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.

[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).

[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.

[Killer Bee]

in my 68 GMC, rested my hand on the shifter around town

hooked a short bungee from the shifter to the seat bottom on the highway

bought it that way - sold it that way

[D'Monsta]

Popped out twice yesterday on the highway.. I may try this bungee strap idea while I locate another trans.. Clutch is getting noisy too, so time for some work..