Phlash

I continue to try to get my TH400 to shift right. I replaced the accumulator today, but i actually made it a little worse. The old one would shift from 2-3 at 32 mph and the new one at 35mph. The vacuum at 1000 rpm is 15 inches at 3250 ft elevation. I have also tweaked the screw inside the vacuum connector on the acumulator, but it doesn't seem to affect the shift point. How many turns can you adjust this screw. I have also check the kickdown switch above the gas pedal, and it does have power and does energize when the pedal is pressed to the floor. How can I check to see if the solenoid in the tranny is working (should I hear it click, or can I check it with an ohm meter). It won't down shift below 50 mph, and I can't get a 3-1 shift at any speed. I haven't found this addressed in the technical area of the forum, just a few comments in different posts. Thanks for your help!

Phlash 72 BB Roadster :seeya

Smokehouse69

I'm not a tranmisson expert by any means, but when you talk about the 2-3 shift at 35 mph, how much throttle are we talking about? Is this under very light acceloration or wide open? I don't think your modulator is the problem, but if you are unsure, switch that sucker out, they are very cheap.
It is my understanding that shift points are determined by the governor weights and springs. The governor is located at the right rear side of the transmission, just about opposite of the speedometer cable connection, if you are shifting at too low a speed then your problem can be in the governor itself.
B&M makes a spring and weight kit that allows you to customize your shift points by swapping out springs and weights. I just put one in my car (I have not driven it yet, so I can give no opinion of how it works.)
At what RPM are your shifts taking place?
According to my chassis service manual for my 69 with a Base 350/300hp and the TH400 here is a representation of the table in the book of shift points with 3.08 rearend and F70-15 tires.
Throttle Position 1-2 Up 2-1 Dn 2-3 UP 3-2 Dn
Closed 7-10 10-8 14-18 16-11
Thru Detent 35-47 41-27 64-79 74-58

I have no real idea of what they mean by Throttle position, I assume that that is the with a closed throttle (15-19 in of vacuum) and a coast situation, this is where the modulator would allow the shift to take place.
Thru Detent is the kick-down solenoid.
I hope this helps you out.

7tea1

I can hear mine actually "click" in the tranny (naturally, the key is "on" and the motor off) when I press the pedal to the floor.

KJL

I can here mine click as well with key on and motor off... I am not sure what you mean by a 3-1 shift, It will do that when the car comes to a near stop but never under power like a kick down, I think that is only 3 to 2. There was a thread a while back where a member said with a hole saw he cut a 5" diameter hole through the tunnel under the center console to access the governor cover. I think that is the only way you can remove the gov. without dropping the transmission.

Matt Gruber

sounds like the kickdown is not working.
in D mine will drop to 1st WOT at 40 mph.
Stock should be at least 30-35 WOT to 1st.
tips in sig.

KJL

Hey Matt,
I found your message and I guess that was a 3 1/2" diameter hole. I just install a transgo kit in my turbo 400 and thought while I was under there thought I would try to identify where a clicking sound was coming from. It turns out it is coming from the governor. Is there trick to locating this 3 1/2" hole correctly before I drill?

Thanks

Matt Gruber

KJL
on my 72 the line where the rug meets the console is the top of the hole.
i did not remove the seat; that is the approximate rear of the hole. i put a square plate over the round hole, so precision is not needed. trim if necessary :cheers:

Phlash

Smokehouse69,

I did replace my modulator and it seemed to hurt the shift point. It went from 32mph to 35mph. No matter how light you use the throttle it will not upshift into 3rd. In fact it will downshift from 3rd to 2nd at about 27 mph, even though you are coasting down. It is possible that there has been a kit or modifications done to the tranny, but I have no way to find out. Also, even at at 35 mph and wot, it won't drop to 1st gear, but only a slight change in throttle it will go from 3rd to 2nd. My 68 that I had many years ago was pure stock, and it would shift to third at any speed if the throttle was light enougn, and it would downshift from 3rd to 1st if your speed was low enough.

As far as the solenoid, I will need to get my wife out and let her see if it clicks. I just can't hear as well as when I was a young pup. Any other possibilities or suggestions would be greatly appreciated.

