TH400 coolant lines

[don t. - 72gmc]

1970 survivor that had an aux tranny cooler when I got it. I pitched that cuz I will never need it. It leaked big time. New radiator w/tranny line connections on passenger side. New pre bent lines from Inline.

I have searched this site. Im confused on which rad port goes to what tranny port.

Is there an easy way to route the new lines w/o ruining them?

Thanks much...Don.

[Steeveedee]

I would think that it doesn't matter which line goes to which port on the radiator. All I'd look for is a stress-free connection. My truck has an auxiliary cooler since it was set up for towing.

[don t. - 72gmc]

Originally Posted by Steeveedee

I would think that it doesn't matter which line goes to which port on the radiator.

And the confusion continues(real big grin).

Some say, the tranny has a pressure and return line.

I have zero clue but sure don't wanna torch my tranny w/hot fluid if it can be avoided.

Thanks much....Don.

[geezer#99]

IIRC the bottom port on the trans is the pressure/hot and the port above it is the return. And hook the pressure line to the bottom port on the rad.

[PbFut]

Correct. Important to hook pressure to bottom of the cooler so the air in the cooler pushes up and out the return line.

[Steeveedee]

It's a complete circuit back to the transmission, not dead-headed like brake lines. Any air is going to be pushed out by the solid line of fluid coming from the trans, back to the trans.

[Accelo]

The bottom transmission line runs to the top of the radiator.
The top transmission line runs to the bottom of the radiator

[Ironangel]

Originally Posted by Accelo

The bottom transmission line runs to the top of the radiator.
The top transmission line runs to the bottom of the radiator

This is correct! I know the web shows both ways but the THM manuals show they cross with the hot fluid entering the top. I think the reasoning is that the top of the radiator is hotter than the bottom because the flow is the same. Hot in at the top and cooled out at the bottom...I do all my own work on transmissions rear axles and motors and have researched the flow on THM 350's and 400's and simply remember that the lines cross for radiator cooled GM transmissions. When you install a cooler, that's when reasoning and flow charts change... You can hook them up backwards and it wont hurt anything other than run the transmission a few degrees hotter. Same theory applies for the radiator coolant hoses. If you reverse the flow through the radiator it will still cool, just not as efficiently... A transmission needs 160+ degree fluid to function properly, that's the primary function of the trans cooler in the radiator. It's not until the trans fluid reaches temps above 200 degrees does the so called cooler even begin to work cooling the trans fluid.

[don t. - 72gmc]

Thank you very much...Don.

[nightrider388]

I was always told on a turbo 400 bottom to bottom (out) and top to top(return)

[dmjlambert]

It had not yet occurred to me to be confused about this, since I haven't hooked mine up yet and I still have crappy bent-by-hand brake lines for the transmission fluid, going to rubber lines then to an external cooler. So I now look forward to seeing how you guys conclude this. If the engine, transmission and radiator are installed and you have pre-bent lines ready to go, is it possible to install the pre-bent lines? Or is this something to do while the engine is out? I tried to figure it out and gave up, but I will admit I only tried for about 20 minutes.

[Steeveedee]

Originally Posted by dmjlambert

It had not yet occurred to me to be confused about this, since I haven't hooked mine up yet and I still have crappy bent-by-hand brake lines for the transmission fluid, going to rubber lines then to an external cooler. So I now look forward to seeing how you guys conclude this. If the engine, transmission and radiator are installed and you have pre-bent lines ready to go, is it possible to install the pre-bent lines? Or is this something to do while the engine is out? I tried to figure it out and gave up, but I will admit I only tried for about 20 minutes.

I get where you are coming from, as I inherited my truck with an external oil cooler for the trans. I'm not convinced that it makes any difference whether the hot trans fluid goes in the bottom or the top of the radiator intercooler, until an actual GM engineer comes and tells us.

[don t. - 72gmc]

Originally Posted by dmjlambert

If the engine, transmission and radiator are installed and you have pre-bent lines ready to go, is it possible to install the pre-bent lines?

I installed the tranny lines this way....
The bottom transmission line runs to the top of the radiator.
The top transmission line runs to the bottom of the radiator
...as mentioned above.

I taped the ends to make sure crud did not enter. Ran them along the oil pan.
My neighbor helped, stuck them down the top side while I was under. Hooked the tranny side first. Top rad line needed to be bent a bit but, for the most part, fit.

