Slow Climb To The Top

Hello all, I have an '84 C20 350 automatic, I believe it's a TH400, it seems to shift well from a stop, but when I get to a slight grade it will not drop into a lower gear. It used to, but no longer and as a result of course the engine will rev but that's it. Of course I can manually shift into a lower gear, then up into second and on into drive after picking up speed or cresting the hill but it won't get it on its own and the same goes for down shifting when attempting to pass or just merging into traffic, engine revs with no push from dropping into a lower gear. Torque converter? Fluid is fine, looks and smells good. Anyway, wondering what the brain trust can come up with, I miss that old performance. Thanks very much in advance.

What transmission do you have?
@NickTransmissions please check this out

Modulator valve dead? Or vacuum line popped/disconnected? I don't have a lot of experience with transmissions (TH400), but when mine became disconnected, my trans acted really goofy. Also, there's a yellow wire under the dash that I disconnected on accident. Had some weird effect on the trans as well

I believe it's a TH400 but I'll find out for sure, thanks.

gill finn said:

I believe it's a TH400 but I'll find out for sure, thanks.

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Hey man, responded to your post on Youtube - here's the copy:

I'm thinking either kick down cable possibly out of adjustment, kick-down valve sticking in its bore in the valve body or vacuum modulator system not functioning...I'd first check vacuum at the modulator to ensure you have at least 15hg. If you do (and the hose isn't cracked or damaged in anyway), I'd test the modulator...You'll have to look up the test procedure but you can use a hand-held vacuum pump to test it. Once the modulator is off, take a pencil magnet and remove the modulator valve - see if it's stuck or dragging real bad...If so, take bench buddies (small abrasive hones) and polish the bore (bench buddies are used with a drill)...Also, if the valve has real bad scoring or feels rough to the touch, put it in the drill and polish each land with sand paper (800-1000 grit) - spray it and the bench buddies with WD40.

Never heard of bench buddies? Is it a powder like bonami or Ajax? I've used bonami for cleaning Cylinder walls up from rust. Hasn't scratched yet lol

squaredeal91 said:

Never heard of bench buddies? Is it a powder like bonami or Ajax? I've used bonami for cleaning Cylinder walls up from rust. Hasn't scratched yet lol

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They are small abrasive hones for use on valve body and pump bores where valves are located. Same concept as bottle brush hones for engine cylinders, and same procedure...

Thanks a lot Nick, appreciate the response and good info!

There should be an electric switch at the throttle pedal for kick down. Check adjustment and continuity.

On a TH400 there is an electrical switch that goes through the case near the valve body. I don't remember exactly, but it hooks onto something under the pan, and that controls the kick down. Maybe unhooked or needs a replacement switch.
Its been a few years since I had a TH400, but I drove it for a long time without the kick down. Before I sold it I dropped the pan to change filter and fluid and saw the wiring issue. I think I had to replace a switch in there, and it started working.

TH400 kick down solenoid has a single wire connection on the drivers side of the unit.
I believe the wire is routed along with the starter wires.
The adjustable switch is located above the throttle pedal.

gill finn said:

Thanks a lot Nick, appreciate the response and good info!

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You're welcome...For some reason i had it in my head you have a TH350 but since you have a TH400, you wont have any kick down cable but instead, a detent solenoid. Check that detent solenoid by dropping the pan and visually inspecting the solenoid and its wire for wear. There's a little metal gasket between the solenoid and spacer plate/case. The detent and detent regulator valve can also cause the same issues by sticking/dragging in the bore (they're both in the same bore in the valve body).