TH400 skips 2nd gears goes right for 3rd WTF

[fire-chickens]

Alright I just bought a '84 C10 with a TH400 and a BBC and for some reason 2nd gear works fine when shifted manually but but accelerating in drive it'll go from 1st to 3rd.

what would cause this issue?

2nd gear will work if manually shifted to either gear. from 3rd to 2nd it will downshift, and vice versa.

no matter how much throttle i give the truck it goes from 1st to 3rd making this truck feel like a SLUG!

 

[HotRodPC]

Hope it's the governor valve stuck in the governor since you can change that on the outside of the case without dropping the pan or pulling the trans. I have a feeling that is what it is. Check that out then report back on what you find out.

It's a square plate with 4 bolts on the side of the trans case near the rear where that case meets the tailshaft housing.

 

[chengny]

Check the basics first.

Proper ATF level?

Is the modulator vacuum line connected at both ends (the intake manifold and the modulator valve nipple)? Are the rubber hoses at the ends of the steel tubing cracked or collapsed?

The modulator and governor work together to control shift points and spacing.

The governor sends a variable oil pressure signal (as a function of the truck's road speed) to the modulator valve.

The modulator is acted upon by that signal - along with an engine load signal (generated by intake manifold pressure/vacuum) - which causes it to shuttle and direct mainline pressure from the pump to the various shift valves.

Modulator Operation

A vacuum modulator is used to sense engine torque input to the transmission automatically. The vacuum modulator transmits this signal to the pressure regulator, which controls line pressure, so that all torque requirements of the transmission are met and proper shift spacing is obtained at all throttle openings

Governor pressure acts on the modulator valve to cause modulator pressure to decrease as vehicle speed increases. Generates a speed sensitive oil pressure that increases with output shaft or vehicle speed. Governor pressure is used to control the shift points and modulator pressure regulation.

 

[fire-chickens]

turns out I was incorrect after some careful and lighter driving it seems to be working but the shifts are very soft going from 2nd to 3rd gear, and its a bit early shift kinda. which i realized my worst problem could be adjusting the vacuum modulator.

the truck sat a lot too for the past several years, only just over 3000 miles in almost 4 years

 

[HotRodPC]

Yep, turn the screw in on the modulator about 1/2 turn and see if that delays the shift a bit and should also firm it up a tad too. Now don't go making it to late, but it does sound as if it's shifting to early also or you wouldn't think it was skipping 2nd.

If the fluid looks good, maybe a good trans seal conditioner product might work too. I'm not big on additives at all, but for auto transmissions that sit for a long time, they do benefit for a seal conditioner treatment. You might not notice it for a few days. It may not even help, but it sure won't hurt. Don't go for the cheapest one you can find either. SeaFoam does make a trans conditioner, so does Lucas. Those are products that seem to work for other things, so they should be good too and not just a gimmick like a lot of STP or Gumount cheap **** is.

 

[da_raabi]

I've actually been having the same issue. 454 T400 that's been sitting for a while. It shifts a tad slow across the spectrum (takes a bit getting into D and R), but I have noticed that 2nd seems non-existent when driving. Where is this modulator screw located so I can adjust it?

 

[HotRodPC]

I assume you have one of the smaller gold bell type modualtors since the screw is obvious on big canister type. It will be inside the vac port. You'll need a tiny screwdriver, and pull the vac line off to get to the screw. In most cases when you've lost 2nd gear, it's the intermediate roller clutch/sprag has gone bad. Not real common the Th400's though. It does happen though. The roller clutch going bad on my Th400 is what literally cooked my trans and how I bought the truck.