Transmission will not shift until it warms up by driving it for approx. 15 mins

At this point you have nothing to lose by trying more chemical cures, such as the various seal conditioners and stop leaks that are sold. You can also add a couple cans of STP to thicken the ATF. I put in a couple cans of the seal conditioner and that helped my S-10 for a while, then it started having delayed shifts more and more so I put in

2 or maybe 3 cans of the STP to thicken the ATF. That also helped. I would estimate that I got an extra two years out of the transmission by doing that. The STP won't hurt anything, it just makes the ATF a bit more viscous so it doesn't leak past the worn internal seals as easily. The higher viscosity does create slightly more internal power loss in the tranny but not enough to matter for normal people.

Do not use additives, this is do to a partially clogged trans oil filter, t hick additives will make it worse, (when the oil warms up it get thinner an d gets threw the partially clogged filter easier do to getting thinner at h otter temps, you need to have the trasmission fuel and filter changed to re pair the problem , if after that if it comes back the problem (the engine w as probally overheated, resulting in band damage form the tranmission coole r), I'm a registered journeymen mechanic and know the things, email me dire ct at snipped-for-privacy@aol.com for any further help, i'll see if I can save you a bunch of money....

To add to the discussion: Many shops can now power flush your tranny which might resolve any fluid problems being discussed here. IMHO if you keep changing out fluids without doing this you may not get to the root of the problem. FYI, I'm not a mechanic but I believe Scott is on the right track.

A so called "power flush" will not solve a plugged filter, if that is the problem. Check the condition of the fluid. Not hard to tell if it is thick just on your fingers. Smell it - stinks? possibly burned. Colour? Red OK, brown or black - burned.

If the fluid doesn't stink and is a nice pink coulour, with a good slippery feel, it is extremely unlikey to be a fluid problem.

I too am an automotive technician - licenced for over 40 years - and the RIGHT additive can often do the job very effectively - for a very small fraction of the cost of "shotgunning" the problem with a full transmission overhaul.

Just another possibility - is the fluid level just a WEE bit low??? On a 1989/90 Aeostar the dipstick was marked aboit 1/4 inch low - worked fine in warm weather but was dicey when cold - worked fine when overfilled 1/4".

It really sounds to me like one of the valves in the valve body is sticky - and the friction modifier additive addresses this quite effectively. If it doesn't work, it won't hurt anything and you can still rebuild the tranny.

been a long time since I saw a car that old, but wasn't shift point purely vacuum related back then? Seems a failing external shift modulator could act differently at diff. temps as stated. GW

I don't think any cars were ONLY controlled by the vac mod. Usually there is a throttle valve that senses the position of the gas pedal, and a Vac Modulator valve that senses how hard the engine is pulling. Between the two of them working together PLUS the spinning speed sensing gizmo on the output shaft that raises pressure the faster the output shaft is going, all of that together determines when to shift. Or something like that.

oh, boy.... ancient trans theory vs. my memory...I think TV cables replaced vac modulators as a "load sensor" and the speed sensor was called a governor, in the dark ages b4 computer control. GW

That's what I was trying to think of .. "governor"! Now that you mention it, I do think some trans used a vac mod plus gov and others used a TV plus gov and some used all three. So I think your original thinking was right, you just didn't mention the gov.

If a trans had just a governor, it would always shift at a predermined RPM. The TV-cable or vac.mod. gives the valve body a "sliding scale" input to decide to shift sooner or later. Since the OP asked about late shifting...until warm...and already changed the fluid...and the trans does not slip. the first thing I would do is look at the vacuum modulator, if it has one. If it's leaking internally it will have fluid in the hose. (which usually leads to the hose swelling and falling off) GW

And puffs of white smoke from the exhaust (1964 Plymouth Fury, or 1964 Buick Skylark, I don't remember which). I can remember fixing it before the hose swelled and fell off :-)

lol. Yes I just remembered what a great "internal engine cleaner" trans fluid makes. and it's cheaper than SeaFoam!

GW

Yep, long before the days of catalytic converters. I don't think it would be very good for them :-)

Likely the buick because the old Mopar used a kickdown linkage and no modulator valve.

You're probably right. I had a 70-something Nova that also had the modulator go out. Late 60's, early 70's Fords had the kick-down rod also. One of my jobs at the Wayne Assembly Plant (now known as Wayne Stamping & Assembly) was putting transmissions on the engines and hooking the kick-down rod to the transmission linkage. Good times . . . NOT!! It was good money for the time, though (especially when we were working those 80-hour weeks).

Many (or at least some) FORDs used both. The modulator adjusted line pressure and the kickdown operated passing gear.IIRC

Yep. Hence the name "kick-down" :-) I know that the C4 and C6 that went on the Galaxies had both; the kick down was adjusted as it was connected to the carb linkage, then tested at EOL (end of line). The modulator was vacuum operated, and, IIRC, set the shifting point by the amount of vacuum created by the engine RPMs. That was external. They also had an internal governor on the output shaft. Been a LOOOONG time since I worked on anything that old :-)

Warning - the stuff STINKS!!!!!!