Tranny Problem--TH400

[prairie]

A few months ago I bought a pickup with a TH400 transmission with 160K. Drove it 180 miles home on a 45 degree day with no problems. Now that things have gotten colder a small problem has shown up. When temp is below zero, it takes 5-10 minutes of warm up before the transmission will work, then works good. Between 0-10 degrees it may or may not work after a short 1-2 minute warm up, but will always be going after 5 minutes.
Changed tranny fluid/filter recently and seems to be a bit better, but have not put it to a real cold test yet.
What your diagnosis and prognosis?

 

[diesel4me]

old age...

It likely has dried up "lip"seals in the forward clutch,that soften up and expand enough to seal, and allow the forward gears to work once it warms up some..I had a TH350 with similar symptoms--in cold weather I couldn't go forward until the engine warmed up,but it had reverse OK..I drove it like that for years,but almost got fired for being late for work too often!..I put a SM465 in that K5 eventually..nothing I tried made any difference,as far as additives went..

I would say change the filter and fluid,but since you already did,I would say to also ensure your modulator is getting vacuum,and doesn't have a ruptured diaphram (if ATF pours out of the vacuum hose nipple its junk!)..

You could try adding some "Lucas" tranny treatment,or add a pint of brake fluid to the tranny fluid to help swell up and soften the hardened seals inside the tranny..it could be a weak pump too,but I'd put my money on dried up seals,or maybe a clogged brass "cone"screen under the valve body,possibly the pressure regulator valve might be sticking too..(none of which can be easily fixed at home unfortunately)...

 

[prairie]

will have the modulator vacuum and diaphram checked out. We have a local mechanic who only works on older vehicles without all the computer stuff.
I am mechanically challenged when it comes to automobiles, but my 8 year old son is very interested and seems to be mechanically inclined and a very fast learner. This truck is going to be kind of a project for us, will try and do as much work as we feel confident and capable of, and farm the rest out. Plan on a mud/dirt/snow wheeler with 4-6" lift and 37" tires.
Checked with a very reputable transmission shop about possible rebuilding if needed. Was quoted about $900 to factory specs, and about $1200 built for big tires and off road use. Does that sound reasonable? They do a lot of rebuilds for the HotRod crowd and heavy use farm/work/towing pickups.

 

[diesel4me]

sounds about right..

Unfortunately,it does cost close to a grand,(if not more),if you have a reputable tranny shop do ALL the work..

I used to know a few guys with tranny shops,but one died, the other moved away!..I used to save a lot of money by pulling the tranny out myself,and bringing it to them,rather than have them R&R it,AND rebuild it..if I brought my own rebuild kit,they would only charge me 150-250 bucks to slap it together,IF no other parts like clutch drums or gears were needed..(It usually only took them about 2 hrs to do it!)..

A TH400 I hear is not that diffucult to rebuild--I have never rebuilt an automatic myself,or done more than filter changes,or replace modulators,other than take one apart to get the 4x4 output shaft out for another tranny,but it didn't appear to be that hard to put together,though you may need a few "special" tools to put it together..there are VHS videos of step by step instructions on how to do it,if your feeling courageous!...a rebuild kit might cost 150-200 bucks....

I might have to learn how myself,seeing my friends I had rebuild mine are no longer here..hope my TH400 never croaks!..