Turbo350 w/shift kit slamming 2nd gear

[KeithM]

Hi, I just had a new trans installed in my 78GMC C15. I know it has a shift kit since it came out of a 67 Nova show car. It had a 3500rpm stall converter which was swapped out for one better suited for an old truck. My question is how can I get it to stop slamming 2nd gear? Would the modulator be able to adjust this out a bit? Or is there anything else that I could do?
Thanks in advance!

 

[NickTransmissions]

Hi, I just had a new trans installed in my 78GMC C15. I know it has a shift kit since it came out of a 67 Nova show car. It had a 3500rpm stall converter which was swapped out for one better suited for an old truck. My question is how can I get it to stop slamming 2nd gear? Would the modulator be able to adjust this out a bit? Or is there anything else that I could do?
Thanks in advance!

Does your converter have a stock-level stall speed?

If so, im not surprised that the 1-2 shift is super harsh as it was set up for that much higher 3500 stall. Check the 1-2 accumulator to see if the spring was omitted. If so, install a new spring and that should tame it. If the spring is installed, you're dropping the valve body and looking at check balls and spacer plate hole sizes...dont keep driving it like it is now, you may crack the outer race on the intermediate one-way clutch and lose 2nd gear all together.

 

[KeithM]

Thanks Nic! My bet is it has a close to stock converter. Also the speedo reads about 100mph when I'm doing about 40mph. I went and pulled the speedo gear out of my old trans to see if that would fix it and it had a red gear just like my stock one so no change. Not sure what else would cause this? I'll look for that spring to see if that's gone.

 

[Rusty Nail]

Why didn't the "transmission guy" or anyone else suggest adjusting the detent cable?
That may be your issue _  especially after a recent install. Duh.
The modulator will change the shift point mph, that's the wrong tree to be barking at, like the convertor.

Probably very likely the time for detent cable replacement - allegedly they adjust themselves to some point but ....yours may be all bunched up after install.
Prolly oughta check it.
Take a pix.
More common than you think.

Uhhh ...you prolly oughta verify correct fluid level of DEXTRON 6 with engine at operating temperature-
Check the vacuum hose connected to the modulator...
Hope this helps.

 

[NickTransmissions]

Why didn't the "transmission guy" or anyone else suggest adjusting the detent cable?
That may be your issue _  especially after a recent install. Duh.
The modulator will change the shift point mph, that's the wrong tree to be barking at, like the convertor.

Probably very likely the time for detent cable replacement - allegedly they adjust themselves to some point but ....yours may be all bunched up after install.
Prolly oughta check it.
Take a pix.
More common than you think.

Uhhh ...you prolly oughta verify correct fluid level of DEXTRON 6 with engine at operating temperature-
Check the vacuum hose connected to the modulator...
Hope this helps.

Are you referring to me? None of what you suggested will fix his 1-2 shift and Dex 6 is totally unnecessary in a TH350, was engineered for later transmissions with different friction compositions which won't be applicable for these older units.

The kick down cable controls, well "kick down", aka downshifting. Not upshifting and he never mentioned anything about a no-upshift condition (usually from 2nd to 3rd) which would be present if there's a problem the detent system. The vacuum modulator controls part throttle upshift and the adjustment is subtle, nothing like the mechanical recalibration for the 1-2 shift he needs to perform. Sure, he can replace it and the modulator and the filter and perform a general service but none of those things will fix a super-hard 1-2 shift that he's experiencing by taking a transmission mechanically calibrated for a high-stall application in a completely different vehicle and running a stock converter in his truck.

How many TH350s have you built and for what applications?

Thanks Nic! My bet is it has a close to stock converter. Also the speedo reads about 100mph when I'm doing about 40mph. I went and pulled the speedo gear out of my old trans to see if that would fix it and it had a red gear just like my stock one so no change. Not sure what else would cause this? I'll look for that spring to see if that's gone.

You're welcome...Speedo drive and driven gear will need to be swapped out; the driven gear just slips into the housing but the drive gear is held onto the output shaft in a quasi-press fit so you'll need a puller of sorts to remove it then you can use a pipe of the appropriate diameter and soft dead-blow hammer to tap on the new gear.

TCI's drive/driven gear calculator can help you determine the number of teeth you need for each.

let me know when you've had a chance to inspect the 1-2 accumulator housing...There may be either no spring present or a broken spring which is fairly common in these transmissions, especially when behind high horsepower/torque engines.

 

[bucket]

Are you referring to me? None of what you suggested will fix his 1-2 shift and Dex 6 is totally unnecessary in a TH350, was engineered for later transmissions with different friction compositions which won't be applicable for these older units.

The kick down cable controls, well "kick down", aka downshifting. Not upshifting and he never mentioned anything about a no-upshift condition (usually from 2nd to 3rd) which would be present if there's a problem the detent system. The vacuum modulator controls part throttle upshift and the adjustment is subtle, nothing like the mechanical recalibration for the 1-2 shift he needs to perform. Sure, he can replace it and the modulator and the filter and perform a general service but none of those things will fix a super-hard 1-2 shift that he's experiencing by taking a transmission mechanically calibrated for a high-stall application in a completely different vehicle and running a stock converter in his truck.

