Is it just me? Trans issues 87 4wd 2.6 5 speed long bed 150k miles

[96meo]

So got my rig for free from a family member right before they passed. I went the whole 9 yards on the engine new non air injected head, bigger valves, bored over, balanced, weber, yada yada. New u joints, and center support bearing, fresh clutch, changed all fluids, went bumper to bumper to have it road safe and daily drivable. 1st gear always sounded straight cut. Towed a small uhaul trailer over a mountain once no issues. After about 6 months was pulling onto the freeway about 3/4 throttle in 3rd gear shifted to 4th and the truck was knocking(not from the engine thankfully). Isolated it to the trans and parked it for a few months till I tracked down a parts truck(87 short bed 4wd 5 speed missing the cylinder head). Swapped over trans the transfer case was not the same so I used my old one everything bolted up fine trans was quiet and shifted smooth. 2 months later 5th gear started to make noise. Another month all gears but 4th sounded straight cut. Changed the fluid seemed a little dark but nothing jumped out or was shiny(first time since installing the used trans which I had drained and filled during install about 10k miles had passed). Driving to work thought Id missed a gear shifting 3-4 pushed in the clutch put it in fourth got onto the highway shifted to 5th and cranked up the tunes to tune out the noise from the trans(45 mins all highway speeds 70-85). Got off work hopped in my truck took off and trans is no longer making noise, however it now pops out of 4th gear after 3-4 shift unless held there for a few second, also pops out decelerating in 4th gear.

So thats where Im at now its my daily driver, I am an Auto Tech who is capable however I do not have the time to tear it down and investigate also its my only vehicle currently. Truck has heavy steel wheels, and stock size K02’s, is that in combination with being a long bed too much for the 5speed? Is there somewhere to source a new/rebuilt trans or even a beefed up one? I can swap the trans on a day off, tear down is not an option.

I really want to keep this truck and continue fixing it up but it has to stay reliable and being at the end of trans #2 has me discouraged unless trans 3 comes with resolve to the issues I'm having. Help?

[tortron]

Bent shift forks maybe

[SubGothius]

What spec of gear oil did you fill it with? These trucks spec GL-4 "or higher" but GL-5 wasn't yet developed then.

When GL-5 came out, it turned out to be unsuitable in many ways for gearboxes spec'd for GL-4 or earlier -- too slippery for many older-style syncros to work properly, and some of the additives can corrode yellow-metal (brass/bronze/copper) components.

The general rule now is to only use GL-5 for gears that specify it, and only GL-4 for gears that specify that, but gears spec'd for GL-1 through GL-3 can safely update to GL-4.

[geezer101]

It's possible the front bearing has completely failed and the gear set thrust is making it jump out of gear. I think you will need to remove the transmission sump cover plate and get a small pry bar and lever against the gear sets to see how bad the shaft thrust play is to determine where it has failed.

[96meo]

What spec of gear oil did you fill it with? These trucks spec GL-4 "or higher" but GL-5 wasn't yet developed then.

When GL-5 came out, it turned out to be unsuitable in many ways for gearboxes spec'd for GL-4 or earlier -- too slippery for many older-style syncros to work properly, and some of the additives can corrode yellow-metal (brass/bronze/copper) components.

The general rule now is to only use GL-5 for gears that specify it, and only GL-4 for gears that specify that, but gears spec'd for GL-1 through GL-3 can safely update to GL-4.

Kinda was afraid of this I have been using GL-5 as it’s what my work has in bulk and what the parts store had in stock was told GL-5 was superior and supersedes GL-4. I’ll get some GL-4 in it and start looking for another trans as the damage has been done. Once this ones out I’ll tear it down as well as trans 1 see if I cant rebuild a good spare. Hoping I can get the correct fluid and limp it through winter. It’s hard to find a trans that doesn't come with the whole truck and I ran out of storage space. One of these months I’ll get some photos, document the truck, and donate. This site has made it possible to bring this truck back to life and daily drivable. Thank you for the response.

[geezer101]

You can buy new synchro sets for the KM145 w/bearings etc but make sure it is specifically for the KM145 and not KM145/KM132. The 2 transmissions don't share synchros and gear sets. Found this on site after a google search - http://www.mightyram50.net/vbulletin/showthread.php/2308-KM-145-Tranny-Info

[SubGothius]

Kinda was afraid of this I have been using GL-5 as it’s what my work has in bulk and what the parts store had in stock was told GL-5 was superior and supersedes GL-4. I’ll get some GL-4 in it and start looking for another trans as the damage has been done. Once this ones out I’ll tear it down as well as trans 1 see if I cant rebuild a good spare. Hoping I can get the correct fluid and limp it through winter. It’s hard to find a trans that doesn't come with the whole truck and I ran out of storage space. One of these months I’ll get some photos, document the truck, and donate. This site has made it possible to bring this truck back to life and daily drivable. Thank you for the response.

If it hasn't been running the GL-5 very long, there probably won't be much if any damage/wear due to that alone, mostly just poor syncro function in use, which could explain the straight-cut impression and even the slipping out of gear. Redline, AMSOil and Royal Purple all offer top-quality synthetic GL-4 gear oils.