M5OD has begun popping out of first and sometimes second

[Shran]

I will preface this discussion by stating that I rebuilt the trans in question, I have successfully rebuilt several others and have had a bunch of them apart and am looking for some experienced guidance on how to proceed.

This is all in a 1992 Explorer and the goal was to get a trans with lower 1, 2, 3rd gears. My build consisted of a 1994 4.0 M5OD case, 1988 2.3 tailshaft/1/2/3 gears, 1997 5th/reverse cluster, new input and countershafts to match the 2.3L components, a bearing and syncro kit obtained from Allstate Gear, and a top shifter housing from the 1988 2.3L trans.

I inspected everything as well as my fairly untrained eyes could throughout the build process and replaced anything that looked worn. Everything on the main shaft looked to be in good condition and the build process was pretty smooth. I used the shifter housing from the older trans because the shift forks were in good shape and it has the neutral switch, which my Explorer also has.

I put roughly 2000 miles on the trans with zero issues other than 4th gear being slightly hard to engage right out of the box, which I attributed to break-in and it has since been much better. However during my Utah trip last week, I was going up a small hill on a dirt road in 2nd gear, 2wd and it popped out of gear. I initially figured I was just lugging it too hard but the next day it happened again in 1st, 2wd. Frustrating. I wheeled all of Hell's Revenge and another ~500 miles of dirt and highway and had zero issues until a steep dirt road where it again popped out twice in 1st, 2wd. After that it has been much worse... even on the street pulling away from stop lights it will pop out of first. I will note that it has never happened in 4 Low (I'm assuming the gear reduction takes some load off whatever is happening in the trans.)

Looking for some guidance on what to look for - I will for sure check the 1/2 shift collar, IIRC that one is directional but I'm 99.99999% sure it's oriented correctly since I marked it as the donor was disassembled. 1/2 shift fork is a possibility but it looked fine, just some light scuffing was all. I read another thread that I can't find now where someone mentioned shimming the countershaft tighter fixed a similar issue. Seems to me that I had to shim it a hair loose.

@adsm08 - you were having this issue with yours right?
@PlumCrazy
@AllanD
@Transman304
@bobbywalter
@BlackBII

 

[ford4wd08]

I owned a M5OD in a '95 F150 behind the 4.9 straight six.

It did the exact same thing that you described popping out of 1st gear. I could beat it into 1st and it would stay, but not well.

During my M5OD rebuild, the trans guy showed me the parts. The shifting forks were worn down to nothing......

I don't know if that is your issue or not, but some experience I had with the M5OD I owned.

 

[Eddo Rogue]

I know a transmission guru that pretty good at phone diagnosis. I used help him out w/ the R&R's when he got slammed, and would watch him rebuild these things w/ his eyes closed. Everything was always spotless during assembly. A lot of guys/shops that had a crappy rebuild or unique problem, or need a race build will seek him. Might be worth it to call and pick his brain.

S K Performance Transmission · 9900 Glenoaks Blvd STE 1, Sun Valley, CA 91352

★★★★★ · Transmission shop

goo.gl

 

[Shran]

A worn shift fork would absolutely cause this but IF that ends up being the case, how did it happen? Difference in top cover between 1988 and 1994? Shift collar installed backwards although I think I did it right? Variations in the shift fork itself between years? Something else?

I need to pull it and tear it down I guess... I am gonna be very confused if the 1/2 fork is fine and the shift collar is oriented correctly. FWIW I changed the fluid about 400 miles in and saw nothing I would consider abnormal although I did have the problem of excess pressure in the case which caused an ATF flood on the floor...see my other thread regarding that mess....

 

[BlackBII]

First guess is a worn shift fork, as that would make it not go completely into gear.

Here's the relevant pages from my 1992 Manual.

It's been a while since I was in an M5OD. Does the 1st and 2nd gear hub have the little half circle spring and tabs that help them lock into gear?

 

[Transman304]

Not model specific but same principle. You will find wear in the speed gear or slider. I rarely see shift forks worn under normal use but if you drive with your hand resting on shifter it will wear.

 

[adsm08]

I still am because I lack the motivation to do anything about it. Mine pops out of Reverse, so I can live without it with some careful planning.