How many TH350s have you built and for what applications?

You're welcome...Speedo drive and driven gear will need to be swapped out; the driven gear just slips into the housing but the drive gear is held onto the output shaft in a quasi-press fit so you'll need a puller of sorts to remove it then you can use a pipe of the appropriate diameter and soft dead-blow hammer to tap on the new gear.

TCI's drive/driven gear calculator can help you determine the number of teeth you need for each.

let me know when you've had a chance to inspect the 1-2 accumulator housing...There may be either no spring present or a broken spring which is fairly common in these transmissions, especially when behind high horsepower/torque engines.

I'm just going to point it out before someone else does, but the th350 kickdown/detent cable does indeed have a minor effect on upshift mph, under full throttle. Or at least it does on a stock, late 60's version. You can adjust it to vary the upshift speed by 5-10 mph. I used to play around with that adjustment all the time on my first car.

The 1-2 accumulator, is that the one that is under the round cover on the passenger side? If so, can that spring be removed to get a little firmer shift without doing anything else? Or will that potentially cause other issues?

 

[NickTransmissions]

I'm just going to point it out before someone else does, but the th350 kickdown/detent cable does indeed have a minor effect on upshift mph, under full throttle. Or at least it does on a stock, late 60's version. You can adjust it to vary the upshift speed by 5-10 mph. I used to play around with that adjustment all the time on my first car.

The 1-2 accumulator, is that the one that is under the round cover on the passenger side? If so, can that spring be removed to get a little firmer shift without doing anything else? Or will that potentially cause other issues?

Yes but it wont cause a neck-snapping, back breaking 1-2 shift that the OP is describing (slamming into gear). To your point, it can and sometimes will prevent upshift if it's not functioning as i mentioned in that post above. And I have no issues with someone questioning what I say but there's a right and wrong way to do that, which is one of the overarching points that I was trying to make in my response to Rusted Nail.

To answer your question, yes - the 1-2 acc piston and spring is on the pass side of the trans behind the cover. Every so often the spring breaks which leads to a very harsh 1-2 shift but no corresponding harsh 2-3 shift, which if the cable was either misadjusted or missing would impact both 1-2 and 2-3 shift timing/feel. The spring is often left out in high stall applications, along with all the check balls in the case, except for the modulator check ball.

 

[bucket]

Yes but it wont cause a neck-snapping, back breaking 1-2 shift that the OP is describing (slamming into gear). To your point, it can and sometimes will prevent upshift if it's not functioning as i mentioned in that post above. And I have no issues with someone questioning what I say but there's a right and wrong way to do that, which is one of the overarching points that I was trying to make in my response to Rusted Nail.

To answer your question, yes - the 1-2 acc piston and spring is on the pass side of the trans behind the cover. Every so often the spring breaks which leads to a very harsh 1-2 shift but no corresponding harsh 2-3 shift, which if the cable was either misadjusted or missing would impact both 1-2 and 2-3 shift timing/feel. The spring is often left out in high stall applications, along with all the check balls in the case, except for the modulator check ball.

Yes, I didn't mean to question you about the detent cable or suggest that it may be part of the OP's problem, just simply point out that it can have a minor effect on road speed at which the trans shifts. And only at full throttle.

So is it safe to leave out the 1-2 accumulator spring on an an otherwise stock th350? I only ask because my Chevelle has a very powerful sounding exhaust (although the car is far from powerful, lol) that sounds out of place when the trans simply glides into 2nd gear. A more positive shift would match the sound better.

Sorry about getting a little off topic.

 

[NickTransmissions]

Yes, I didn't mean to question you about the detent cable or suggest that it may be part of the OP's problem, just simply point out that it can have a minor effect on road speed at which the trans shifts. And only at full throttle.

So is it safe to leave out the 1-2 accumulator spring on an an otherwise stock th350? I only ask because my Chevelle has a very powerful sounding exhaust (although the car is far from powerful, lol) that sounds out of place when the trans simply glides into 2nd gear. A more positive shift would match the sound better.

Sorry about getting a little off topic.

No, sorry - i wasnt referring to you questioning me...i generally advise against leaving the 1-2 acc piston out when converter stall and trans are stock out for two reasons:

1. The delta in shift firmness with it removed is usually too aggressive, esp in a lighter weight vehicle (ie Nova) compared to a squarebody truck and can make the vehicle borderline undriveable for most

2. The th350 1-2 accumulator's job is to soften shifts by slowing the speed of the shift down so the intermediate roller one way clutch (OWC) can go from freewheeling to fully holding before the intermediate clutch pack fully applies. If the clutch is at full compression before the intermediate OWC is holding, the OWC and/or outer race can break.

