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1991 Chevrolet k1500 Extended Cab short box Auto Trans

I picked up a 1991 chevy k1500 extended cab 4x4 pickup yesterday that wont shift out of 1st unless it is revved to the hilt and moving about 30 mph.

It bangs into 2 hard as heck. Reverse and first go into gear fine-No slip that I can tell. pop it into neutral and back to drive at normal speed it went into 2nd but would not go into 3 or 4th as I picked up speed. I was a little afraid to wind it up again like the first time. Is there anything I can check before a shop visit? Wish it could be something simple but have doubts.

Trans is full of fluid-smells and looks new. No leaks detected. Thinking of pulling pan to see if the magnet has chunks attached but wanted simple first. The guy I got it from said it started acting like this 2 weeks ago.

Is there a checklist of things to try…cheapest moving down the list to shop visit?

It sounds like the seals in the transmission are hard and dried so not enough hydraulic pressure can be produced for the transmission to shift correctly.


Anything a little cheaper it could be?

Sounds like the governor in the transmission is stuck.

and how might one un stuck it?

Oh forgot to say I am 100 percent New to automatic Transmissions. Searching internet didn’t find much as far as things to check and how a newby can check them. I really need you’ll help.

I saw something about an additive to use to soften up seals. Any + or - Ideas on good ones to use that wont make things worse? do people let the additives run a month or two then change trans and converter oil after the additives worked a bit?

Roughly how they work, there’s a pump inside driven by the output shaft, so the pump’s output pressure is proportional to vehicle speed. That’s how they know when to shift at certain speeds. The engine load has to be taken into account also in the shift algorithm, and how that’s done varies design to design. On my 70s Ford truck there’s a device that screws into the transmission called a “vacuum modulator” which connects to the intake manifold vacuum to do that function. On yours. being newer than mine, there’s probably some electric solenoids involved, controlled by a computer which knows the intake manifold vacuum and vehicle speed.

To effect a shift, the force of the transmission fluid pressure is applied to either the clutches or the bands in the gear set (usually these are a planetary design, meaning the gears orbit around in circles like the earth goes around the sun). The gear set performs a certain function (e.g. reverse, neutral, forward in a certain gear ratio, etc) depending on the band/clutch fluid pressure inputs. However it’s done, it takes really high fluid pressures to create the force needed to effect the shifts. In your case I doubt the shift algorithm implementation is the problem. Instead, either the internal fluid pressures aren’t high enough b/c something’s (usually the seals) leaking internally, or the clutches/bands are worn out. The former is sort of like being unable to brake b/c the master cylinder is leaking internally. The latter is sort of like being unable to brake b/c the pads are worn out.

Your best bet is to ask around who’s the best transmission shop in town and get an assessment. They’ll likely measure the fluid pressures and hook up some electronic gadget to see if the solenoids are doing their thing.

Thank you George for taking me by the hand. It is really appreciated. I have R&R ford & chevy 70-85 but never got into the guts. I do think I will take your advice and have a shop take a look. Maybe find a used tranny reasonably priced…Thanks again for your time…

I’d rather have the original rebuilt, versus taking chances on somebody’s word that their mystery transmission is good to go

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