TH400 shift points
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
TH400 shift points
Good afternoon everyone,
75 vette with freshen up motor and TH400 auto.
Finally got enough miles on my new motor to feel
comfortable flooring it. It was great except,
firsr gear rode out to 5000 rpm and shifted to second. I would prefer 5500.
second gear only last 500 rpm, from 3500 to 4000 and then it shifted to third.
How do I change when the trans shifts? Second gear is the real problem, I can live with 5k shifting, but 4K is too low.
thanks
75 vette with freshen up motor and TH400 auto.
Finally got enough miles on my new motor to feel
comfortable flooring it. It was great except,
firsr gear rode out to 5000 rpm and shifted to second. I would prefer 5500.
second gear only last 500 rpm, from 3500 to 4000 and then it shifted to third.
How do I change when the trans shifts? Second gear is the real problem, I can live with 5k shifting, but 4K is too low.
thanks
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Sorry it grabbed the wrong one earlier. Personally in the past I just bought the aftermarket governors with the rpm setting I ordered.
I used a 5000 for street and 6500 for race day. They are easy to change in and out.
I used a 5000 for street and 6500 for race day. They are easy to change in and out.
Last edited by gkull; Nov 7, 2020 at 06:26 PM.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
You might want to get a small screw driver and pull the vac. line off the modulator and try tweaking it. because you have a different rpm in different gears. I had my 700R4 modded for NO kick down going to WOT even in OD. I would let it do an auto 1-2 shift by placing it in 2 when I took off and then manually shift into 3 &4
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
I had the same thought, about why it would be different. I ordered a governor kit anyways to at least bump it up to 5500 and I’ll adjust the modulator some too. I’m heading to a drag strip soon and my plan is just to shift it manually from 2-3.
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
If you put the shifter in 1st and leave it, it will get a "mercy shift" to 2nd at very high rpm. This is a TH400 thing. After the mercy shift, put it in 2nd then when ready, shift to 3rd. Try it, it's a hoot!
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
If the shift points are all very similar for the various shifts, then adjusting or replacing the governor can raise or lower them as you need. But if they are at different RPM's for each range, modifying the governor will not help you. The shift points for each range are set with spring rates on each shift valve in the valve body. These must be adjusted so that the shifts for all ranges are close to the same RPM. THEN you select/adjust the governor to set final shift points.
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Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
It also is not consistent. I’m wondering if the flyweights are gunked up or the spring is weak and not consistent. I got on it again today and it shifted from first to second at 4200 rpm. Yesterday it was 5000. If the flyweights don’t work I’ll replace the vacuum modulator, maybe that’s old and inconsistent.
I have a hard time thinking the engine vacuum would be different from yesterday to today
I have a hard time thinking the engine vacuum would be different from yesterday to today
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
7T1vette, is correct about the valve body. Was the 400 also freshened up? A very easy job is to buy something like a Hughs shift kit. Take the time to use polishing compound on each shift valve and throttle valve sleeve. You want them to really smooth to actaute as fast as possible
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
I don’t know too much about the history of the transmission. I believe it is from a 76 Chevy truck. I know I personally have done nothing to it other than change the filter and oil once. Probably could use a rebuild but I’ll wait til it start giving me trouble.
Racer
Joined: Aug 2018
Posts: 380
Likes: 170
From: Erath County Texas
this is pretty close to what I'm seeing ...
71 L48/Auto (TH400? or 350?) ...
cruising at 3000 RPM, go to accelerate for a pass ... Tranny downshifts to 2nd ... lotta RPM, but no pull. it does better without a WOT, just keep it in Hi gear ... but it's not "great" acceleration.
I know I don't have a "horse" with the 270 HP (and I'm almost certainly not getting all that even with a Lars QJet overhaul) and the wipers/headlight vacuum bypassed (ie no vacuum leaks in those systems) ... but it should accelerate with a downshift better than remaining in high gear.
Thanks.
I checked the fluid level (I do have a couple of leaks I need to fix) ... serviced to full .... no help.
71 L48/Auto (TH400? or 350?) ...
cruising at 3000 RPM, go to accelerate for a pass ... Tranny downshifts to 2nd ... lotta RPM, but no pull. it does better without a WOT, just keep it in Hi gear ... but it's not "great" acceleration.
