Transmission jumps out of 3rd gear

[Martin64]

The transmission in my truck jumps out of 3rd gear occassionally, usually when it's under power (meaning not when it's coasting or when I let up off the gas). Any ideas?

[shortbed70]

TH350? time to crack it open when mine did that it was shot.

[Martin64]

3 speed manual.

[markeb01]

I've never had a 3 speed pop out of 3rd so I looked up the troubleshooting section of my 1966 Motors Manual. They show 2 types of transmissions. I'll list them both since I don't know which yours is.

A. GM transmission with 2nd speed gear located at rear of mainshaft.
1. Transmission mounting bolts loose.
2. Control rods interfere with engine mounts or clutch release lever.
3. Control linkage does not work freely.
4. Gear does not fully engage.
5. Damaged mainshaft pilot bearing.
6. Clutch gear bearing retainer broken or loose.
7. Dirt between transmission case and clutch housing.
8. Misalignment of transmission.

B. All transmissions with low-reverse located at rear of mainshaft:
1. Improper shift procedure.
2. Linkage parts worn, bent, broken or out of adjustment.
3. Excessive end play caused by wear in shift forks, sliding gear fork grooves, thrust washers, mainshaft and countershaft bearings, or clutch pilot bushing.
4. Misalignment or excessive clearance between sliding gear and mainshaft.
5. Damaged synchronizer.

My experience with a manual trans popping out of gear has always been a bad synchro, but it's normally on 2nd gear in a 3 speed, or 2nd or 3rd in a 4 speed. Hopefully something hear will help diagnose the problem.

[Martin64]

Thanks for what you posted. I'm actually leaning toward the linkage not being right. There's really no "slop" in it at all, really to the point of it feeling a little tight...especially in 3rd gear. I'm just wonding if it I can't work some "play" into it to be sure it's getting all the way into 3rd. It doesn't immediately jump out of 3rd, but if I don't rest my hand on the shifter it will come out. I noticed it more-so the other day under accelleration going up a hill.

[markeb01]

That really does sound like the gears aren't fully engaging. If they aren't achieving full alignment, increasing the torque by accelerating will tend to force them apart.

[Martin64]

Reverse, 1st and 2nd are fine. I'd experienced 2nd gear being bad in an old ranch Jeep years ago, but it only came out of 2nd when you let up off the gas pedal....(that was fun going down hill!). But I thought it odd that 3rd would jump out, especially during accelleration, but I wanted to make sure this wasn't characteristic of the 3 speed. Ok, this gives me something to do this weekend....have my son move the shift lever in the cab while I watch from underneath and see what's going on.

[chevyguys]

I read somewhere that 3rd gear popping out is usually caused by a bad front bearing... It was somewhere on this board but I can't find it now...

[Martin64]

I've heard that one too. But I'm going to check the easy (and free) stuff 1st. It wouldn't suprise me if the clutch is wearing thin also. I've had the truck a little over a month and I'm sure some things are going to need replaced along with a few obvious things I've already spotted. I already started tracing a few "mystery wires" that start at one end of the truck and end at the other...and neither end is hooked to anything. I even found a toggle switch with wires leading to it but weren't attached to anything.

[Tx Firefighter]

Those 3 speeds are dead simple to work on. If the linkage adjustment doesn't fix the problem, I'd remove the side cover and look at the shift forks for wear.

[Martin64]

Hey Kevin, I may be tapping our knowledge on these if I can. I found another 3 speed down here in Chula Vista for $50 that's suppose to be good. My thoughts are (if it is good) to put that one in my truck and rebuild mine. I gotta go look at it today. My plans for checking it are to pull the covers and gander in there and see if something doesn't look right and maybe run a magnet in the bottom through the drain plug and make sure there's no bearing or chunks that come out.

Since mine's still in my truck and I can't count them, does anyone know what the spline counts on the shafts are suppose to be were there variations and it'll be a "crap shoot"?

[markeb01]

If it's original it should have a 10 spline 1 1/8" input shaft. These were used from the mid 50's the mid 80's on various models. The later design has a 26 spline input shaft. If you found the deal of the century on a trans with a 26 spline shaft, the only thing you'd have to do is install a matching clutch disc (and of course a new pressure plate so they would wear evenly).

