TH400 slushy shifting.

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84 M1008

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Gentlemen, my newly acquired 84 K30 doesn't seem to shift right. It's hard to tell when it shifts. It's more or less sliding when it upshifts. Could it need a new modulator or could the 37 inch tires be the cause?
 

HotRodPC

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All of the above could be the cause. Probably doesn't need a new Vac modulator. If it's an adjustable one, try turning the screw in half to a full turn and see if that makes a difference. My 400 in my C20 was pretty soft and soggy shifting too. I prefer a more positive shift myself so I did adjust the modulator to delay the shift a bit, and when it did shift it was a bit more positive. If the trans is working properly, a Stage 1 Shift Improver Kit to gain a more positive shift.
 

84 M1008

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Thanks HotRod, this trans was suposedly rebuild 10K miles ago but who knows. I have to find out the line pressure parameters and see if this thing is sick before I start throwing parts at it like I usually do. I'm thinking it may need a different stall converter because of the tall tires, but I don't know too much about how that effects performance. It's a 6.2 so it is what it is. Thanks for the info.
 

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Oh yeah, if it was built 10K miles ago and shifting soft, I'd for sure tighten the screw up on the Vac Modulator and see what that does.
 

84 M1008

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It should have been serviced three or four times since also which it probably wasn't.
 

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Converter stall only has to do with the engine's power curve. @$$uming you have the correct transmission components you should have a lower RPM stall converter for the diesel to take advantage of the low RPM torque and the governor should upshift sooner because of the diesel's lower maximum RPM.

My experience with incorrect components was the truck accelerated poorly and if you kept your foot in it you would hit the engine governor which did nothing for acceleration, to say the least.

I don't think either of these items would affect how firm the shifts are.
 

84 M1008

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It accelerates as I expect a na 6.2 to, but I'd like a crisper shift. I know TH400s were never known for that.
 

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Shift kit will make it crisp. I rebuilt my 400 in January and put a TransGo TH400 1&2 kit in it with towing settings. The shifts are progressively firmer the more throttle you give it, but always crisp. Reverse is a bit harsh but otherwise the shifts are EXACTLY what I like. Plus the dual-feeding the kit provides will make your direct clutches last longer.

If your trans is otherwise good this kit would be very helpful, and can be done in the truck. Take a look. Its a $40 well spent!
 

84 M1008

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Thanks da_raabi! I'll look into it.
 

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Shift kit will make it crisp. I rebuilt my 400 in January and put a TransGo TH400 1&2 kit in it with towing settings. The shifts are progressively firmer the more throttle you give it, but always crisp. Reverse is a bit harsh but otherwise the shifts are EXACTLY what I like. Plus the dual-feeding the kit provides will make your direct clutches last longer.

If your trans is otherwise good this kit would be very helpful, and can be done in the truck. Take a look. Its a $40 well spent!

Does the 1-2 kit shift manually when not in D? Meaning if the selector is in 2, then the tranny is in 2, so if you're at a stop it'll be taking off in 2 and not 1? I'm wanting to get a Transgo Kit but debating on the SK 400 or the 1-2.
 

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Does the 1-2 kit shift manually when not in D? Meaning if the selector is in 2, then the tranny is in 2, so if you're at a stop it'll be taking off in 2 and not 1? I'm wanting to get a Transgo Kit but debating on the SK 400 or the 1-2.

You have the option. I like the 1-2 kit as it has a lot more options and upgrades, particularly if you are rebuilding the transmission, and not just doing a valve body refresh.

I chose to keep the standard auto shifting with my build, and it has worked well for me.
 

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You have the option. I like the 1-2 kit as it has a lot more options and upgrades, particularly if you are rebuilding the transmission, and not just doing a valve body refresh.

I chose to keep the standard auto shifting with my build, and it has worked well for me.

Right on. I'm undecided at the moment if I want to get the 1-2 kit, or just stick with the SK 400 kit. Since I am not doing a full rebuild, I'll probably end up getting the SK 400 kit.
 

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Use the 1-2 kit. It includes the special spacer plate to dual feed the direct clutches. That is a major benefit in durability. Use the tow drill chart, you can always make the feed holes bigger, can’t easily make them smaller.
 

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Well you two have swayed me to get 1-2 kit.
 

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Yes, the 1-2 Kit is best. BUT, just do it for Stage 1. Stage 2 is harsh. I loved the stage in my Street/Strip truck but anything over a light throttle take off I'd bark the 33x12.50 tires with the Dana 60 LSD diff with 4.56's. Half throttle take off it would slam 2nd and easily kick the ass end of the truck sideway as it barked the tires. That truck was fun as hell. But as said, just a daily or work truck do the 1 kit. It's firms and crisps up the shift. Consider the 1 as an RV kit and Stage 2 is Street/Strip.

What you were talking about, about having to shift it to whatever gear you wanted, is reffered to as a Full Manual Valve Body. Meaning, you have to manually shift it and no auto function whatsover. Sometimes reffered to as Stage 3 kit and considered Strip Only Kit, as in Dragstrip. You'll like a Stage 1 kit. When you get on it hard, like maybe a full throttle take off, you just might get a slight bark of the tire if you're a peg legger but nothing that's not tolerable, and that's only if you're on it hard.

(Optional Reading)
My dad is a tree hugger slow driving type. He didn't like riding in my street/strip 64GMC. Speed scares the **** out of him. I think I've told the story here before. He asked me for a ride to the Chevy Stealership to pick up his truck one time so he gets in. Starts asking what's all the switches for, what's the buzzing noise, my fuel pump, why does the shifter, what's that gauge for, why does the engine sound like it's missing at idle, (big cam) blah blah blah. I took it easy on the surface streets but as said, at half throttle take offs it would bark tires pretty good and sit you back in the seat. He didn't mind that much, kinda liked it but said, you're gonna get a ticket. LMAO. Getting on a long freeway onramp, I got all over it. I thought he was going to hyperventilate. He didn't have time to say **** but catch his breath in full throttle first gear when the tires finally hooked, then hit 2nd the ass end up the truck went sideways, by this time he's screaming, LET ME OUT, LET ME OUTTA THIS BITCH, **** THIS, LET ME OUT, I"LL WALK. :happy175: So by this time I'm now on the freeway doing about 90 in 2nd at 6500rpm with them 4.56's engine screaming. Hit 3rd and let it settle back down to 3000 rpm which is where I cruised on the freeway which was about 60mph. Remember speed limit was 55 back then. Of course with headers and Turbo Mufflers it wasn't quiet at all. He didn't like that truck ATALL !!! :lol: He saying ****, you better slow your ass down or I'll just get out and walk. :happy175:

I'll save that story, but my dad end up wanting an older truck. I found him a nice shape 68 3/4 ton that didn't run for cheap. I ended up building him an engine and transmission for that truck. I'll just say, he bitched about it all the time cuz I snuck some adders in the engine and transmission. Pretty much tried to keep everything RVish. Notice said, ish :lol: It had one of my previous cams in it that was pretty mild and he had some good compression and a Stage 1 kit in it. :rofl: He used to get home from work and tell me I need to do something with that truck. I asked why? He said, I can't smash the gas cuz it goes to fast, don't want to shift and when it does the tires bark. :rofl: Now who the hell complains about to much power?
 

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