wolfman98

I am having problems with my 69-98 455 and TH400 tranny. The car does not kick down a gear when I step on it. Even when I am only doing 50 mph and floor it , it stays in 3rd gear.The modulater valve on the tranny has a rubber tube that changes into a metal tube and comes up to the top of the motor , then goes back to rubber tube and goes into an inlet to the top of the block in front of the carb. Is this correct and could the valve be the problem? Not really sure how it works but it senses pressure in the motor? I had the carb completely rebuilt but I still have to pump it a few times on a cold start. I did not replace the choke coil spring and maybe it's a little weak? Any ideas on these problems would be greatly appreciated.

70 cutlass s

Th400 uses a electrical detent switch called downshift switch. Its located on the gas pedal. Switch could be bad or need adjustment. Also you may want to check electric wire that goes to the trans make sure its good. As long as your trans is upshifting fine from 1st to 2nd to 3rd than your modulator is fine. Having to pump a carb. when its cold is normal may need adjust the choke a little.

wolfman98

Thanks I will check that wire for the downshift switch tomorrow. I will try a choke adjustment as there are 3 holes you can put the little stem from the coil spring into.thanks again i will let you know what I find out.

wolfman98

Ok I got to check out the funky switch on the gas pedal arm and I concluded that even when pushing the pedal to the floor it did not push the switch lever far enough. I could slide the lever by hand easily about 3 times as far and when I did it seemed that there were 2 distinct though faint clicks as I slid it up. So I cleaned up the switch connection and cut a small rubber hose and put over the gas pedal arm under the switch and it then pushed the switch arm further. Hopefully this will solve the problem but I will have to wait til tomorrow cause it rainy as we are having a noreaster blowing by.

rocketraider

Wolf, you can adjust the downshift switch for proper operation. If you have a 69 chassis manual look on page 6M-44 4-barrel carb section and go to Fig 6M-103. Drawing 15 shows the adjustment.

If you don't have the CSM: Take that rubber hose off, then

Push the plunger in the switch towards the firewall until the end is flush with the switch housing.

Then press the accelerator pedal to wide open throttle position (to the floor in other words) to set the switch.

This should set your downshift point. You might want to use a test light across the switch terminals to make sure it's working. If the switch tests good and still won't downshift, check the GAUGES-TRANS fuse in the fuse block (though if it's blown none of the warning lights or gas gauge should work either) and the wiring to the downshift switch on the transmission itself. Look for black and orange wire.

All the vacuum modulator does is sense normal (non-detent) shift points according to engine load. It determines that by vacuum signal. If a vacuum modulator fails, what will usually happen is the transmission will upshift either late or not at all.

wolfman98

I was wondering if it could be adjusted but could not figure out how , thanks. I do have the 69 service manual and the guages work so the fuse is good. I will try the adjustment first and see if it works , thanks for you expert input .