Q: Car accelerates very slow when it hits 20 mph and neutral shift on dash starts to blink.

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I changed the transmission on my car because before it Wouldn't accelerate when on drive. After i changed that, my car started to accelerate again but now when i start going 20 mph it gets a heavy shake and starts forcing to accelerate right, i check my dash board and my "N" shift is blinking on and off.
My car has an automatic transmission.

Either the transmission has thrust bearings or bushings that are worn causing the internal parts to go out of balance or there is an issue with the fluid is low causing the gears to slip. If the fluid is full and new, then the clutches may be worn causing them to slip. I recommend seeking out a professional, such as one from Your Mechanic, to help you diagnose your transmission shaking issue.

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What you are describing may be a result of a faulty speed sensor, potentially low transmission fluid or a even a faulty transmission control solenoid. As your vehicle travels down the road, the computer analyzes data being sent by vehicle speed sensors and engine speed sensors. Based on this information, the Engine Control Unit (ECU), or the Transmission Control Unit (TCM), executes the appropriate upshifts and downshifts by sending a signal to one of several shift solenoids. These transmission solenoids have spring-loaded plungers inside them, which are wrapped with wire. When this coil of wire receives an electrical charge from the TCM / ECU, it causes these plungers to open, allowing transmission fluid to flow into the valve body and pressurize the desired clutches and bands. When this happens, the transmission changes gears and you continue down the road. The computer controls the transmission solenoid in several ways. If the vehicle is equipped with a dedicated Transmission Control Unit, it can open or close the hydraulic circuit using a direct 12v signal. In some cases, the Engine Control Unit (ECU) can control the solenoid’s plunger by turning the ground circuit on and off. When this is not working properly, this can cause a jerking reaction, a delay in shifting or in some cases may cause the transmission to hesitate in shifting gears. I would recommend having a professional from YourMechanic come to your location to diagnose and inspect your transmission.

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