Worn synchro or speed gear were the only things I remember the book saying could cause this. Unlikely with two gears doing it. Possibly a bad synchronizer hub or a worn fork. Those are the only parts specific to those two gears.

Did you check that end play was in spec before final assembly?

 

[Shran]

Thanks guys. I'll reach out again once I've torn it down.

Did you check that end play was in spec before final assembly?

Hmm, end play on what part?

 

[4x4junkie]

Probably referring to the output shaft assembly. I could foresee too much play in the bearings of the output shaft allowing enough misalignment in the gear assembly to cause the shift collars to 'walk' themselves out of engagement while under load. I think a bad pilot bearing could cause it also.

IIRC there are shims behind the front cover plate that set the end play of the shaft assemblies (it's been some time since the last time I had a M5OD apart).

 

[Shran]

Yeah I can see that happening too. IIRC I was able to shim both the input bearings and the countershaft very close to the tight side of the specs in my book.

I'll check the pilot bearing once I get the trans out, it was new roughly 25k ago and I greased it 2k ago when the trans was out but that means nothing, really.

Someone on Explorer Forum suggested the same thing Transman did, I'm hoping that's the cure. I don't think I inspected the gears on the main shaft as well as I should have and now it's biting me... I figured they worked when the were in my other truck before so they should be fine now but they were also set up differently in another trans.

I haven't had the energy to tear into this yet. I just fixed another one that had a roll pin fall out of the 1-2 shift fork... I can only pull and reinstall so many of these before I'm kinda burnt out.

 

[Shran]

OK dudes... I have this torn down finally and would like some opinions. Some things I noticed:

- both slider rings 1/2 and 3/4 are kinda "loose" and move a little in every direction even when they're locked into a gear. 3/4 is probably looser than 1/2. Normal?
- Those little teeth on the gears themselves have a bit of wear... some of the other transmissions I've torn down were significantly worse. Should I just replace them?
- Shift forks are fine. They have light polished areas where they contact the slider rings but no material missing.
- Also noticed that I have a little bit of play in my input shaft bearing. Was not there when I built the trans, either I did not shim it correctly or there is some wear.

Would the slider rings being as loose as they are, compounded with a bit of wear on the teeth cause this? I am real tempted to just order new 1/2 and 3/4 clusters at this point...

See video:

 

[adsm08]

I have never seen that much play in the shift hubs. But then I've never rebuilt one of these for a real issue, just wear parts, or a 4x4 swap.

@Transman304 hopefully will have a better idea of how normal or not that may be.

 

[Transman304]

The movement of the sliders is normal, the teeth on the speed gears look worn. Really need to get it apart so you can inspect the hub and slider. Definitely need a small parts kit with keys and springs.

 

[Shran]

The movement of the sliders is normal, the teeth on the speed gears look worn. Really need to get it apart so you can inspect the hub and slider. Definitely need a small parts kit with keys and springs.
View attachment 45467View attachment 45468View attachment 45469View attachment 45470

I got this thing torn down the rest of the way tonight. I found one huge, glaring problem - the main shaft center bearing race was crooked inside its pocket and inside the rear bearing retainer. It was worn really bad - visibly so - I measured that it was 0.245" deep on one spot and 0.272" on another. So that needs fixed for sure... I don't know if it was like that before the rebuild and I missed it, or if something else is going on. The fluid was full of ground up aluminum particles, that bearing is thrashed, the input shaft bearing is junk. I don't think the bearing itself was to blame, it was seated flush on the main shaft.

Could that cause or at least contribute to my popping out of gear issue?

I would think that the race should be somewhat loose-ish inside its pocket... correct? I can slip it in and out like I can with the input shaft race... it's not super sloppy, I'm just not sure why it would have spun inside the bearing retainer.

Guess what I'm getting at is, like to just rule out a junk case. I have another one I could probably put these guts into but it's still a functional spare trans so I don't know if I want to give that up.

First gear looks OK, the hubs look OK, sliders both have a bit of wear so I will just replace those. 2nd gear looks a hair worn too, a few teeth are a little rounded off but nothing like your pics, just the very tip is worn. 3rd gear looks good, input/4th still looks new.

 

[Transman304]

That definitely will cause the problem you described. Could you post some pictures of the parts that are in question? Sounds like end play was incorrect.
Brad