[padresag]

Originally Posted by Martin64

Reverse, 1st and 2nd are fine. I'd experienced 2nd gear being bad in an old ranch Jeep years ago, but it only came out of 2nd when you let up off the gas pedal....(that was fun going down hill!). But I thought it odd that 3rd would jump out, especially during accelleration, but I wanted to make sure this wasn't characteristic of the 3 speed. Ok, this gives me something to do this weekend....have my son move the shift lever in the cab while I watch from underneath and see what's going on.

linkage wear is a big factor. a little wear at each connection adds up
drop the gear oil out and see how much crap that is in the bottom of the trans. sounds like it could be a brg and if a brg is on the way out, you will see brg material in the bottom of the trans
the one that it may be though is bad eng mount on the left side,either frt or rear. the reason for this answer is throttle pressure in third gear, esp if you are going up a hill applying more throttle as you go up. this will make the eng rise up on the left side which a bad eng mount will do and in essence perhaps pull third gear out of it's detent and you find yourself in neutral
ron

[Martin64]

Well, when I dropped the oil out of the transmission that's currently in my truck I ran a magnet into the drain hole and gave it a quick check. The magnet came out with plenty of sludge as well as 2 needle bearings. So I'm pretty confident I have a "time bomb" on my hands and sure it'll give up the ghost at the most un-opportune time out in the middle of "No-where" and no cell phone reception.

I'm going to hook up with the guy that's selling the other transmission in an hour or so and see if I'm in luck and can change out transmissions this week end , then I'll get started re-doing the one that's in there now.

[Martin64]

Well that didn't work (I don't think). That transmission was an earlier model. It looked pretty good though. I wrote down the casting numbers if someone wants to cypher them out. I'll post those a little later. I left 'em in the truck and I just got a bunch of forms I gotta fill out and get in the mail.

[Martin64]

Casting numbers:

Left side of case 2GM 591616 or it could be 591815
E218

On a cover it had 3686832GM

On the shaft flange where it bolts to the front:
591620


I spun the shaft and went through the gears and it has all 3 + reverse. No weird knocking or clicking, but then again I wasn't spinning it at 1500 rpm either.

[padresag]

Originally Posted by Martin64

Casting numbers:

Left side of case 2GM 591616 or it could be 591815
E218

On a cover it had 3686832GM

On the shaft flange where it bolts to the front:
591620


I spun the shaft and went through the gears and it has all 3 + reverse. No weird knocking or clicking, but then again I wasn't spinning it at 1500 rpm either.

perhaps those needle brgs have been sitting in there since the last rebuild. what sort of shape were they in?
ron

[padresag]

those needles were more than likely dropped at the time it was last repaired. they can't just fall out
2 needles missing can cause slop in the shaft
ron

[Martin64]

The transmission I looked at tonight has an output that looks similar to this one.

[1Bad62Pro/Street]

Had the same trouble with my Ole' 3 speed. Kept jumping out of 3rd gear. Had to hold it down constantly to drive. Drove it for 3 years like that. The synchronizer was wore slap out. Rebuilt it and she ran like a scalded dog! Have the 3 speed and the linkages for sale! The 3 speed transmission and the linkages are posted on the The 1960-1966 Chevrolet & GMC Pickups Parts Board.

1960-1964 rebuilt 1/2 ton 3 speed transmission
http://67-72chevytrucks.com/vboard/s...d.php?t=349416




Thanks,
1Bad62Pro/Street
1 Bad Mamma Jamma

[D'Monsta]

This started happening to my '66 GMC last week.. Glad I found this thread.. I'll keep an eye out for another trans ..

[G8rN@te61]

I just had my GM 3 speed rebuilt by Jim Cook Racing Transmissions in Concord, NC. He specializes in these. Apparently very common in dirt track cars. He's affordable and did excellent work but you have to take the tranny to him. He's not much of a communicator but my truck runs so smooth on the road now.

[D'Monsta]

Originally Posted by G8rN@te61

I just had my GM 3 speed rebuilt by Jim Cook Racing Transmissions in Concord, NC. He specializes in these. Apparently very common in dirt track cars. He's affordable and did excellent work but you have to take the tranny to him. He's not much of a communicator but my truck runs so smooth on the road now.

Thanks.. I will contact him..

[Lokin4AReason]

Knowledgeable thread . . .

My nova did the same , till i found a replacement transmission for it . And slapped the other to the side as a spare until needed

[justa dumb hick]

Randoms thoughts to consider: 1) Verify run-out of the installed bellhousing to the centerline of crankshaft. 2)Inspect the back-cut on 3rd gear. Manual transmissions often have a slight back-cut that "holds" the gears together during load.