Will a bad pulse genetator output speed sensor make transmission go into fail safe mode?

A bad pulse generator (also known as a speed sensor) on an output shaft can indeed cause the transmission to go into fail-safe mode. Here's how it works:

The pulse generator is a component attached to the output shaft of the transmission. It generates electrical pulses that are used by the transmission control module (TCM) to monitor the transmission's output speed. The TCM uses this information to adjust the transmission's shift points and ensure smooth operation.

If the pulse generator fails or becomes faulty, it may provide inaccurate or inconsistent speed readings to the TCM. This can confuse the TCM and cause it to make incorrect decisions about shifting, resulting in rough shifts or even transmission failure.

To protect the transmission from damage, the TCM may put the transmission into fail-safe mode. In fail-safe mode, the transmission will operate in a limited number of gears, usually second or third gear. This prevents further damage to the transmission and allows the vehicle to be driven to a repair shop.

It's important to note that a faulty pulse generator is just one of several possible causes for a transmission going into fail-safe mode. Other issues such as low transmission fluid, faulty solenoids, or electrical problems can also trigger fail-safe mode. If you suspect a problem with your transmission, it's always best to have it diagnosed by a qualified mechanic to determine the exact cause and get the necessary repairs done.