THM350 Questions

[PbFut]

I have never owned a THM350 and it is clear the present set up I am using is not correct. I am seeing temp at the exit of the cooler line close to the tranny reaching 250 degrees on very hot days. This in turn places stress on the coolant cooling system because of the in radiator tranny cooler. I am suspecting the tranny fluid I selected was a mistake. The newer tranny I had in Trail Blazer SS I own had to use Dextron 6 or it would really act up while towing. Thinking that would be a good fluid to use, I changed the fluid in my THM350 to Valvoline VI. I think the friction modifiers in the fluid is allowing too much slip and generating excess heat. My plan is to drain the THM350 and replace with ATI super F 30w. I know that is a ford fluid but read that it provides excellent results in a THM350. Anyone running this fluid?

My other question is fluid change. To get some of the fluid out of the converter, it is suggested to run the engine with the cooler line un hooKED to pump the fluid in the converter out. Then refilling with fresh. Assuming you don't run too long is this an advisable method for fluid swap?

[Warrens69GMC]

Why not run Dextron 3 ?

[Rust_never_sleeps]

Dex vi is backward compatible for Dex iii

https://thetotalfix.com/dexron-iii-vs-vi/

[Warrens69GMC]

Originally Posted by Rust_never_sleeps

Dex vi is backward compatible for Dex iii

https://thetotalfix.com/dexron-iii-vs-vi/

I know that it backwards compatible, but if he is having problems, why put in type F?

[72c20customcamper]

Originally Posted by Rust_never_sleeps

Dex vi is backward compatible for Dex iii

https://thetotalfix.com/dexron-iii-vs-vi/

It says right in the article that Dextron VI is of lower viscosity than Dextron III. It may be backwards compatible but it isn’t the right fluid for the 350 and states a number of times to use the manufacturers recommended fluid . I’m suspecting a lot of slippage causing the higher temps . I would never run the car with a trans line disconnected the worst thing for a transmission is low fluid levels . If you want to get the old out have it flushed . Be aware that everyone I know that had the transmission flushed ended up with problems .

Drop the pan and refill run a few miles and do it again is the way I’ve always done it if I wanted to flush the transmission.

[PbFut]

Originally Posted by Rust_never_sleeps

Dex vi is backward compatible for Dex iii

https://thetotalfix.com/dexron-iii-vs-vi/

Originally Posted by Warrens69GMC

I know that it backwards compatible, but if he is having problems, why put in type F?

I agree with 72C20CustomCamper. I think the friction modifiers in Dextron VI and the thinner properties are allowing too much slip. Type F actually has less slip than standard Dextron III and there was a anti foaming property to the original Type F fluids as well. It was known to be used in the GM drag racing circles for a slight advantage. Thinking about type F, maybe I would be over correcting. I think I will search a bit more for some newer Synthetic based Dextron 3 30w. Assuming they blend the correct friction modifiers, it will be as close as I can get to the original fluid properties. It certainly worked for billions of miles back in the day.

I picked up a Derlane pan with the cooling tubes. Looking at the design, I don't think it will fit the factory cross member and it will certainly limit air flow in the tubes. Summit has a inexpensive replacement crossmember that appears may correct. It arrives today. Plan is to swap the fluid and see if it helps any. Then change again while swapping pan. That way I'll know if the pan works or is just a marketing gimmick. As a last resort, I have a Hayden cooler that never got used on another project. I don't want to put a cooler in front of a AC exchanger but it may end up being my only option.
I will report back in a few days, the results.

[Rust_never_sleeps]

If you search the forum, plenty of folks use dex 6 in TH350, TH400 trannies.
It might shift differently due to viscosity(and TypeF shifts differently as well)
If the friction modifiers were causing it to slip, wouldn't that mean less heat, not more?

[Rust_never_sleeps]

In any case, Dex3 might be the right fluid, Ford fluid might be the right fluid, BUT I fear you have other problems going on that aren't due to the Dex6.

https://www.motortrend.com/how-to/ch...rformance-car/

[Bigdav160]

The heat is made in the torque converter, not the clutches. If the clutches were slipping you would need a tow.

The fluid is not your problem.

[72SB]

DEX 3, now DEX/MERC is what a TH350 uses. You really don't want to mix old and new gen trans fluid. Sounds like OP has quite a cocktail of wrong fluids. A general drain/filter only is 4qts so there are 6 qts still in trans & TC.

There are shops that can, with a specific machine, "flush" the entire system.

Unless the trans has a high stall TC, temps should not be 250. 180 is normal.

[PbFut]

Got to agree with the cocktail. Don't know what previous owner had in it, but sure it was cheap stuff. Walmart has a great price on Valvoline Dex/Merc. 17 a gal. I just picked up 2 gal and will see if I can do a couple changes. Then pick up another gal and swap the pan. That should get me a better percentage of fluid I am familiar with into the system.

[Getter-Done]

Since the mid 1980's I have used Type F fluid in all my Chevy automatic transmissions.

Reason being is I always put a B&M shift kit in everyone of them.

That is what they Recommend, and I thought it could not hurt.


This have been a Debate for years and still is

[{Type F can be used is any tranny and does improve shifts in GM and Chrysler transmissions. B&M Trick Shift is basically Type F with their secret elixirs (blue dye) added. Type F in a GM is a very old hot rod hop up trick and should actually help tranny life due to the firmer shifts.}]

Link: https://www.hotrodders.com/threads/t...rmenr%20shifts.

[68 P.O.S.]

An old post from the master :

https://67-72chevytrucks.com/vboard/...d.php?t=523334

[PbFut]

Originally Posted by 68 P.O.S.

An old post from the master :

https://67-72chevytrucks.com/vboard/...d.php?t=523334

Ten year old info. Lots of changes to Synthetics since then and non-synthetics are nearly extinct, I'll bet, or have been required to change the formula due to EPA rules.

[Bob B.]

Originally Posted by Bigdav160

The heat is made in the torque converter, not the clutches. If the clutches were slipping you would need a tow.

The fluid is not your problem.

I would agree with that. Have to ask, are you running a stock torque converter? High stall converters tend to heat up the fluid more.

[68 P.O.S.]

Originally Posted by PbFut

Ten year old info.

...And still completely relevant

[Accelo]

Originally Posted by Bigdav160

The heat is made in the torque converter, not the clutches. If the clutches were slipping you would need a tow.

The fluid is not your problem.

I agree, you would have smoked the transmission if it was slipping enough to heat the oil that much. How did you measure the fluid temp? If you have a infrared gun get a pan temperature also.
Is the converter a high stall?

My other question is fluid change. To get some of the fluid out of the converter, it is suggested to run the engine with the cooler line un hooKED to pump the fluid in the converter out. Then refilling with fresh. Assuming you don't run too long is this an advisable method for fluid swap?

You can't pump fluid out of the converter, it has to be replaced with new oil

Some empty the pan of the old oil, refill with the new oil. ThenDisconnect the return line to the transmission and add a hose to a container. Start the motor. It pump the new oil in and the old out. Of course there is mixing of the two fluids. While the motor is running add more oil to the transmission till you are happy with the exchange as necessary. Do not add more than 5 quarts to an empty THM350 transmission with an empty pan. It will come out the vent.
Cheers

[PbFut]

I multi changed the fluid using 3 gallons in the process. Went to do the final lowering of the lift and had a chain work off the Ram. Now I got 2 tons of truck stuck on the lift.
When it rains it poors.