Help Transmission slipping

[chevytruckfan]

This may sound wierd but my TH350 transmission started slipping. It started off fine out of the neighborhood below 2500RPM Then it started to slip after that. I thought it had slipped into N so I shifted one to the left hit the gas and I was in fact in N so I shifted one to the right again hit the gas and the engine reved like it was in neutral. I thought I was screwed and have to get a tow home but just to get me home I thought I would limp back in low gear. The trans kicked in and took off again. Shifted back into drive same thing slipping again. Pulled into a gas station to check the fluid level, everything was ok. So I get back on the road again got going in drive until I hit above 2500RPM then shift into low, except I missed and went into 2 low. I didnt want to completely burn the trans going down the highway in 2 low so I shift back to drive again and the trans catches and it was all good like it had never happend. Any body know what would have caused this? I want to check it out before I take it back on the road again but don't know what to look for.

[JimKshortstep4x4]

It sounds like a loss of pressure on the front clutches so it could be a few problems such as the modulator valve, low fluid, bad filter, the pump itself, the governor, shift valve, and probably other problems.

I would start by changing the fluid and filter and the modulator valve, (if you have not done so already).

Good luck!
Jim

[Longhorn Man]

Check the vacume line going to the modulator, and up at the carb. There should be a hard line too, so make sure it didn't pop off the hardline.
When you shift the tranny manually, as long as you are at or over the normal shifting point, you are doing the job of the modulator, so it'll shift.

[1972C10]

Modulator can be unhooked and it will not cause slipping , Sounds like wear on the high gear drum/ Band Does the fluid stink? How is reverse Say if you bury the brake and try to stall it up ?

[Brainchild]

The trans in my 4x4 did the same thing.I never had anymore trouble with it after that one episode,but I didn't trust it either,but it worked without fail hot or cold weather for 2 years.

Could just have had an air bubble in the line,or a piece of dirt in the valve body,maybe a sticky valve or something.I'm not muck on transmissions,so this is pure speculation.

How warm is it outside and do you have a good cooler on the trans?

[Longhorn Man]

Most ppl confuse slipping with spooling, when you let off the gas, and the engine slows down, but the tranny won't slow the truck down.
I may well be WAY off, but from his description, I say crawl under and look, it takes 10 min and it is free.

[1972C10]

Yeah its not a bad thing to check cause it does happen ,

I had a Th400 rebuilt for my 85c20 and hadnt put too many miles on it ,

One day i left going to go to the autoparts store wich is maybe 1 mile from my house if that. It wouldnt shift i immediatly thought my trans went out again and started to get kinda mad that it was acting up cause it had no miles on it. So i tryed to manual shift and it worked got to the parts house and it was idleing a little low i went to check the fluid and when i did i seen the hose dangling around.

Turned out the rubber hose that came with the new modulator had cracked i dont understand what caused it to crack in such a short time but i got me some new vaccum hose problem solved.

[Longhorn Man]

I had to drive mine manually for about a month last winter. There was no way I was gonna lay in the slush and crap to fix it.

[TugOwar]

Ok guys, a little help please.

My th350 is also slipping in 3rd, but ONLY after I drive it for awhile and the fluid gets hot. Otherwise it runs out just fine.

When I bought the truck it had a tranny cooler on it. It sits in the space between the radiator and the grille, sorta propped up against the radiator.

I have a 160* thermostat and it runs 160* until the tranny starts slipping, then the water temp will gradually start to climb. When the tranny starts to slip I'm looking to park it anyway so I don't know how high the water temp would get, not over 180* yet.

I've changed the tranny filter, the fluid is clean and smells good, the vacuum to the modulator is good.

BTW, this thing has a small diameter modulator (that way when I bought the truck) never seen one that small, if that means anything.

This evening I changed the thermostat (160*) just to be sure it wasn't sticking, and moved the tranny cooler so it's more in the air flow through the grille and not touching the radiator. Could it have been transfering heat from the radiator? Where and how is a tranny cooler supposed to be mounted?

Even though it only slips when hot could it still be the modulator valve?

Your thoughts?

Thanks

[TugOwar]

Ttt

[Longhorn Man]

tug...all the mod valve does is tell the tranny when to shift.
If you're slippin, double check the fluid level, and if good, you probably need to look into a rebuild.
it isn't going to be heat related...160 degrees is the lower side of ideal.