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Diagnosing Symptoms of a Bad Manual Transmission

Updated on July 18, 2017
Dan Ferrell profile image

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

Manual transmissions can develop problems after miles of service.
Manual transmissions can develop problems after miles of service. | Source

Manual transmission problems can develop due to high mileage, abuse, or lack of proper maintenance. But they are rather rare. Most manual transmission issues originate not with the transmission itself, but from related components like the clutch assembly, linkage, or driveline: the components that transmit turning power from the transmission to the wheels. Also, as we discuss below, symptoms that seem to be coming from the transmission can come from unrelated parts of the car.

The key to diagnosing your manual transmission problem is to gather detailed information about the particular problem. For example, does the problem appear in only one of the gears, only at a certain speed, only when turning, only when downshifting, or after having serviced the clutch or another transmission component? Can you feel a vibration? Can you hear a clunking or grinding noise?

Symptoms of a Bad Transmission

Here are some symptoms that can indicate a worn-out transmission, each one of which we discuss below.

  1. Odd sounds (whirring, squealing, bumping, or thumping)
  2. Grinding noise
  3. Transmission jumps out of gear (into neutral)
  4. Difficulty shifting gears
  5. Car stuck in one gear
  6. Car that can't get into gear
  7. Leaking transmission oil

This guide will get you started on identifying the source of these problems. You'll need to investigate further and remember that manual transmission configurations can change from one model to the next.

Remember that this guide only deals with symptoms coming from the manual transmission (or transaxle) itself. Some symptoms that may appear to originate in the transmission actually come from the clutch (or another system), and vice versa. To get a more accurate diagnosis, take a look at the Bad Clutch Symptoms guide as well.

Your noise may not be coming from the transmission; a bad CV joint can produce noises when you accelerate or make a turn.
Your noise may not be coming from the transmission; a bad CV joint can produce noises when you accelerate or make a turn. | Source

1. Odd Noises That May Come From the Transmission

The most common cause of a noisy transmission is insufficient oil, causing the gears or internal assembly to hum or whir. If a noisy transmission does have enough oil, the lubricant may be contaminated with metal shavings or particles.

Insufficient or contaminated oil may cause the transmission to become noisy in some or all of the gears. But if you hear noises in a specific gear, that gear's teeth or synchronizer may be worn or damaged.

Sources inside the transmission that can cause noise:

  • A worn-out synchronizer
  • The gears on the speedometer drive
  • Misaligned transmission
  • A worn or damaged input shaft bearing, if your transmission makes noises only in neutral (sometimes a bumping sound)
  • Worn-out gears
  • Output shaft pilot bearing problems
  • Metal shavings in the oil

Sources outside the transmission that can cause noise:

Some noises that seem to come from the transmission are actually coming from an outside though possibly related source. For example, if you hear a thumping noise when you accelerate or decelerate, check first for these problems before you blame the transmission:

  • A loose or damaged engine or transmission mount
  • A worn or damaged drive axle inner CV joint
  • Problems with the differential case

Noises that manifest when turning could point to a problem with the CV joint. Knocking noises when driving at low speeds could come from the differential case or the CV joint.

A Three-Step Test for Transmission Bearing Noises

That noise you keep hearing may come from a bearing. This three-step test is simple and can reveal problems with one or more transmission bearings.

  1. Fire up the engine and set the transmission to neutral. If you can hear a noise in neutral, suspect a worn-out input shaft bearing. Watch the video at the bottom of this article to see how internal problems can affect input shaft operation.
  2. Now, with the engine running, depress the clutch pedal just enough to remove the free play, to the point where you feel you need to apply force to the pedal to push it down further. You may need to push the pedal a little more. If you hear a squealing noise, you may have a worn out release bearing.
  3. Fully depress the clutch pedal to release the clutch. If you hear a whirring sound then, you probably are dealing with a bad pilot bearing.

A bad synchronizer can cause a grinding noise.
A bad synchronizer can cause a grinding noise. | Source

2. The Transmission Makes a Grinding Noise

Problems with the transmission can also be revealed through a grinding noise.

A grinding noise may come from clashing gears. The clashing may happen because of linkage problems like wear or need of adjustment.

Other potential sources could be a worn or damaged synchronizer, shift fork or rail and bearing shafts.

If you can hear the gears clashing only when downshifting, the problem may come from the synchronizer (too much play at the output shaft end).

However, a grinding noise may also come from a dragging clutch.

Shift linkage problems can cause your transmission to jump out of gear.
Shift linkage problems can cause your transmission to jump out of gear. | Source

3. The Transmission Jumps Into Neutral

This seems to be a common problem on worn-out transmissions. You shift into gear, and the transmission jumps out of gear.

