Th400 shifting.

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Falkasorus

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I recently bought an 87 chevy c10. Ran and shifted great. Now all the sudden when I'm driving through town at 30-35 cruising speed it kicks down to 1st. Let off the gas and it shifts back up. Start cruising and it drops down again.
This doesn't happen all the time. Any thoughts before I start tossing money at it ?? Normal driving and acceleration to 50 shifts smoothOnly thing I've recently done is put plugs in it.
 
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AKguy

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Have you checked the usual suspects like the vaccum lines and fluid level?
 

Falkasorus

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Yea fluid level was good doesn't smell burnt. Checked the vaccum line appears to be fine didn't put a reader on it.
 

yevgenievich

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Kick down electrical switch? But that should shift to second
 

Matt69olds

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As mentioned, the kickdown solenoid is only suppose to kickdown into 2nd gear at wot. There are very few TH400 transmission calibrations that will do a 3-1 downshift about 20ish mph (the Buick Stage one, W-30 Olds 442, Ram Air GTO, some of the high performance Chevelle/Camaro/Impala) but those are very rare. I have been looking for that specific valve body for 20 years, with zero luck. I suppose it’s possible but very unlikely that someone swapped parts into your truck sometime in its history.

I would start by unplugging the kickdown connector at the transmission. That will eliminate the wiring and switch outside the transmission. Other than no wot kickdown, the transmission should function normally.

Still does it? Time to dig deeper.

Pull the govenor from the transmission. It’s located behind a stamped steel square cover on the passenger side of the transmission, near the rear of the trans. You will lose a very tiny amount of fluid. Make sure you have a replacement gasket, usually they tear taking the cover off. Inspect the governor gear and weights. Make sure the gear isn’t chewed up or loose on the shaft. Make sure the bore the govenor spins on in the case isn’t chewed up. The govenor should fit into the bore with no up/down wobble. If it moves around, the case is worn.


If it still does it, drop the transmission pan. Make sure the kickdown solenoid is tight. Depending on what style solenoid is used, it may or may not have a gasket. The oval can style used a small stamped steel gasket. The square open style solenoid has a molded seal on the bottom, and doesn’t use a gasket. If it was assemble incorrectly, it can cause a internal leak. Usually that causes a late upshift, or no 3rd gear. The solenoids aren’t expensive, it’s probably worth rolling the dice on replacing it. You will get the later style that uses no gasket when ordering a replacement.


If everything checks out, it’s time for valve body inspection. Or depending on how much crud is in the pan, transmission removal and autopsy might be needed.
 

Falkasorus

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Thanks for the information very helpful. Also it's not dropping from 3rd to 1st rather 2nd to 1st in and out at the 20-30 speed range. I will try all the things you say. But does my vaccum accumulator have any effect on that?
 

Matt69olds

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The vacuum modulator controls part throttle shift points. High engine load=low vacuum. Low vacuum=high modulator pressure inside the transmission. That moves the shift points up.

Make sure the vacuum line is in good shape. Don’t use long pieces rubber hose, I have seen many cases where rubber hose gets soft from the exhaust heat, engine vacuum cases the hose to collapse and pinched off the vacuum supply to the modulator.

Modulators usually either work or they don’t. Sometimes they get bent, causing the valve inside to not move. Or the diaphragm breaks, then engine vacuum will slowly drain the transmission of fluid.

It couldn’t hurt to try a new modulator, you can get a replacement from AutoZone for about 15 bucks. I think the number is MV100, it’s adjustable to change part throttle shift points 2-4 mph up or down by turning the little screw inside the vacuum hose fitting. Don’t turn the screw more than 4 turns, each turn is about 1mph change.
 

Falkasorus

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Dude much appreciated. Glad I joined and asked the question. I have a new vacuum modulator and will probably just get a governor. And check the kick down switch. Will post my updates thanks
 

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