Is my transmission about to go?

To start off, the vehicle in question is a 77’ Chevy C-10 Pickup(250ci./350 auto trans). When I accelerate from a stop, My truck shudders as if it was spinning the tires, when the tires are not. It only happens in 1st gear and only from a complete stop. I checked the fluid level and its a little overfull. Its been six years since the tranny was rebuilt, but the transmission before it was the original and it lasted for 20 years. I use this truck daily and the problem is getting worse. Any help is greatly appreciated.

Might be a rear motor mount or a center support thats bad and allows the trans to flop around. I would check those and the Ujoints first. LEE

Thanks man, i’ll go take a look.

All appears good in the driveline, at least i couldn’t move anything by hand, and all the bolts appear to be in place.

The oneway roller clutch of the reaction planetary carrier may be worn and slipping. One way to verify this is to shift to manual 1st and see if the problem still exists on start out. If the problem disappears in manual low, the transmission will have to be disassembled to look at that oneway clutch. The mechanic should also inspect the bushings that support the rear planetary because the grab/slip will have stressed the support bearings. You might have to replace the planetary carrier as I think the outside race for the oneway clutch is the carrier itself – the inner is replaceable separately.

Usually when the transmission is rebuilt, unless the transmission was renewed, the oneway roller and sprague clutches are not replaced unless there are signs of wear or deterioration. They usually do last the life of the vehicle. They are not wear parts like frictions, seal rings, and gaskets.

If the problem goes away with manual low, you could just manually shift the transmission unless that is too onerous.

Whatever happens get back to us with follow up.

Sure sounds like a problem. To get your fluid level down, just pull the modulator valve out. It has a bolt through a clip to hold it on. It has the vacuum line on it so you should be able to find it. The fluid will splash a little and drain down to the proper level. If the vacuum line has fluid in it, the modulator valve has to be replaced. It probably won’t fix the problem but at least you know some other bits of info.

That sure sounds like the sprag (Roller clutch) has given out. Looks like it going to have to come apart. These transmissions are relatively cheap to rebuild.


ALWAYS, ALWAYS, ALWAYS replace sprags (Roller clutches) during overhaul. I commonly see rollers which show no visible signs of wear but still slip. I make it a point to replace roller clutches on all my rebuilds. Less comebacks = happy customer = much happier me not to have to rebuild again at MY expense.


When checking the motor mounts you should raise the hood, hold the brakes and put the transmission in drive and gently accelerate. If the mount is broken the motor will move a great deal more than normal. It may seem to jump quickly, moving out of its moorings. If you can’t see the engine from the driver’s seat, get someone to look for you, but stay to the side, not in front of the vehicle since it could go forward if the brakes aren’t held tightly.