[many birds]

My Th400 with an unknown shift kit installed by the PO keeps shifting back and forth between 2nd and 3rd gear under light throttle at about 30 to 35 MPH. Happily stays in third above this speed and stays in second below this speed.
I have the vacuum regulator connected to the manifold.
No problems with the 1-2 shift.
Fluid level is full
The kick down is not connected

Any ideas? Bad regulator?

[steve25]

A needed check ball may have been left out during the shift kit install.

[promptcritical]

Did it just start doing this or has it always done this (since you've had it)?

[many birds]

Originally Posted by promptcritical

Did it just start doing this or has it always done this (since you've had it)?

I’m not sure. I’ve had car operable for only about one now. Most of that time has spent tuning and troubleshooting. It was a 15 yr resto project started by the previous owner.

To me, the shifting issue seems new, but I could be wrong.

[many birds]

That was supposed to read operable for one month....

[TAQuest]

I would guess it's the kickdown solenoid. I've seen them fail like you described.
Here's a video to see what it takes to change it:
https://www.youtube.com/watch?v=Pb8ieNRVbGg

[many birds]

Originally Posted by TAQuest

I would guess it's the kickdown solenoid. I've seen them fail like you described.
Here's a video to see what it takes to change it:
https://www.youtube.com/watch?v=Pb8ieNRVbGg

My kick down switch isn’t connected. It’s the carb mounted kind and the carb I have doesn’t have the throttle shaft for the kick down switch. I planned to get that figured out sometime in the future.

[Kenth]

Make sure vacuum modulator is connected to a manifold vacuum source.

[TAQuest]

Originally Posted by many birds

My kick down switch isn’t connected. It’s the carb mounted kind and the carb I have doesn’t have the throttle shaft for the kick down switch. I planned to get that figured out sometime in the future.

They can malfunction even not hooked up. If they leak they shift like you described. The two bolts come loose, they leak and it shifts back and forth. Or they can leak because they are not sealing right.

[Half-Inch Stud]

leaky or particle-jambed detent switch has done that

[OCMDGTO]

I have the same exact issue going on right now. I figured it was due to an engine swap resulting in a vacuum or stall change(it did not do this with my old engine). You can take the vacuum line off the modulator at the trans, insert a small flat screwdriver to make adjustments. Clockwise will raise the shift point. I have been experimenting with mine to find a happy medium so I can wind it out manually. T400 downshift is electric triggered by the gas pedal with nothing to do with the carb. I believe the T350 downshift is triggered by a cable attached to the throttle linkage at the carb(been many yrs since I had a T350). How wild is your setup?

[many birds]

The setup is relatively mild. 1968 400 bored 0.030 over, refreshed, stock 1968 #15 heads with a lunati voodoo 703 hft cam and reworked qjet. Idle vacuum somewhere between 11 and 14 mmHg.

On the first gen Firebirds, the kickdown switch is mounted to the intake and a lever on the carb activates it at heavy throttle. My carb is from a 1972 or 1973 and it doesn’t have that lever on the throttle shaft. It would be great if I could rig it to work from the gas pedal under the dash.

[steve25]

Here's a question I need to ask as after all of these years info not know the answer that might effect the OPs issue, that being is the TB 400 kick down switch on the gas pedal a normally open switch, or a normally closed switch?

[Hikin Mike]

Originally Posted by many birds

It would be great if I could rig it to work from the gas pedal under the dash.

I never had a downshift bracket on mine, so I found this at a local wrecking yard about 25 years ago. Modified it to fit my car. Yes, I have shrink wrap on it (not shown here).

[lust4speed]

Normally open on the switch. The original switch on our '65 2+2 fell apart and replacements should have been gold plated with diamonds for the money they were asking. Went with a nitrous switch that closed when the secondary lever contacted it on the passenger side of the carb.

[dataway]

When I was a kid I pulled the kickdown switch off my GTO and wired it to a toggle ... if memory serves me .... it would do weird things when I clicked it on and left it on ... felt a lot like a higher stall converter or in some way it was letting the engine rev higher before a rolling launch at say 30 mph.... don't remember if it had any effect from a standing start.

Course that was 40 years ago and I was an excitable child.

[steve25]

One big thing to check is to disconnect the shift cable at the trans and make sure that it's adjustment is placing the shift lever firmly in the D position.

[many birds]

I realize this is old thread...I just now restarted working on the car. The 2-3 hunting still continues. I have readjusted the governor for about 5500 rpm WOT shifts. This has had no effect on part throttle shifts. I have connected the downshift switch and adjusted the modulator, but the 2-3 hunting still continues. Playing with the modulator only changed the speed when the hunting was occurring. I’m going to pull the pan and change the downshift solenoid next. While I have the pan off, is there anything else I need to check?

[OCMDGTO]

For me the cure was changing to a different vacuum source and that immediately solved it.