I have aSP400 in my build. In drive, the trans works as it should, goes through all 3 gears. The trans has a B&M shift kit with a Genie shifter. I can go through all 3 gears on a manual upshift. The problem is with a manual down shift from 3rd. down shifting to 2nd , the trans stays in 3rd, yet if I put my foot into it, it will shift down to 2 nd. I can down shift to 1st., with no problem. I have checked all the shift linkage etc. The tranny has about 2,000 miles on it. Can anybody offer any solutions here. Appreciate your suggestions.
Do you have the kickdown hooked up, it’d be an electric switch? On regular turbo 400 the solenoid in the trans can go bad and not downshift.
A switch pitch 400 should have a kickdown switch with two prongs; like this one. The top one controls the switch pitch and the bottom one controls the kickdown for passing. Hope this helps. Only thought on not down shifting with your shifter is that the linkage needs to be further fine tuned. Good luck. Al
I can tell you how a stock 400 works, but not one with a shift kit. I'd be calling B&M and telling them your problem.
I have the down shift and the switch pitch wired to a togle on my shift lever. I can togle the down shift to 2nd , but it does not kick in until I put my foot into it. I have been using the down shift to 2nd. to slow down at a stop light, and it has worked just fine until lately. I appreciate any advice here.
Make sure the vacuum modulator is working. Pull the hose off of it and see if is full of fluid. If it is, replace the hose and modulator.
So you are using the kickdown solenoid for a 3-2 downshift? I thought you were manually pulling the shift lever to 2nd. That might require a bit of throttle pressure work. With high vacuum you have very little throttle pressure (vacuum modulator). The kickdown solenoid was designed to work at wide open throttle so it would typically see pretty good throttle pressure.
It sounds like you are saying there is no engine breaking in manual second. Sounds to me like the intermediate brake band is not holding for some reason. Could have something to do with the shift kit I guess.
I was always told manual downshifting was a weak spot for most automatic transmissions. I never really understood why. The explanations came to "harsh engagement". Even putting the transmission into gear at idle is "harsh engagement". That being said I have not replaced a transmission since I stopped downshifting them.
I think I'd be looking at the 2-3 shift valve. Take it out. Spin it in a drill. Break the sharp edges with some 320 grit paper. Clean the hole out with carb cleaner. Make sure it operates freely without the spring and then with it. If this just started, it could be a crud build up.
I've always heard the same thing...you can't drive an auto trans like it's a standard/stick, it wasn't designed to do that...leave the shifter alone and let the trans do it's job.
What year is the transmission? It should have a tag on the side with letters/numbers that tell you some useful info. I know they did a minor redesign in 65. If you have a 64 Buick unit, it might be set up differently? I haven't looked into them. I know in my Chevy II with a full manual valve body, it downshifts from 3rd to 2nd just fine, all the time. I also know that it's on the ragged edge of reliability, as it gets hammered pretty hard at the drag strip, then has to drive thousands of miles back home. I end up overhauling it more often than I should. It went about five years, before I had to put new direct clutches in it again last summer. And it's buzzing now when cold, after putting only 6k miles and another 10 or so passes on it...I think the pump is getting worn out, and they're getting hard to find.
The 64 ST400 is a different animal of one year only. I did not realize the pumps are hard to find. Need one?
I have both a fixed pitch and a switch-pitch TH425 on the shelf. If you want to talk about rare parts, there are some in there! I am going to build the fixed pitch one. I do like the idea of an SP425, but not the idea of spending the coin to build one that might not be rebuildable when I break it.
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