In a high stall application (3500+ generally speaking), the much looser converter absorbs most of that shift energy hence why its much safer to leave it out (and block the 2-3 accumulator and dual feed the high clutch).

The above also broadly applies to the TH400 and 4L80E.

In your case, assuming your converter is stock, id drop the valve body and see what the 1-2 and 2-3 feed hole sizes in the spacer plate are drilled to (if at all) and open those up a little bit first. A modest increase in feed hole size will make a big difference if the plate hasnt been touched while not putting that OWC in any danger of breaking...let me know if want to get into it and ill help out however I can.

 

[KeithM]

Well Bingo! I took the spring out of my old trans and when I popped the cover off the new one I found a spring that was cut off an inch shorter! Had fun getting that split ring back on though
She shifts real nice now. Nice little bump into second that’s still stronger feeling than stock but not harsh at all.
Also checked old trans and found the clip for the speedo drive gear missing and gear was no we’re near where it should be. I’ll work on that next.
Thanks so much Nick!

 

[NickTransmissions]

Well Bingo! I took the spring out of my old trans and when I popped the cover off the new one I found a spring that was cut off an inch shorter! Had fun getting that split ring back on though
She shifts real nice now. Nice little bump into second that’s still stronger feeling than stock but not harsh at all.
Also checked old trans and found the clip for the speedo drive gear missing and gear was no we’re near where it should be. I’ll work on that next.
Thanks so much Nick!

Nice job, Keith and you're very welcome!

Now you know the relationship between that spring and piston and shift feel insofar as the 1-2 shift is concerned in the TH350, and to a great extent, same for the 2-3 and similar in pretty much every other GM transmission that uses a similar accumulation strategy for control of shift speed and feel.

No amount of screwing around with the kick down cable or monkeying with the modulator would have fixed that problem hence why I didn't suggest you bother with those two things...I can't tell you how many times I've heard my TH350 customers tell me how their transmission slamming into 2nd gear and they don't know why. But mystery gets solved as soon as I take off the 1-2 accumulator cover...

I'd suspect your 1-2 feed hole in the spacer plate has been opened up as well so you may still have an uncomfortable 1-2 shift even after replacing the accumulator spring so to fully fix it, you may need to replace the plate with another, unaltered spacer plate. Let me know if you have to cross that bridge and I'll help you as best as I can remotely.

 

[MikeB]

I once had a TH350 with a harsh 1-2 shift after I installed a B&M "Shift Improver" (ha!) kit. I called TransGo and their guy told me why the cheap B&M kit caused my problems. Ended up going with a TransGo 350-1&2 kit, which gave me firm but not harsh shifts. I've used two more of their kits since then, and they've always worked great.

Maybe our Nick's Transmissions guy can refresh my memory on why the B&M kit's crude mods caused the problem, and why the TransGo kit worked so well. I do know it was a valve body/separator plate thing.

 

[NickTransmissions]

I once had a TH350 with a harsh 1-2 shift after I installed a B&M "Shift Improver" (ha!) kit. I called TransGo and their guy told me why the cheap B&M kit caused my problems. Ended up going with a TransGo 350-1&2 kit, which gave me firm but not harsh shifts. I've used two more of their kits since then, and they've always worked great.

Maybe our Nick's Transmissions guy can refresh my memory on why the B&M kit's crude mods caused the problem, and why the TransGo kit worked so well. I do know it was a valve body/separator plate thing.

Compare the instructions for each shift kit as it pertains to clutch feed hole sizes and you'll understand. It may also occur to you why spending money on shift kits for these transmissions is a waste to begin with, especially if you're rebuilding the transmission

 

[MikeB]

Compare the instructions for each shift kit as it pertains to clutch feed hole sizes and you'll understand. It may also occur to you why spending money on shift kits for these transmissions is a waste to begin with, especially if you're rebuilding the transmission

You hit the nail on the head about the issue being hole sizes. That was 20+ years ago, but some of it is coming back to me!

Please explain how lazy shifts be firmed up with and without a rebuild? Thanks.

 

[NickTransmissions]

You hit the nail on the head about the issue being hole sizes. That was 20+ years ago, but some of it is coming back to me!

Please explain how lazy shifts be firmed up with and without a rebuild? Thanks.

Simple - drill the feed holes out....

The art vs science is knowing how much to drill each. You first have to take a variety of factors into consideration (and ill make this specific to the TH350)....They are as follows:
1. Vehicle application
2. Final Drive ratio
3. Converter Stall (perhaps the most important single factor)
4. Other things done to the transmission (dual feeding, check balls, accumulator modifications, etc)
5. Preferred / desired shift feel

The above list isn't exhaustive but those are the main things to take into account.

Generally speaking, the higher the stall speed, all other things equal, the larger the feed hole sizes should be to ensure each upshift is crisp/quick without being harsh/back-breaking.

I tell folks that, if you're not sure and don't have a body of experience to draw from, just follow Transgo's instructions regardless of what shift kit you use (or whether or not you use one at all). This way, you don't go too far w/the re-sizing.

That's basically it.