The Stovebolt Site Forums The Shop Area Driveline I may have broke my pressure plate ?

Canadian 1950 GMC 1/2 . 3 in the tree and a 216. The truck has been up and running for some time now. 2500 miles or so. I had everything working down to the clock. And yes even the vac wipers and emergency brake. Well long story made short . My son came home and we went for a ride. He was doing well learning to double clutch. Motoring down the pavement around 50 mph. He decides to take a left. Slows a bit and [oh no] he abruptly shifts from 3rd to 1st. Well the gear went in. The clutch still to the floor the tires lockup and squeal on the pavement. Ok now engine keeps running but no power transfer to the back wheels. So the question is what is broken ? Feels like I can shift as always into any gear. In fact I got her moving under her own power in reverse only just enough to back her off the pavement . and onto the gravel side road. Thats all she had tho. Now nothing in any gear. Maybe a little chatter when releasing the clutch.Very little. I should just be happy she backed off the pavement. I'm guessing pressure plate troubles? What are your ideas? Thanks in advance.

Start by looking for a broken axle shaft. Jack up the rear wheels one at a time and try to turn the lifted wheel with the engine stopped and the transmission in gear. It should not turn. The axles were not nearly as durable back then as with today's steel alloys. Another possibility will be the universal joint under the torque tube ball at the back of the transmission. If it's broken, either of the rear wheels will turn, but there might be some grinding or rattling inside the pivot ball. Both of these items can be checked with a minimum of disassembly. Of course, the cover plate at the back of the rear end can be removed for a quick look-see at the gears. Start at the back wheels and work your way forward until you find something broken.
Jerry

Thanks for the help. I have more info after further inspection.Speedo works only when being towed. I can start truck. I can shift in and out of gears. with clutch down. if I try shifting without the clutch I will feel the gears grind until I step in the clutch.It will then slide into gear. It all feels right.Just doesn't move. If my universal or axle is broken I should still have a working speedo. Correct ? No working speedo when in gear and 2000 rpm on tack should mean it is ahead of my universal should it not ? Remember my speedo works when being towed. Thanks

And more. So left tire is off the ground. It spins by hand after one revolution it gets hard to spin. I then started the truck put in gear and left wheel off the ground will free wheel . It acts as a broken shaft .Slow to start but with some coaxing will spin quite fast. When it spins the speedo registers . So that means the speedo is running from power off the back tire not the front of the trans ? what the heck..

Ok so this is where I'm at. The speedo works when the back tires move. Either from being towed or from engine power. Must have a broken shaft because rpm seems faster than the tire is turning when one tire is jacked off the ground. Must not be a posi rear end but is a torque tube. All original truck. as far as i can tell. Since the speedo moves only when the tires turn my break must be before the speedo.

sounds like it is time to pull that axle shaft, I'd start with left axle, if it is fine, go with right, then look at spider gears in the rear end.

If you run it in gear with the wheels on the ground, the truck doesn't move but the speedometer registers, something behind the transmission is broken. If it doesn't register, something is broken ahead of the output shaft of the transmission. Your speedometer runs off a gear at the rear of the transmission output shaft.

Why was he double clutching?

By chance is your truck a long bed?

No truck is a short bed. And double clutch gears . A pause works as well to keep it in the next gear but not always effective. Just gives the trans a moment to effectively line up. I'm not sure exactly but have drove lots of 50's era 1/2s and larger trucks and all gms tend to not always match if you don't double clutch. Maybe others can chime in with their ideas.

Ya the speedo runs only when the back tires rotate. I would bet that the clutch plate is severed. Friday i will first check the drive shaft u-joint .then almost certainly drop the trans if it't not the input shaft. Which i highly doubt . It will be the clutch plate. Can i get a clutch plate at napa ????

Depends on the diameter.

The reason I asked about the double clutching is that by 1950, the GMC had synchromesh gears in both the 3 speed and the 4 speed. However if the synchros don't work very well, double clutching can do the job.

Well there you go. Good to learn something new. When she is up and running i will need to do a bit of trial and error as in shifting both ways. I know as a kid we would always double clutch the chev 49 3 ton. By the way i still have it also. It will be my next old project. It'in very nice but original farm grain truck shape.

Update on my clutch problems. Well I kinda thought my problem was ahead of the trans. because the speedo did't register . So I just bit the bullet and pulled out my trans . A 3 in the tree. When out I could see the universal was fine and also the trans seemed fine. Out came the clutch and soon found that one side of the clutch plate was disintegrated . In went a new clutch and after some adjustment all is working fine . I now do not need to double clutch any more . Also the old truck seems to have a little more go off the line . After cleaning 65 yrs of grease and gravel off the trans. it could be just the weight difference that is making it quicker . Ha Ha. I would like to thank everyone for all the helpful advice I received along the way . Thank you.

Good job!!!