TH400 TURBO PROBLEM

Hi,

I have a 1970 chevy truck with a TH400 transmission and it was overhauled around 1 year ago and I keep it at home for the same period of time (with no oil), after I got the truck painted I put the tranny on the truck but now I'm having problems with the third gear shift, first and second are shifting working good but It never get the third shift make, even when the RPM's are too high, checked oil level, vacumm hose, and they seems to be ok. Oil is new and clean but I haven't remove the transmission oil pan yet. Any comments are welcome.    

Check the linkage adjusment, when the indicator says drive you might still be on the 2nd gear detent or partially in between.

Pull off the cable or rod linkage from the lever on the side of the trans then manually shift into neutral then place your gear selector into neutral and see if the linkage slides on freely (without moving the detent or using any force), make the necessary adjustments to the linkage if necessary. Your indicator and trans detent should now be in sync.  

I appreciate your quick response, but I think this is not the problem because I can count every shift position from the column. Any other suggestion?  

Non electronic turbo gm tranmissions shift from a signal sent by the governor and vacuum modulator. As engine speed increases the governor uses centrifugal weights which expand much like a mechanical advance on a distributor. These weights will reach their limit as rpm in that gear increases and tells the valve body to shift to the next higher gear. The modulator will keep this shift from happening until later under a low vacuum signal (heavy load or accelerating). This system is very reliable and fairly simple compared to the newer electronic transmissions which are computer controlled.

If your modulator diaphram is bad the symptom will usually be a early shift under heavy load. Since yours is not shifting at all into high gear but the 1-2 shift is working. I still think it is the detent.

You said you can feel the shifter go into each position at the lever but this is not a true indication of what is happening at the trans. If you pull the linkage off you can manually shift the bracket using a screwdriver or crescent wrench. There will be very distinct "clicks" or detents these must be in exact positions in every gear or posssibly damage or eratic operation will occur.

If you eliminate these possibilities you could have pressure problems in the valve body which is way to complicated to explain in this forum.

As for mechanical issues the symptoms can range from strange noises to ghost shifting. In a turbo 350 or 400 the weak link is by far the 2nd gear sprag when they break you will have 1st and 3rd gear only and sometimes a rattling noise.

If you are  sure about the detents and they are adjusted correctly your problem could be diagnosed fairly easily by an experienced trans technician There are several things they can check and adjust without removing the trans.  

the shifter mech at teh column has it's own "detents". What you may be feeling is those, which is not at the tranny!

Also, the kickdown cable being off or not right could cause this couldn't it?  

There is no kick down cable on a th400.  You may have a bad solenoid.  I was just putting a shift kit in mine and the directions said that if the solenoid is not sealing well it will always be in passing gear.  Maybe thats what it is.  always in passing gear??  

Well. after some time we found out that I had problems with some moisture inside the transmission, I had to change some bad parts including oil and oil filter mesh, I think it was due to the long time with no use. Thanks to all.