1969 GMC Clutch Issues


#1

Hello. Here’s an oldie but a goodie. I have a 1969 GMC K3500 3/4 ton pickup that I use as my driveway plow. The last time I used it, I noticed that the clutch stopped engaging after a particularly long plow session. I stepped on the pedal and it wouldn’t engage, and the gears would just grind. I parked it, and started it up again the next morning to finish plowing and it was fine. I figured something had just gotten frozen from the heavy, wet snow. I did get it stuck in snow banks a few times. I left it parked for a few months and then started it up during the summer and the clutch would not engage again.

The bottom plate that covers the clutch access has always been missing, and I figured some snow and ice had frozen up the linkage, but that’s apparently not the problem. I thought maybe the friction plate had no material left on it, but then why would it be fine the next morning? Does this thing need a new clutch, or is there something frozen in the mechanism that might be easily repaired?

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#2

Using a truck with a manual transmission as plow truck burns the clutch out in no time. So the clutch is probably shot.

Everyone I know who has a plow truck, has an automatic transmission with a tranny cooler.

Tester


#3

I’m confused whether the clutch won’t engage or won’t disengage. When you step on the pedal, to the floor, the clutch should disengage, allowing you to shift into gear without grinding. When you release the pedal the clutch engages and the engine makes the truck go. Could you please make your problem more clear?


#4

I’m sure OP’s clutch is not Disengaging.


#5

I got a standard transmission on the advice of my mechanic. I have seen several automatics require a rebuild after plowing. I got 7 years of plowing out of this clutch and I had no idea how old it was.


#6

Sorry, it looks like my terminology is mixed up. The clutch doesn’t appear to be working when I step on the pedal. The gears grind when I try to shift. The pressure on the pedal seems OK.


#7

Since you engage (push) the clutch pedal to disengage the clutch confusing the terminology is very common.


#8

To my unschooled mind this sounds like the mechanism that releases the clutch has given up the ghost. I’ll bet it’s a mechanical linkage. If you are trying to fix this yourself you could open the hood and watch what happens when someone pushes down the clutch pedal. Then slide underneath and watch what happens there. If everything seems to move logically, then the problem is inside the clutch housing and the transmission has to come out. On a 4 wd that’s a serious business.

The only other thing I can imagine that would possibly be cheap to deal with is that the clutch disk is stuck to the flywheel. They do rust together sometimes. If it was mine I’d try to roll it up to something really solid, like a big tree, put it in neutral, start it, step down on the clutch pedal and then try to jam it into 2nd gear. If it goes into gear it should jump a bunch and either stall or the clutch disk will come loose or break. I’m sure my fellow responders will tell you it’s a terrible idea, so I’m ready for the abuse.

Is it worth doing? Only you can say.


#9

I don’t know what’s up with the clutch, but that’s a pretty cool old truck. Would be worth a fair amount of money to some folks if it weren’t for the rust issues.


#10

I presume this is a cable operated clutch of some kind. My old VW Rabbit had that configuration and one time the cable broke, resulting in a limp clutch pedal & no forward progress. A replacement cable was an easy fix. Suggest to battery-up your flashlight so it’s nice and bright and start by tracing the entire mechanism from the clutch pedal to the release lever while a helper operates the clutch pedal. Apply lube along the way. You may find somethings that’s binding. However if the pedal feels like it takes the same amount force to press as always, the transmission is probably going to have to come out for a inspection, and if you do that you’ll probably be installing a new clutch. Not an unexpected thing a truck of that vintage used as a plow vehicle. Good for you for keeping the ol’ gal running & employed.


#11

Welcome to upstate NY :slight_smile:


#12

I think the linkage goes from the pedal directly to a lever on the side of the transmission. It seemed to be moving with the pedal, but I will check again. If it is surface rust on the disk and flywheel, maybe I can get a little bit of PB Blaster up in there, of course hopefully without messing up the friction material on the disk.


#13

That gravel guard on the bell housing is needed in the best of driving conditions and it’s likely critical when plowing snow. And if you have a 4 speed (granny low) transmission it will take a heck of a beating and keep coming back. The outrageously low ratio in first on those transmissions virtually eliminates clutch slippage.

And the clutch linkage on ole pickups was a lever operated rod from the pedal dropping down to the crossover linkage where a short rod extended to the clutch fork. There was a grease alemite on the crossover on those I recall working on.

And the K3500 was a one ton truck.


#14

Its a 4 speed, and 1st is very low, about the length of the truck before you have to shift. I believe the transmission is a Rockwell T-221. The code on the VIN indicated a 3/4 ton Fleetside, but perhaps that’s wrong.

Anyway, how you describe the linkage seems correct. Perhaps the fork is hung up and I can get some penetrating oil past the rubber boot where the linkage goes into the bell housing. I will look for a grease fitting.


#15

Really bad idea!!


#16

The clutch linkage sounds like the same setup as the old Jeep CJ’s I owned. On those, the rotating part (I think it was called a “bell crank assembly”) contained plastic bushings that would wear out or break. This would cause the clutch fork not to get pressed as far as needed. The clutch “free play” would basically be out of adjustment at that point. So that might be something the OP could check for the grinding. Eventually, the bushings would wear enough (or break) and allow the push rod to slip off the clutch fork. Then the return spring would pull the clutch pedal to the floor, and you had to limp back home shifting without a clutch, or buy the bushings and some grease and repair it in the parking lot of an abandoned store :grin:. I’ve done both, and prefer the limp it home approach.


#17

Those old trucks were geared so low that it was easy to use the starter to get them off to a start and then shift up through the gears at low speed, even shifting into 4th at 15 t0 20 mph.

And yes. If that crank bushing was gone the stroke of the push rod would never extend far enough to disengage the clutch. Also, if the left motor mount was bad releasing the clutch caused a lot of lurching, likewise in reverse if the right side mount was bad.

The 8 bolt rims and K3500 fender marker indicate a full 1 ton but the heaviest version of the 3/4 ton had the same axles as the 1 ton with 8 bolts on the wheels. But it doesn’t matter much in discussing it here. With the 8 lug wheels and granny low 4 speed it’s close to being a farm tractor.