HTH...Don.

[Richard]

Has been said in other posts the output from trans is the lower fitting. On stock setups this line would be connected to the lower fitting in radiator. If an auxiliary cooler is added I always connect it inline from the top fitting out of radiator to lower temps a bit more as fluid returns to trans. One thing about the stock cooler is that it also acts to bring trans temp up to operating temps faster in cooler weather.

[dmjlambert]

Thanks. Can you show pictures of the route the tubing takes?

[71CHEVYSHORTBED402]

Originally Posted by dmjlambert

Thanks. Can you show pictures of the route the tubing takes?

I can only speak for a BBC 402/400TH for small details. Top to bottom and etc. as stated above. News tubes should have tape on them showing where you need to straighten them. Nice to have a tube straightener plyers, tape the plyers to prevent jacking up your new stuff. New tubes come with say two clips to bind them together. Mark where they go for reference, then pull them off.

I installed mine without the radiator, and grille and condenser for that matter, slid them in from the front. Routing should be at least somewhat apparent. Speaking for my drive train, they run along the oil pan's upper RH side into the engine compartment.

Install the tubes to the transmission loosely. They must screw on easily. Good idea to lube all four nuts with trans fluid.

My tubes retain with a clip mounted by the oil pan’s 3rd 5/16-18 bolt from the front on the RH side.

You'll see the tube nuts are in perfect position to screw onto the radiator. JK, you have to bend the ends kindly until they're PERFECTLY straight and screw into the radiator easily, by hand.

For new lines, you may find it easiest to hold the radiator up a pinch, in such a way to install a line into the radiator. So long as it all comes together as intended. Tighten up the tube nuts, do not over or under tighten them.

Install the bind clips, which reminds me, I haven't done that yet

[Steeveedee]

^ "You'll see the tube nuts are in perfect position to screw onto the radiator. JK, you have to bend the ends kindly until they're PERFECTLY straight and screw into the radiator easily, by hand."

This. It's called a stress-free installation. If an installation of any sort has stress at the start of its life, it will fail sooner than if it wasn't stressed.

[71CHEVYSHORTBED402]

Originally Posted by Steeveedee

^ "You'll see the tube nuts are in perfect position to screw onto the radiator. JK, you have to bend the ends kindly until they're PERFECTLY straight and screw into the radiator easily, by hand."

This. It's called a stress-free installation. If an installation of any sort has stress at the start of its life, it will fail sooner than if it wasn't stressed.

Right, and if you cross thread in any manner you're screwed, ask me how I know. Junk radiator didn't help, I was happy to throw it away.

[dmjlambert]

Thanks guys. 71CHEVYSHORTBED402, is that your truck with the shiny engine? It sure is pretty. Too clean though, I can't find my way around without underside grime to let me know what's what. :-)

[71CHEVYSHORTBED402]

Originally Posted by dmjlambert

Thanks guys. 71CHEVYSHORTBED402, is that your truck with the shiny engine? It sure is pretty. Too clean though, I can't find my way around without underside grime to let me know what's what. :-)

Yes. It kind of looks like this now, but I've cleaned it up since, added some decals and straps.

[dmjlambert]

That's crazy clean, super nice!

[71CHEVYSHORTBED402]

Originally Posted by dmjlambert

That's crazy clean, super nice!

Thank you, I'm real happy with it. And have to be careful. I'm drawn to look under the hood, but don't want to wear the hood springs out

[dmjlambert]

That is interesting. Do you have one of those manuals that you can show us the page from? I'm a believer and noticed the assembly manual shows the lines going in that fashion, but you have to follow the line closely and it is not very obvious. There is conflicting information on the web. It would be nice to put the subject to rest.

[71CHEVYSHORTBED402]

Page 414 of the assembly manual for a V8. Page 413 shows a V6 which is the same, though a V6 varies, because some show a radiator with both of its fittings near the bottom.

This is approx. how I found my 400TH, and it was reassembled the same. I say approx., only because the prior owner cut both lines to the radiator and installed hoses that connected to an auxiliary

Bottom trans. fitting to the radiator's top fitting is how I've always heard it. Sure it might be counter-intuitive, because hot fluid is often a top line. Still is, just happens to run from the transmission's bottom fitting.

[nightrider388]

I honestly don't think the transmission or cooler cares which way it goes through there. Its a heat exchanger and will be in there the same time no matter which way it flows.