly.
  • The cable linkage is a common system today. It has fewer parts than the shaft and lever linkage. One end of the cable connects to the clutch pedal and the other end connects to the clutch fork. Most modern systems are self-adjusting. This removes clutch pedal play, as necessary, as the clutch disc wears or the cable stretches.

    • Although the system is self-adjusting, system issues may require repairing and setting the system again for proper operation. If necessary, consult your vehicle repair manual.
  • Another common assembly in used today is the hydraulic clutch system. In this system, fluid pressure transmits motion from the clutch master cylinder to the clutch slave cylinder through a hydraulic line to perform the same job as the shaft and lever linkage and the cable system type.

    • A common problem in hydraulic systems is leakage. A quick simple check for a potential low fluid level is pumping the clutch a few times before shifting into the grinding gear. If the grinding is reduced or gone, there could be a leak inside the master cylinder seals.
    • A hydraulic system, just like a self-adjusting linkage system, may provide thousands of miles of trouble-free operation. But system problems, like fluid leakage, cylinder worn seals and other issues may cause gears to grind and other clutch system problems. Consult your vehicle repair manual to troubleshoot and make the necessary repairs.

On some models, you can adjust clutch pedal free travel. However, other clutch linkage configurations exist and require an appropriate inspection and trouble shooting procedure when the clutch doesn't seem to release properly. Consult your vehicle repair manual, if your gears are grinding.

On some models, you can adjust the clutch linkage or clutch pedal.
On some models, you can adjust the clutch linkage or clutch pedal. | Source

3. Binding or Unadjusted Gearshift Linkage

Problems with the linkage can prevent gears from shifting correctly. You may need to raise the vehicle and support it on jack stands to inspect the linkage.

Over time, linkage components may wear and fell out of adjustment. However, only transmissions with an external linkage can be adjusted, whether column or floor mounted. Also, external gearshift levers may need adjustment.

Consult your vehicle repair manual for the correct procedure to adjust the linkage, if necessary.

Clutch components may wear or get damaged and cause gears to grind.
Clutch components may wear or get damaged and cause gears to grind. | Source

4. Drag or Binding Clutch

When the clutch remains partially engaged, even if the clutch pedal is fully depressed, you'll hear the gears clashing or grinding, especially when you try to engage reverse. This is called dragging or binding clutch.

A binding or dragging clutch may cause other symptoms like hard starting and vehicle movement when starting the engine since the engine and transmission input shaft are still engaged.

You can check for a dragging clutch following this simple procedure:

  1. Start the engine.

  2. Fully depress the clutch pedal.

  3. Shift into first gear.

  4. Without releasing the clutch pedal, shift into neutral.

  5. Wait for five seconds.

  6. Shift into reverse.

    • If you can hear the gears grinding when shifting into reverse: check for clutch pedal free travel and check the clutch linkage for bent, binding or broken parts as necessary. If everything seems in order, most likely the problem is in the clutch assembly.

A worn or damaged synchronizer can fail to equalize speed and cause gears to grind.
A worn or damaged synchronizer can fail to equalize speed and cause gears to grind. | Source

5. Damaged or Worn Synchronizer (Blocking Ring)

Basically, synchronizers equalize rotating components to a matching speed so that you can smoothly shift gears. It then locks the appropriate components together, depending on the gear you are shifting into. This action prevents gears from clashing or grinding every time you shift gears.

For the most part, transmission components can work for thousands of miles trouble-free throughout the life of the vehicle. But this smooth operation can be disrupted by poor maintenance, abuse, poor design, or unanticipated part failure. Thus, synchronizer components may wear or fail prematurely.

Problems with one synchronizer (usually a blocker ring) will cause one gear to grind. If more the one synchronizer has a fault, possibly a wearing issue, more than one gear may grind.

Diagnosing or repairing this type of failure requires disassembling and inspection of the transmission. Sometimes, installing a new or rebuilt unit is more practical.

The next video explains how the synchronizer works and what happens when the blocker rings wears out, causing a set of gears to grind because of the difference in speed.

Worn Synchronizer Causing Gear to Grind

Excessively worn or damaged gears can grind when engaged.
Excessively worn or damaged gears can grind when engaged. | Source

6. My Reverse Gear Grinds

Forward gears have a synchronizer to equalize gear speed when shifting gears. Usually, a reverse gear doesn't have a synchronizer, since you are at a complete stop when engaging reverse.

However, if you hear a grinding noise every time you try to engage reverse, the transmission may not be completely disengaged from the engine.

Try shifting first into a forward gear to stop gear rotation and then engage reverse gear. If you can still hear reverse gear grinding, the splines the clutch disc slides on might be rusty or damaged. You may also be dealing with a warped clutch disc or other assembly issues.

Proper transmission maintenance can prevent serious component problems.
Proper transmission maintenance can prevent serious component problems. | Source

7. Dealing With a Grinding Manual Transmission

These are the most common problems that may cause one or more gears to grind or clash. However, other internal transmission issues may cause gears to grind during shifting as well:

  • a worn or damaged rail
  • worn bearings
  • a damaged shift fork
  • shaft problems

It's difficult to diagnose these type of issues without dropping and disassembling the transmission. When necessary, consult with a professional transmission shop before committing to a transmission repair. Evaluate your options, especially if you are dealing with a high-mileage transmission. Sometimes, it's better to replace the transmission with a rebuilt or new unit than trying to repair a single component.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

    © 2020 Dan Ferrell

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