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Jul 21st, 2025 | | | | Joined: Jul 2009 Posts: 42 Apprentice | | Apprentice Joined: Jul 2009 Posts: 42 | My turbo 400 in my '53 gmc kicks down to 1st gear sometimes when I'm accelerating in second. Does anyone know what the problem could be and how to fix it? I'm thinking it has something to do with the vaccuum advance. However, it also doesn't hold "park". COuld there be a linkage misadjustment causing this?
I know zilch about auto trannies, among other things!
Later,
Jason | | | | | Joined: Feb 2000 Posts: 4,886 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 4,886 | Park is operated by linkage, get it to work before messing with the kick down. Unhook all your linkage with the wheels blocked, and manually shift the trans to park. Remove the wheel chalks and confirm its in park. If its not, you have internal problems. Place your shifter in park and check the linkage line up, then move the trans to low by hand and then the shifter, check the linkage again. You should be able to tell if what you have is going to work or not by how the hole and shift rod line up. Now is the time to correct any short comings.
The kick down is controlled by a electric switch on the throttle or carburetor linkage. You can adjust the position of the switch to trip at any point by moving it along the travel of the linkage.
Joe | | | | | Joined: Jul 2009 Posts: 42 Apprentice | | Apprentice Joined: Jul 2009 Posts: 42 | Joe,
Thanks for the info. I will take the linkages loose and check it manually like you suggested. One thing----when I took the pan off to change the filter and fluid, I noticed an electrical connection with a wire on it inside the pan. There is no electrical wire going to the transmission from the outside so I know it's not hooked up. As far as I can tell, the only thing going to the transmission is a vaccuum line and the shift linkage.
Could that be my problem, or does this transmission have a vaccuum and/or electrical kickdown? I have never actually pressed the gas pedal hard enough to cause it to kick down, except starting off and I didn't push the accelerator hard enough to cause a kickdown (it shouldn't have done it--that's what I was wondering about).
Thanks,
Jason | | | | | Joined: Feb 2000 Posts: 4,886 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 4,886 | Jason,
The 400 will down shift on its own if it sees the vacuum drop from the carburetor. The modulator and governor control normal day to day operations in the transmission. They will work with out a switch hooked up, but if racing or towing, you should have one.
The kick down switch will override the governor and downshift the trans any time the switch is tripped. The kick down switch also raises the hydraulic pressure so under a hard shift the clutch packs don't slip plus it holds the trans in gear longer for more power. You will notice at full throttle shifts that the rpm is way to low with out the switch.
Joe | | | | | Joined: May 2009 Posts: 6 New Guy | | New Guy Joined: May 2009 Posts: 6 | If the vacuum source to the vacuum modulator is lower than it should be (leak in the line somewhere or low engine vacuum) than the transmission will think the throttle is opened more than it is and call for a downshift.
The vacuum modulators are also adjustable to custom tailor part throttle shifts.
It is also possible your modulator is failing. | | |
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