Thread: Chevy 350 TH Shifting Problems and More???

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First off, a disclaimer: I don’t know a lot about automatic transmissions. I want to learn more about them which is why I am here as opposed to going to the shop to have it repaired. With that said, here is the problem.

Engine: 350 SBC

Transmission: 350 TH

Vehicle: 1947 Studebaker Pickup

Current Condition: Power plant combo was installed in truck in the mid 90’s. Mileage
on the engine since install is less than 10,000 miles (the truck
was not driven often, mainly car shows).

Problem:

It feels like it takes about twice as long than what a “normal shift” from first to second would be. In addition, once the transmission gets ready to shift to second it jolts hard. This will happen every time. The other shifts are not as pronounced and I would almost believe that it is only from 1st to 2nd that is the problem. I also have a problem going from a dead stop and accelerating over a hill. The truck will crawl and almost die up the hill. During this time you can feel the transmission “wind” up as if it is catching up and then a jolt and away we go up the hill! I can’t count how many times I had to wave the cars behind me to pass me.

Since these problems have occurred I have replaced the vacuum modulator in the transmission, verified that the fluid is at a good level and that the transmission is not burnt. I have also removed the governor, but do not know what to look for to determine if the part is bad. The plastic gear on the governor looks brand new.

I am sure I have left out some information. I appreciate the help and look forward to the replies.

Originally Posted by DennyW

Do you know what the engine vacuum is warmed up, and idling ?
Do you have the vacuum hose going to the right place ?
Is the trans modulator adjustable ?
Does the trans have a shift kit in it ?

I did not check the vacuum. What is the optimum range I should see at the carburetor?

I have the vacuum going to one of the small ports on the front of the carburetor. The one on the back of the carburetor is plumbed to the power brake booster.

The modulator is adjustable, and I failed to mention previously that I had adjusted it all the way out in increments of a 1/2-1 turn and did not get any noticeable difference. The vacuum modulator is a single red band adjustable.

I don't believe this transmission has a shift kit. I will have to look back for receipts since my Dad past away last year and he would've been the only one who knew.

you need a good source of strong vacuum going to the trans .. sounds like you may be using ported vacuum .. most GM trans pull vacuum straight from the back of the intake manifold .. my typing is slow ... what denny said ... HA !!

THIS IS A GREAT EXAMPLE OF WHAT WE ALL SHOULD DO: Since the truck wasn't built by Stude47, it sure would have been helpful if the builder (in his case, his dad) had documented the parts used in the build. A simple document listing all the parts with perhaps the date acquired, from where, the cost, and the part number would be helpful to the new owner trying to resurrect the vehicle..... easy to do when the build is going on. Print the document and put it in a plastic bag under the seat. Future generations will thank you.

Have you checked the tv cable adjustment it controls the line presure from the front pump as you open the throttle you pull the tv cable ond it increases the rail presure and also the shift timing whe you are crusing in 3rd gear stand on the on the pedel and see if it shifts down to 2nd gear ( passing gear )

Originally Posted by DennyW

((I don't know what cam you are using for one. If it's stock or close to it. You should be getting 14 to 18 inches.))

I have the vacuum going to one of the small ports on the front of the carburetor. The one on the back of the carburetor is plumbed to the power brake booster.
(( There again, I do not know what carburetor you are running. But, the port you hook to, should be main vacuum, not ported. The best place is to port it right to the intake manifold. This can be done by using a vacuum Tee.))

The modulator is adjustable, and I failed to mention previously that I had adjusted it all the way out in increments of a 1/2-1 turn and did not get any noticeable difference. The vacuum modulator is a single red band adjustable.
((Once you check things out, you can then adjust the modulator to the stock settings...))

Below are three pictures of the setup. The vacuum line is going from the modulator on the transmission to the port on the front of the carburetor (smaller of the two black lines in the circled area). The carburetor is a Summit Racing M2008 Series. The last picture is the connection that goes to the power brake booster.

Originally Posted by DennyW

You have the trans modulator hose on the ported port. Wrong one.

https://static.summitracing.com/glob...rb%20instr.pdf
The blue cap is full vacuum.

I will get it hooked up and try that out, thank you. I will let you know of the results when I can.