1996 Recurrent transmission issues

[HERSHEY10]

So I just am wondering if any people who are more mechanically inclined than I am would know why I am having the same issues in my ranger time and time again. Since owning the truck I have been through 3 transmissions, which I know is ridiculous. Each time the same thing occurs, Im cruising on the freeway in 5th gear (overdrive), and the gear noise starts getting progressively louder until a loud clunk occurs and I am no longer 'in gear'. I shift into 4th, which works, and get the car to a stop. I try to put the truck in first but the gear isn't there. 1st, 2nd, 3rd, 5th, and reverse are essentially gone and the truck will shift into them without the clutch and will just rev when in placed within the respective gear. This exact thing has now happened 3 times and all with the same diagnosis, 'catastrophic failure of the transmission'. The truck currently has 120,xxx miles on it. The first time it happened it had 102,xxx and I figured I would get it replaced because I wasn't going to find a new car for the price of a transmission. The second occurred at 116,xxx miles and a month after a move for school from Portland, Or to Los Angeles, Ca and then to Louisville, Ky (so about a 3,000 mile total drive). The truck was fine the entire trip and then a month later the above described happened. I just moved the truck across the country and I just started medical school so I didn't have time to get a new car so I had another used but rebuilt transmission put in. Now less than 3,000 miles later, the same thing happened again and I was at wits end. But I am in med school, I have no money and even less time to be shopping for a new car and then dealing with what comes along with it. So currently it is In the shop, getting another used rebuilt transmission put in. My question is, is this coincidence or is something else causing this issue? How can I prevent this from happening again as I am so sick dumping money into a truck that's worth close to nothing? The top end was rebuilt at 100,000 miles so the truck should last me through med school but it seems despite the mechanics being almost new, it keeps dying and its very frustrating. Advice or ideas of why?

 

[RonD]

First a little about manual transmissions.
You have a Mazda M5OD-R1 transmission but they all work essentially the same way.

In a 5 speed, 4th gear is not a "gear", it has 1:1 ratio so no gear is needed, input shaft and output shaft turn at the same speed, 1 to 1 ratio, so they are connected directly together when 4th "gear" is selected, no need for a gear.

1st, 2nd, 3rd, 5th and reverse are gears, and gears are needed because ratio is not 1:1.
The input shaft has a drive gear that turns the counter shaft gear, the above gears are all powered by this counter shaft.

When transmission has 4th "gear" but no other gear then the input shaft gear has been damaged, it is no longer turning/powering the counter shaft gears.

Manual transmissions rarely have problems but input shaft gear stripping is one of the more common problems when there is one.

Why is yours failing?

Can't say, could be an issue with pilot bearing/flywheel causing a wobble on the input shaft, putting stress in the shaft and drive gear.

The input shaft is supported in the back by the transmission, and in the front by the flywheel, clutch system is in between.
When you push in the clutch pedal the input shaft must be free to rotate at a different speed than the engine(flywheel) or you wouldn't be able to shift gears.

The pilot bearing sits in the flywheel and the input shaft tip goes into pilot bearing, this bearing supports the end of the input shaft but allows it to spin at a different speed when clutch pedal is down.

Or could be the rebuilder didn't fix a problem with internal shaft alignment causing stress on the input shaft gear or too much of a gap between the gears.

Or put in the wrong transmission fluid, M5OD uses automatic transmission fluid(ATF) not regular gear oil.

Or didn't replace the rubber shift rail plugs with metal ones and fluid got low, input shaft gear is first to heat up and fail.

To many variables really.

Your 3 times is not common, M5OD has been a very reliable transmission, it was used from 1988 to 2011 in Rangers, so millions are out there on the road, still, and while there will always be failures in mechanical systems, it just isn't that common for the M5OD-R1

Picture here: http://www.nationalnewstoday.com/au...-an-expert.php/attachment/leah-schoenfeld-10b

On the left you can see Input shaft drive gear is completely ground down, counter shaft gear still has some teeth but obviously damaged
This was from an M5OD-R2, used in Ford F-series full size trucks, same lay out as yours, this one ran low on fluid