I've posted a few times about the strange issue I have with the trans on my 64 Fleetwood. When it gets warm and up to temp, I loose 3rd gear, but I never loose reverse. According to multiple sources, 3rd should be find as long as I have reverse. When the trans DOES shift, it does so with ease. I check and maintain the fluid level nearly daily as it does have a leak. But I have yet to find any answers in regards to this issue. 

I have a new pan seal and filter as a last Hail Mary to see if that is the issue, but I am not keeping my hopes up. The way I see it, if the filter doesn't solve my issues, I have two choices. 

1) I purchase a used trans from Jason and have it freighted to me here in Illinois. I order some new seals and have a trans shop swap them around for me. I'm not sure the exact cost for this option, but I am thinking it may save a little money vs. doing a rebuild. 

2) I just save for the rebuild. I am leaning towards this option as then I know the trans is worry free for the foreseeable future and I can drive and enjoy the car. However, I am getting quotes in the $2,500 range which is a little more than I want to spend in the next 6 mos to a year. IF I do go this route, I'm looking for someone who has recently had their 64 "TH400" rebuilt. I had two shops in my area tell me they won't touch a 64 because the internals are different than that of the later years. What overhaul kit did you purchase? How long did it take? Just factfinding at this point and would love your input!



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As a follow up because I didn't want to make the post too long. I have replaced the vacuum modulator, serviced the governor and inspected that it was moving freely. Those were the two most common issues I have seen others have. Outside of replacing the filter-I am out of ideas and so is the trans shop that states they can rebuild a 64 THM400.  


The trans in our year Cadillacs will not shift correctly—- if the trans fluid level drops - I think a pint or two 

As you say you are losing fluid ( not sure how much  )  I think this might be some of the issue with your 3rd gear not shifting as it should —-but just a best guess knowing that 


It is true that 64 TH400 is not 100% identical to later years, but those differences are minor. Even here in Finland, far away from Caddy home, I can get 64 specific parts without problem. 
Unfortunately I am not able to tell the solution for the problem in you caddy.

Can you change from 2nd to 3rd gear manually?  ie. With warm engine drive fast (>50 mph)  with stick in 2nd gear and then move it to 3rd position. If it changes ok then valves and bands seem to work ok with warm engine. In that case I would search the problem from governor even you have changed that. 

From my research, everything internally seems to be OK. When it DOES shift to 3rd, it shifts very well. Reverse functions nice and strong when the engine is warm as well. Perhaps when I replace the filter and pan seal, Ill check all of those areas again as well. Certainly won't hurt anything!



The  vacuum shift modulator off the back of the carb on the Fleetwood has  something to do with the  correct shifting of the th-400 trans 

Might have already been considered by Logan already 

I do know that Logan examined and cleaned  the governor some ( not sure how much )  but sure he did not yet  replace it 

Looks like the governor is the major control point for correct trans shifting and if clogged from debri  stops working as it should 

Just picking up a few pointers from prior threads 

 I Enjoy  knowing this stuff 


I’m unaware of any vacuum shifter on the back of the carb. To my knowledge, there is a vacuum line there that goes to the modulator on the side of the trans. But no component other than the line. 

When I change the pan seal and filter, I’ll certainly check the gov again. Might replace the entire vacuum line as well. Logan


There is a vacuum modulator that controls trans shifting on the side of the trans— at the end of the carbs rear vacuum hose - to the trans

That modulator  is replaceable and could be your problem 

Be sure the vacuum hose  is not leaking air and the metal connection at the back of the carb are not leaking air —so the vacuum is working as it should 

Those metal fitting need to be tight and snug 

I used plumbers tape on the treads so the fittings got snug were I wanted them to end up and work 


The vacuum modual is on the trans with a hose to the back of the carb where  the vacuum originates 


Per she shop manual on pg 10A-22, Fig 10A-16 look at H-I-K-N



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