The Stovebolt Site Forums The Shop Area Driveline Truck runs, but won't move

On my way home tonight, I was stopped at a light. When I let the clutch out to go, something let go (small bang, as if something snapped) and the truck didn't move. Could I have broken a u-joint? The engine runs fine, I can move the shift lever, and the clutch pedal feels fine. It just acts like it's in neutral all the time. Nothing seems to sound unusual.

I had it towed home, and I'll check it out a little better this weekend. Anyone have any suggestions on what to look for?

Thanks!

Axle shaft?

Do you have an open or closed driveline? A broken U joint on an open driveshaft is easy to find- - - -the driveshaft will be lying on the ground! The broken axle shaft suggestion is probably a good one.
Jerry

It's a closed driveshaft. I suppose the easiest thing may be to pull the cover off the differential, and look to see what moves when I put it in gear...

I had that happen to my 54 years ago.the spline end of the drives haft twisted off right at u joint. Easy to see if you pull the torque tube ball back from Trans. I had another different to put in .to change shafts is a bit if a job if that's your problem.

Put it in gear and give it some gas, see if speedometer works.
If it does rule out clutch and tranny.
More than likely it is the u-joint or front yoke or rear yoke.
That is the weak spot in the drive train.
Busted rear yoke
4 speed front yoke, u-joint, rear yoke

I had the same thing happen to me.
But I was not present.
I figured someone revved up the engine then slammed it in gear. It has a 1954 factory automatic tranny. Hydramatic.
But after reading elsewhere (last post on page 1 of thread) it does happen if you do not release the emergency brake completely.
The rear brakes are dragging down on the drive train and the first thing that snaps is the weakest spot in the drive train.
The u-joint or front yoke or rear yoke.
Basically its like your giving the engine gas with the tranny in gear and at the same time you have your foot on the brake pedal.

Sometimes the rear yoke gets hot and it snaps.
Could be that they never put U-Joint Locking Clips on the bolts. (only applies to 1941-54 U-Joint)
1929-40 U-joint illustration used 4 bolts & lock washers & nuts, scroll down on left side.
1941-54 U-Joint on right side.

1938 Chevrolet Shop Manual for All Passenger Cars and Trucks
UJ 1 Repair 1/2 ton bottom of page
UJ 2 Repair 1/2 ton
UJ 3 Repair 1/2 ton

I had something similar happen years ago on my '47 1/2 ton (three speed top loader transmission). In this case, one of the pins came out of the shift interlock. Luckily the pin fell to the bottom of the transmission and did not go through the mesh.

A local machine shop repaired the interlock, but I've since replaced it with a NOS.

Good luck.

Bill

Thanks for the tips guys. Edited this post with some new info as I mess around with it some more...

This morning is the first chance I had to look at it again. So when I put it in gear, the speedometer at first didn't move. The clutch pedal felt okay, and nothing looks out of the ordinary from a quick look underneath.

As I played around a little more, shifting and listening, it sounds like the clutch is grabbing because I can hear some sounds from the tranny as if things are spinning.

The speedometer will move sometimes, a little anyway and not always, and when I shift I can grind some gears a little bit (very little). Still, the truck doesn't even inch forward or backward at all.

Does this sound like the clutch isn't fully grabbing? Could some shaft in the transmission have broken? I tried all four gears and reverse, and the same thing happens - not much of anything.

Either pull the torque tube ball back and inspect the U joint, or drop the rear end cover plate and see if the ring gear spins. I'm going to go with the broken axle option over the U joint, as there would probably be some rattling noises from a broken universal.

With a broken axle, the ring and pinion will spin, one axle will keep stationary, and the spider gears will rotate around the stopped axle.
Jerry

Pulled the rear end cover and nothing's moving, so it's upstream from there. Working on getting at the u joint. I've got the bolts out of the torque tube ball retainer, but the retainer is catching on the rear mount. Maybe I can try to jack the tranny a little to get some clearance. I expected to get some oil from when I loosened the torque tube retainer and got none, so that may not be a good sign.

There sure is a lot of North Dakota prairie up under that truck. Also learned about the rule of 25: I could stand to be about 25 years younger and 25 pounds lighter. It's not as easy sliding in and out from under a truck as it used to be.

To make sure.
Your truck is a 1938 Chevrolet 1/2 ton w/4 four speed trans.

When in fourth gear with the clutch out and giving the engine gas.
Can you keep the speedometer at a steady 35 mph for about 5 seconds?

If you can not keep the speedometer at a steady 35 mph.
This might be the problem:
In order of what to check first.
A. Front yoke is busted.(speedo gear is attached to rear yoke)
B. Tranny is broke.
C. Clutch, Pressure plate.

If you can keep the speedometer at a steady 35 mph.
This might be the problem:
In order of what to check first.
A. Front yoke is busted.
B. U-Joint is busted.
C. Rear yoke is busted. (A,B & C can all be inspected by sliding back Ball Housing)
D. Ring & pinion gear busted.
E. Propeller Shaft busted.
F. Axle Shaft busted.

The reason for grinding when shifting gears could be because,
if the u-joint assembly is busted, there is nothing holding (i.e. Ring & pinion) the transmissions rear main output shaft from spinning.
Crosssection of 4 speed Transmission

Finally got around to doing a little more work. The u-joint looks fine. With the transmission in neutral and wheels on the ground I can spin it by hand. So I guess that narrows it down to the propeller shaft or pinion gear shaft.

Any recommendations on what the next best thing to do is? I guess I'm heading toward pulling off the rear end, but am hoping I can avoid that if possible.

Chock tires.
Put pan under the axle housing.
Remove the cover from the back of the axle housing.
When lube comes pouring out check for metal chunks.
Start engine.
Put transmission in gear.
See if ring & pinion turn.
If ring & pinion gears turn normally, then one of the axle shafts are busted.
If ring & pinion gears do not turn normally, you could have a broken tooth on ether the ring or pinion gears.
If nothing moves in the axle housing, you could have a busted propeller shaft or the splines on one of the ends of the propeller shaft are stripped.
More than likely it is a broken tooth on ether the ring or pinion gears.
1938 Axle & partial torque tube
1938 Ring & Pinion and partial torque tube
Exploded torque tube
Torque tube cutaway bottom of page.

Thanks jorb. Yeah, I'd pulled the rear cover off earlier, and everything looks good. No broken pieces. When I put the truck in gear, nothing moves in the rear end. When I roll the truck, the ring gear and pinion gear move fine. I guess the only thing to do is to pull the rear end off and see what's broken. Might take me a while, but I'll post what I find. Thanks again everyone for the comments.

I don't understand. If you have it running in gear and the speedometer is not working, the problem is forward of the U joint and either in the transmission or the clutch. I am assuming that the speedometer was working before.

My bet is a broken input shaft in the transmission. This would help explain the intermittent movement of the speedometer. When the speedometer is moving, the broken ends of the shaft have enough friction against each other to move the speedometer.
Carl