My 1989 ranger five speed transmission has a chip or broken tooth on
overdrive (5th) gear. I believe it is a mitsubishi or Mazda transmission, it was replaced about 40,000 mi ago. Anyhow, I am wondering how difficult it would be to replace that gear? Do I need to pull the entire transmission or is it reasonably easy to do?

 

I have torn apart the Mazda tranny (M5OD or M5R1) but I'm not familiar with the Mitsubishi. There is a YouTube channel named Watch Wes Work where I believe it is the Mitsubishi that he did a two-part video series on fixing one. There are several good videos on tearing down and assembling the M5R1, and it's F150 cousin, the M5R2, on YouTube. There is also a pdf of a M5OD shop manual (it's title uses M5OD) on The Ranger Station. For the M5R1 I think you can do what you want to do with the tranny in the truck. You can access the gears for 5th and reverse, and their common synchro assembly, with only removing the case's tailpiece. The part I'm unsure about is the lever for that synchro because the standard procedure is to start by removing the top cover, which you won't be able to do, and a lot of the shifting mechanism is attached to the top cover. (NOTE: If you remove the reverse or neutral switches, plug the holes so the rods and ***** in there don't come out.)
Attached is a photo looking at what you see with the tailpiece removed. The two gears you see meshed are the reverse gear and idler (to make the output shaft rotate the opposite of forward). The lever on the left moves the synchro collar. Forward of the synchro is where one of the gears involved in OD is and the other would be on the output shaft. You will need to replace any of the two large nuts you remove, but they're only $10-15. (EDIT: A hurtle you'll need to find a way around is locking the tranny shafts so you can remove the large nuts. You usually engage two synchros from the top, 1-2 and 3-4. You might be able to shift into one gear and put the 5-R synchro into a gear.) The second photo is the mechanism that pushes on the synchro collar. The little gear on the output shaft is probably the OD drive gear.
Good luck. I just subscribed to this forum so I'll look for questions.


M5R1 with tailpiece removed, also countershaft nut and rear bearing.


Mechanism that moves M5R1 5th-reverse synchro collar, removed from case.