Phlash 72 BB Roadster :confused:

KJL

Thanks Matt, That is a good place to start, I figure I will drill some small test holes in that location and see where I come out.

KJL

Phlash,
I feel your pain....unfortunately as far as tranmissions go there are only a few individuals that can help you on the forum. It is a "black box" to most and for good reason. :eek: I got a Turbo 400 manual from a local trany shop and in it there is a trouble shooting section. When a trany starts to shift erratically there can be many causes. I suggest bringing your car to a good Trany mechanic and have them drive test the car and do a hydraulic pressure test. The test can help zero in on problem areas in the transmission. I doubt they would charge more that a 100 bucks for the diagnostic work. A good oil pressure test kit will cost 100+ bucks.

Phlash

Thanks KJL,

One thing about this tranny, it is very repeatable, and shift are good and crisp but not harsh. They just aren't quite at the conditions I want. I know a couple of shops to talk too, but just knew that one of the forum members would have the answer.

Phlash 72 BB Roadster

goinbroke

Shift points for TH400

Light throttle: 1-2 8-13 mph
2-3 21-38 mph

Wide open throttle: 1-2 37-53 mph
2-3 73-91 mph

Matt Gruber

so did you test the kickdown circuit yet?
i ran a wire direct to the solenoid to a temporary doorbell switch to +. give it half throttle at 30mph, hit the button, WHAM 1st gear.
in this case, switch substitues for WIFE

Smokehouse69

That's a good way to check the solenoid and switch, I'll have to remember it! :flag

Phlash

Rip,

Will your TH400 shift out of 2nd at 20 mph when you let off the gas, or does it downshift back to 2nd? Mine just doesn't seem to be quite right, or at least what I remember on my 68.

Matt,

My car is at the muffler shop this week getting new pipes. Once I get it back tomorrow, I will be tweaking it again.

What mph does other TH400 owners shift at? 1-2, 2-3, 3-2 and when will it no longer shift from 3-1?

Phlash 72 BB Roadster :seeya

goinbroke

If i'm in 2nd at 20mph and let off the gas, it stays in second. As far as the 3 to 1 downshift I really don't recall what the speed was. I really don't like doing it because the RPM's just jump wild. I went through the same thing you are, thinking the tranny was shifting too slow and too soon. After looking at normal shift points in GM manual, mine seems to be right in the middle of the range. I guess it's just the nature of the beast.

Hope this helps

Pampa? Up in the panhandle. Right?

Phlash

I may be just think I have a problem and don't. :rolleyes: Rip74, When does yours downshift from 3-2?

Yep up in the panhandle, 60 miles northeast of Amarillo. I have been through Ennis a few times on the way south.

Phlash 72 BB Roadster

jerryp58

All I know about transmissions is what I saw when I put my B&M Shift Improver Kit in (read: I know VERY LITTLE).

I'm a little confused though. Did you change the accumulator & modulator or just the modulator?

I have no idea how to change the accumulator (which looked to me like a piston), but I did open the accumulator cover to remove the spring per my B&M instructions ( http://users.erols.com/jerryp58/corvette/BNM6.jpg ) and was VERY surprised to see that the spring was broken into three pieces :eek:

I don't know what effect that would have on the shifting characteristics of the trans, but if you suspect yours isn't right, that may be something to check.

Phlash

Thanks Jerry,

All I replaced was the modulator. Sorry for the confusion. From others above, I may not have a problem. I guess I need to drop by a tranny shop and visit with another OLD-TIMER like me. :lol:

Phlash 72 BB Roadster

goinbroke

Never really checked to see just how fast I could go and still get a 3 to 2 downshift. I only drive this car about once a week, and don't like to "rough it up" too much. I've got a lot of time and money tied up in this thing and the last thing I want is engine parts scattered along the highway. :eek: I think I remember going 3 to 2 at around 50mph. That's just a guess. If still in doubt, I'd go ahead and let a good tranny shop check it out.

Rip

Went to Palo Duro canyon last year. Beautiful place