Top 7 Diagnosing Symptoms of a Failing Manual Transmission
Manual transmission problems can arise due to high mileage, abuse, or lack of proper maintenance. But they are quite rare. Most manual transmission problems do not originate with the transmission itself, but with related components such as the clutch assembly, linkage, or driveline - the components that transmit the turning power of the transmission to the wheels.
Also, as we discuss below, symptoms that appear to be coming from the transmission may be coming from unrelated parts of the car.
The key to diagnosing your manual transmission problem is gathering 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 cornering, only when downshifting, or after you have checked the clutch or other transmission component? Can you feel a vibration? Can you hear a click or squeak?
Symptoms of a bad transmission
This guide will help you begin to identify the source of these problems. You will need to do more research and remember that manual transmission configurations can change from model to model.
Remember that this guide only addresses symptoms that come from the manual transmission (or transaxle) itself. Some symptoms that may seem to originate from the transmission are actually coming from the clutch (or other system) and vice versa. For a more accurate diagnosis, see also the Bad Clutch Symptoms guide.
1. Strange noises that may come from the transmission
The most common cause of a noisy transmission is a lack of oil, which causes the gears or internal assembly to hum or buzz. If a noisy transmission has enough oil, the lubricant may be contaminated with metal chips or particles.
Insufficient or contaminated oil can cause the transmission to become noisy in some or all gears. But if you hear noises in a specific gear, the teeth or synchronizer for that gear may be worn or damaged.
Non-transmission sources that can cause noise:
Some noises that appear to be coming from the transmission are actually coming from an external, though possibly related, source. For example, if you hear a thudding noise when accelerating or decelerating, check for these issues first before blaming the transmission:
- A loose or damaged engine or transmission mount
- A worn or damaged driveshaft inner CV joint
- Problems with the differential case
Turning noises may indicate a problem with the CV joint. Knocking noises when driving at low speeds can come from the differential case or CV joint. However, a worn bearing can also produce bumps or knocks.
A three-step test for transmission bearing noise
That noise you keep hearing may be coming from a bearing. This three-step test is simple and can reveal problems with one or more transmission bearings.
Start the engine and put the transmission in neutral. If you can hear a noise in neutral, suspect a worn input shaft bearing. Watch the video at the end of this article to see how internal problems can affect the operation of the input shaft.
Now, with the engine running, depress the clutch pedal just enough to eliminate free play, to the point where you feel you need to apply force to the pedal to push it further down. You may need to press the pedal down a bit more. If you hear a grinding noise, you may have a worn release bearing.
Fully depress the clutch pedal to release it. If you hear a humming noise, it is probably a faulty pilot bearing.
2. Transmission makes a grinding noise
Problems with the transmission can also be revealed through a grinding noise.
A grinding noise can come from gears colliding. Shock can occur due to worn or damaged gears, joint problems due to wear, or need for adjustment.
Other potential sources could be a worn or damaged synchronizer, shift fork or rail shafts and bearings.
If you can hear the gears clash only when downshifting, the problem may be with the synchronizer (too much play at the output shaft end).
However, a screeching noise can also come from a dragging clutch.
3. Transmission jumps into neutral
This seems to be a common problem with worn transmissions. You shift into gear and the transmission jumps out of gear.
Again, there could be other causes for this problem besides a worn transmission:
A common problem is a worn, stretched, or misadjusted derailleur linkage. A stretched linkage can be caused by a damaged engine or transmission mount. An external linkage can wear or become loose and out of adjustment, causing the transmission to jump out of gear.
Look for rust and bonding. You can try adjusting the link. But in most cases, you will have to rebuild or replace that part of the assembly.
You may be dealing with a weak or broken spring in the derailleur rail. In an internal shift linkage, the spring is part of the spring-loaded ball that locks the transmission in gear (detent). If the ball slips out of the notch, the transmission will jump out of gear.
Also, you may be dealing with a worn pilot bearing (the gap causes the input shaft to vibrate, causing the shift forks or synchromesh to move).
You may be having problems with a worn synchronizer or shift fork assembly or other internal components.
Other possible causes to consider:
- Loose or misaligned transmission (possibly after service)
- Misaligned clutch housing
- Loose shifter cover
- Worn gear teeth
Note: When the transmission just jumps out of a high gear, check to make sure the clutch housing bolts are tight or the transmission is out of alignment.
4. It is difficult to change gears
This problem occurs when you find it difficult to move the shift lever from one gear to another. This usually points to a problem with a loose linkage, worn shift cables, or worn bearings.
Pay attention to this: If you find it difficult to change gear even with the engine stopped, take a look at the linkage. If the harsh shifts only occur when the engine is running, check the clutch. If the transmission has moved recently, check for misalignment.
Also, depending on your particular model, check the clutch hydraulics and make sure it's free of air. Also check clutch adjustment.
Check the binding. You may need to jack up the vehicle and support it securely on jack stands or remove a component to perform the inspection, depending on your vehicle model. You may need to remove a shifter boot.
Get help from an assistant to operate the link while you inspect the link, if necessary. Look for problems like binding, poor lubrication, misaligned components.
To remove components for inspection, refer to your vehicle's repair manual. Haynes is a good aftermarket manual - get the manual for your exact model.
Other causes of a hard-to-shift manual transmission include:
- Worn or loose internal components (shift fork, levers, axles)
- Low oil level (or wrong type of oil)
- Slipper clutch
- Misaligned transmission
- Faulty pilot bearing or bushing
- Problems with the synchronizer
- Too much end play in input shaft or main shaft
5. Transmission is stuck in gear
You may find that you cannot disengage the transmission. This symptom may indicate:
- Low oil level or wrong type of oil
- Problems with the linkage or shift lever assembly. Look for misalignment, wear, or damage to the shift rods, bushings, or arms.
- Internal components: shift rail, seals, forks, or a stuck synchro sleeve
- Worn or broken drive gear teeth
- A switch lane stuck
- Misaligned transmission
6. Transmission won't start
When you have trouble getting the transmission into gear, inspect the shift linkage for tightness, looseness, or damage.
However, remember that the lack of starting can also be caused by the clutch, if the clutch does not release completely or has other problems. The clutch may need adjustment. See my article on bad clutches. Also, consult your vehicle's repair manual.
7. Transmission leak
Manual transmission leaks can be caused by:
- Defective or worn seals or gaskets,
- A damaged case or component,
- Or even loose screws.
To check for a leak, first check the transmission case and oil level. If it leaks after you just replaced the oil, you may have put in too much.
Consult the repair manual for your vehicle.
Locate the source of the leak. Inspect the transmission oil seal and the o-ring on the vehicle speed sensor.
Consult your vehicle's repair manual for the proper procedure for replacing seals, bearings, or gaskets. Some of these repairs don't require a lot of 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 problem. In some cases, you may be able to do the repair yourself.