How the transmission works:
Both manual and automatic transmissions are complex and require a significant number of components in order to operate. Of course, automatic transmissions are the more complicated of the two systems. However, your problem might have nothing to do with the transmission at all. For instance, if you can’t shift out of park with an automatic transmission, chances are that the problem lies elsewhere, perhaps with the brake light switch attached to the brake pedal. In a manual transmission, changing gears requires a working clutch pedal, clutch and other components. You press the clutch pedal, which engages the clutch and stops the transmission from spinning with the engine. This allows you to shift gears. Sequencers allow you to shift smoothly into each gear.
Additionally, there’s the question of your master cylinder if you have a hydraulic clutch (some cars have a clutch cable, but some are hydraulic and will have a fluid-filled master cylinder and slave cylinder that must be in operation in order to shift gears, or the car will act like the clutch pedal isn’t pressed). In an automatic, the transmission does all the work for you. All you have to do is press the accelerator, and the transmission will shift on its own as your speed increases. This requires a number of components not found on a manual transmission.
Possible Root Causes for why you are unable to change gears:
- Low Transmission Fluid Level: Both manual and automatic transmissions require fluid (different types) in order to operate. If the fluid is low, there’s a chance that you won’t be able to change gears, particularly in an automatic transmission. This also causes immense damage to the transmission itself. However, it’s more likely that you would be able to shift, but the transmission would not move the car.
- Water Mixed with the Transmission Fluid: If the vehicle was driven in water logged areas, it is possible that there is water mixed with the transmission fluid. An inspection of the transmission oil can reveal if this is the case. If so, transmission oil must be replaced.
- Worn Out Gear Lever Assembly: A worn out gear lever assembly could create excessive play, due to which gearshifts could become hard. If this is the root case, the gear level assembly must be replaced.
- Low Fluid in Master Cylinder: If you’re driving a manual transmission vehicle and it has a hydraulic clutch, one of the first suspects is low fluid in the clutch master cylinder. This is generally caused by a leak in the system (you may notice fluid on your clutch pedal).
- Air trapped in the clutch hydraulics: If there is air trapped in the clutch hydraulics, the entire movement of the clutch pedal might not be transferred to the clutch plate. In this case, trapped air must be removed using standard procedure.
- Broken Clutch Cable: If you have a cable-operated clutch, it’s possible that the cable has broken. If the pedal goes to the floor without engaging the clutch, this would the one of the primary possibilities.
- Failed Brake Light Switch: If you’re unable to shift out of park with an automatic transmission, the most likely culprit is the brake light switch. It’s mounted to the brake pedal and designed to engage the shift lock solenoid if it detects that your brake lights aren’t working.
- Bad Sequencers: If you’re able to shift out of a gear, but when you attempt to shift into the next sequential gear, you hear a grinding sound, chances are good that the sequencers are failing or have failed. You should be able to shift to the next highest gear without trouble if this is the problem.