Possible reasons:
* Torque converter lock:
* When braking, the torque converter is locked, which reduces engine speed to save fuel.
* Engine braking mode:
* Some vehicles have an engine braking mode that downshifts and uses the engine to slow the vehicle. This also leads to a decrease in engine speed.
* Gearbox malfunction:
* Worn friction plates or other transmission problems can cause engine speed to drop when braking.
*Speed sensor malfunction:
* A faulty speed sensor may send an incorrect signal to the engine control unit (ECU), resulting in a drop in engine speed.
* Brake booster malfunction:
* A vacuum leak in the brake booster can reduce braking force, requiring the driver to apply more pressure to the brake pedal, which can result in a drop in engine speed.
What to do:
1. Check the fluid level in the gearbox. Low fluid levels can cause shifting problems and cause engine speed to drop.
2. Conduct transmission diagnostics. A mechanic can use a scan tool to identify any trouble codes that may indicate transmission problems.
3. Check the speed sensor. Check the speed sensor for any damage or contamination. Replace the sensor if necessary.
4. Check the vacuum brake booster. Start the engine and press the brake pedal several times. If the pedal does not return fully each time you press it, the brake booster may be faulty.
5. Contact a mechanic. If you cannot determine the cause of the engine speed drop when braking, contact a qualified mechanic for further diagnosis and repair.