I know I don't have a "horse" with the 270 HP (and I'm almost certainly not getting all that even with a Lars QJet overhaul) and the wipers/headlight vacuum bypassed (ie no vacuum leaks in those systems) ... but it should accelerate with a downshift better than remaining in high gear.
Thanks.
I checked the fluid level (I do have a couple of leaks I need to fix) ... serviced to full .... no help.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Are you certain that the carburetor's secondaries are opening when you go WOT? You really can't assume that's the case. You have to have someone hold the choke flap open and look down at the throttle plates while someone else pushes the accelerator pedal to the floor. (And, NO, that is not the same thing as exercising the throttle linkage with your hand!)
Racer
Joined: Aug 2018
Posts: 380
Likes: 170
From: Erath County Texas
No, I've not verified they're actually opening. I'll will look at that tomorrow. I bet the missus can hit/release the pedal for me. Thanks.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
She just needs to press it to the floor...as you would do at WOT. With the choke plate open (manually...above primary venturi), you should be able to see the SECONDARY butterflies standing perfectly vertical. If not, you need to find where the proper correction to this fault can be remedied. If a cable 'travel' problem, adjust the cable hold-clamp as mounted to the carb. If that does not allow full movement, remove 'stuff' from under the accelerator pedal (floor insulation, carpeting, floor mat, etc). It could be that the accel pedal linkage is bent, deformed, damaged. If it is fully open (linkage stop on the secondaries is limiting the travel), just bend that stop until a WOT pedal provides vertical throttle plates.
You have no idea how often this issue comes up. The Q-Jet secondary opening doesn't initiate until about 75% of cable travel is reached. So cable/linkage being off just a bit can cut secondary opening to less than half very easily. Also, if the secondary lockout lever (right side, bottom) doesn't release with the choke falls out, the secondaries will NEVER open at all.
Just a few easy things to check for....
You have no idea how often this issue comes up. The Q-Jet secondary opening doesn't initiate until about 75% of cable travel is reached. So cable/linkage being off just a bit can cut secondary opening to less than half very easily. Also, if the secondary lockout lever (right side, bottom) doesn't release with the choke falls out, the secondaries will NEVER open at all.
Just a few easy things to check for....
Racer
Joined: Aug 2018
Posts: 380
Likes: 170
From: Erath County Texas
thank you sir. I've yet to be able to do this check.
Something else I was wondering, though .... could the torque converter be bypassing internally? I mean ... I get the revs .... I just have nothing to go with 'em. Could the TC be failing to convert?
this thing leaks externally like a sieve. I'd fill a catch pan ... so I have 1/4" plywood under Victoria. I need to attend to it, but I've found it's easier to just keep it serviced ... and drive!
Something else I was wondering, though .... could the torque converter be bypassing internally? I mean ... I get the revs .... I just have nothing to go with 'em. Could the TC be failing to convert?
this thing leaks externally like a sieve. I'd fill a catch pan ... so I have 1/4" plywood under Victoria. I need to attend to it, but I've found it's easier to just keep it serviced ... and drive!
Racer
Joined: Aug 2018
Posts: 380
Likes: 170
From: Erath County Texas
The QJet is working fine, though I discovered the clip for the bi-metallic spring arm is missing. Safety wire for now. :o
Ran Victoria yesterday morning with cool temps, and she did fine. Seems like, however, there are occasions when the tranny will downshift climbing a small grade ... turning 3000 RPM. That seems lacking.
I'll keep rolling the acorn.
Ran Victoria yesterday morning with cool temps, and she did fine. Seems like, however, there are occasions when the tranny will downshift climbing a small grade ... turning 3000 RPM. That seems lacking.
I'll keep rolling the acorn.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 1,043
Likes: 414
2023 C3 of the Year Finalist - Modified
I replaced the vacuum modulator and it seemed to have helped a lot. I thought it fixed it, but yesterday at the drag strip the problem reappeared. First gear ran out good, shifted to second, stayed in second for 200 rpm and then shifted to third. Ruined a good pass lol.
but I’m really stumped what would cause an intermittent random early shift from 2-3 while at WOT. Any thoughts?
but I’m really stumped what would cause an intermittent random early shift from 2-3 while at WOT. Any thoughts?