Once again, there could be other several causes for this problem, besides a worn-out transmission:

  • A common problem is a worn out, stretched, or maladjusted shift linkage. A stretched linkage can be caused by a damaged engine or transmission mount. An external linkage can wear out or become loose and maladjusted, causing the transmission to jump out of gear. Look for rust and binding. You can try to adjust the linkage. But in most cases, you'll have to rebuild or replace that part of the assembly.
  • You may be dealing with a weak or broken spring in the shift rail. In an internal shift linkage, the spring is part of the spring-loaded ball that locks the transmission into gear. If the ball slips out of the notch, the transmission will jump out of gear.
  • Also, you may be dealing with a worn-out pilot bearing (the gap causes the input shaft to vibrate, which causes the shift forks or synchronizers to move).
  • You may have problems with a worn synchronizer or shift fork assembly or other internal components.

Other potential causes to keep in mind:

  • Loose or misaligned transmission (possibly after service)
  • Misaligned clutch housing
  • Loose shifter cover
  • Worn-out gear teeth

Note: When the transmission only jumps out of a high gear, check the clutch housing bolts for tightness, or the transmission for misalignment.

A worn out shift fork can make it hard to shift gears.
A worn out shift fork can make it hard to shift gears. | Source

4. It's Hard to Shift Gears

This problem happens when you find it hard to move the shift lever from one gear to another. Usually, this points to a problem with a loose linkage, worn shift cables or worn bearings.

Check the linkage. You may need to raise the vehicle and safely support it on jack stands or remove a component in order to make the inspection, depending on your vehicle model. You may need to remove a shift lever boot.

Get the help of an assistant to operate the linkage while you inspect the linkage, if necessary. Look for problems like binding, poor lubrication, misaligned components.

To remove components for inspection, consult your vehicle repair manual. Haynes is a good aftermarket manual: get the manual for your exact model.

Other causes for a hard-to-shift manual transmission include:

  • Worn or loose internal components (shift fork, levers, shafts)
  • Low oil level (or wrong type of oil)
  • Misaligned transmission
  • Synchronizer problems

Low oil level can prevent gears' free travel.
Low oil level can prevent gears' free travel. | Source

5. The Transmission Is Stuck in Gear

You may find that you can't get the transmission out of gear. This symptom may indicate:

  • Low oil level or wrong type of oil
  • Problems with the linkage or shifter assembly. Look for maladjustment, or wear or damage to rods, bushings, or shifter arms.
  • Internal components: shift rail, detents, forks or a stuck synchronizer sleeve
  • Worn-out or broken drive gear teeth
  • A stuck shift rail
  • Misaligned transmission

6. The Transmission Won't Get Into Gear

When you have trouble getting the transmission into gear, inspect the shift linkage for adjustment, looseness or damage.

However, remember that failure to get into gear can also be caused by the clutch, if the clutch isn't fully releasing or has other problems. The clutch might need adjustment. Check my article on bad clutches. Also, consult your vehicle repair manual.

Inspect gaskets, seals, or loose bolts when looking for oil leaks.
Inspect gaskets, seals, or loose bolts when looking for oil leaks. | Source

7. Leak From the Transmission

Manual transmission leaks can be caused by:

  • bad or worn-out seals or gaskets,
  • a damaged case or component,
  • or even loose bolts.

To verify that a leak exists, first check the transmission case and oil level. If it leaks after you just replaced the oil, you may have put too much. Consult your vehicle repair manual.

Locate the source of the leak. Inspect the transmission oil seal and the O-ring in the vehicle speed sensor.

Consult your vehicle repair manual for the proper procedure to replace seals, bearings, or gaskets. Some of these repairs don't require much work and you may be able to do the job in your own garage with a few common tools.

Knowing about common manual transmission problems can help you diagnose your problems sooner, and possibly save money. This guide helps you identify and explore those common and not-so-common areas. Still, manual transmissions vary between models, so once you have an idea of the possible problem with your transmission, consult your model's vehicle repair manual to troubleshoot that particular issue. You may be able, in some cases, to do the repair yourself.

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    • Dan Ferrell profile image
      Author

      Dan Ferrell 3 months ago

      Hi H,

      Glad to hear you had a spare. Thanks for stopping by.

      Good luck

    • Hippie2000 profile image

      Hippie Untiet 3 months ago from Wisconsin

      Yeah I had a vibration coming from the from the front of my SM465. Turns out the needle bearings on pilot were gone. And it mushroomed the input shaft. I was happy just to have a spare SM465 to replace it. Now I have time to rebuild the old one and replace that shaft.

      So new clutch, new slave cylinder (I like the old rod and spring version better though) and replaced trans. Had it back on the road by night fall and readly to pull heavy hay wagons in the morning.