Fixing gear ratio errors on Mitsubishi F4A50 series transmissions

Jan. 1, 2020

It is very common to hear of gear ratio errors and shift complaints with Mitsubishi’s 4 speed front wheel drive F4A50 series transmissions.

Wayne Colonna

Mitsubishi F4A50 Series Transmissions Harsh or Flared 2-3/3-2 Shifts and/or Gear Ratio Errors

It is very common to hear of gear ratio errors and shift complaints with Mitsubishi’s 4 speed front wheel drive F4A50 series transmissions, simply because there are a variety of reasons that can contribute to these complaints.

One reason for shift complaints especially after repairs or a rebuild is not resetting the shift adapts. Now that may sound like something that a shop would do right away and in most cases it is. The problem is that with 2004 and later only a factory scan tool can reset the shift adapts. So many times this process is left undone in hopes that it will drive itself out.

Other reasons can be related to an actual slipping transmission, solenoid malfunctions, control valve sleeve wear and/or valve bore wear in the valve body.

Another reason that can be quite elusive is an incorrect gear ratio transmission was installed or an incorrect planetary assembly was used during the rebuild process.

There are two different ratios available as the charts in Figure 1 reveals. There it can be seen that the difference in ratio is with 2^nd, 4^th and reverse gears. The difference in ratio is slight enough that the computer may not initially pick up on a gear ratio error. At first it attempts to adjust the shift adapts causing 2-3 and 3-2 shift complaints. Eventually the computer will produce P0734 for a 4^th gear ratio and possibly P0732 for 2^nd.

The difference in these two ratios is related to the sun gear. This sun gear can have either 25 teeth or 31 teeth. This affects the teeth count with the pinion gears in the carrier assembly. The pinion gear teeth count can be 21 or 18. The internal ring gear teeth count (68) remains the same for both.

A quick and simple way to determine if the wrong ratio transmission is the cause, erase all codes and drive the vehicle in the Manual 2 range and hold 2^nd gear on a drive. This will give time for the computer to calculate the gear ratio. If it’s wrong a P0732 code will show up quickly.