How does your tranny shift ?

[adkphoto]

I hate the way my TH400 shifts. I suspect it has a problem with the vacuum modulator, but I know very little about trannys, so I thought I'd check with the pros here on the BB.

First gear shifts almost instantly, even under WOT. Under part throttle the 1 - 2 shift will happen at like 10-15 mph.

The 2 - 3 shift comes up by 30mph. Downshifts are hard to come by and then the shift back to third is too fast for it to do any good.

Manifold vacuum is good. 18 -19 inches if my gauge is right. I've had the downshift solenoid replaced and fluid/filter change within the last two years.

Any suggestions?

David

[COBALT]

Upshift from curb idle to cruise sounds normal, but your kickdown sounds like it needs adjustment, or your kickdown soleniod is going bad. Since your curb idle to cruise is normal I doubt your modulator is having a problem, but they're easy to replace...just don't expect a huge change. Modulator and governor work together to make the shift transitions based on intake vaccuum and driveline speed. It's easy to check both. As long as the diaphram in the modulator isn't broke and the baffle inside changes shape with vaccuum placed on it then it's good. The governor comes out of the side of the case (look for the square cover held in by 4 bolts), and the gear on the end should be in good shape and not cracked, broken, or worn down. The governor uses weights inside it that spread out as the driveline speed increases. If the counter weights are stuck, or the gear is messed up you'll get irratic behavior.

The TH400 uses an electrical circuit activated by a switch either on your carburetor to control kickdown (or under your gas petal). It usually happens just before WOT with the accelerator either to the floor, or the accelerator rod all the way back. There's a red wire from the fuse block that provides 12v to the switch. An orange wire completes the circuit from the switch and runs down to a terminal on the driver's side of the transmission. Inside the transmission (remove the pan and you can see it) is an oval shaped solenoid that connects to that terminal called the kickdown solenoid. It may be malfunctioning and need to be replaced.

You can test this easily. Take a standard auto circuit tester (the screwdriver-shaped probe with the bulb in the handle, and a ground aligator clip coming out) and connect the aligator clip to the terminal on the transmission. Stick the probe in a place where it's grounded, but you can see it from the engine compartment. Pull the accelerator rod back to open the throttles to WOT. If the light turns on right before or on WOT then the circuit is working right (unless the solenoid is bad).

[adkphoto]

I've replaced both the kickdown switch at the pedal and the solenoid in the tranny. Both work. I guess they just don't work the way I'd like them to.

I wonder...is there a way to adjust the modulator so that shift points come up higher? Or is that where a shift kit would come in?


David

[Class of 69]

Sounds like your wanting a shift kit. They come in various shift patterns to suit your desire. Might be time to check them out. I don't know if any of them still use steel balls in the kit, but don't buy one that has steel balls!! They will hammer out the case, and ruin it over time. This was a major problem back when shift kits were newlly introduced for the TH400.

[JimKshortstep4x4]

Some of the modulators are adjustable. Remove the vacuum line
and look inside the tube. If there is a screw in there try tighting it.

The other trick that I have seen done is an adjustable vacuum control that connects into the vacuum line and you can adjust the shift points by adjusting the control. If this sounds like something you are interested in I can get more information on it as a friend just put one on his 71 SS Chevelle.

Jim

[miket]

I just went through this with a new tranny, motor and radiator..here is what I found ( working with a professional transmission shop)...my truck would shift quickly 1-2-3 and stack up to 3rd at a stop.

The modulator is adjustable....but I was told only slightly....anything over 1-2 turns and it creates more shifting inaccuracies (there is a small screw inside where the vaccum line plugs in)....mine had been adjusted but didn't do what I needed...I alos run about 12 in of vaccum. If you have less, a diesel modulator might work...I have not installed the electric full throttle kickdown yet...

The conclusion was....the new radiator I bought still hand some particles in it which made it into the tranny causing a majority of these problems....as well....to get a stronger shift behind the 502...I went to a Hughes shift kit.

[adkphoto]

I'll try turning the adjuster screw in on the vacuum modulator and see where that gets me. Don't see how it can hurt much.

An adjustable modulator sounds interesting. Wouldn't mind hearing more about that. I think I've got a shift kit in my future though.

Thanks!

David