Keeping heat out of Th400

[lil catching fox]

I have a 1983 K10 shortbed with a 305. The truck originally had a 700R4 but the previous owner swapped it with a TH400 with a conversion kit from advanced adapters I believe. The kit came with a new mount and output shaft among other things.

Recently I decided to pull a 5000lb trailer and smoked the Th400 in it (I have my suspicions that the 400 had problems already and this was the tipping point), fortunately I found what I believe to be a TH475 that came out of a mobile home. My plan is to take the output shaft out of my 400 and put it in the 475 and run the 475. My concern is that with an underpowered engine it’s going to do the same thing. I’ve heard the 475 has lower first and second gear which might help. It does have a deeper pan which will help. What other steps should I take to keep this new transmission from over heating? Auxiliary trans cooler? Eventually I would like to swap the engine for a 350, but unfortunately I can’t make it work right now.

 

[legopnuematic]

What tire size and gears does your truck have?

Just the factory in radiator transmission cooler or any auxiliary cooler?

Pulling 5000lbs should not kill a TH400, or even a TH350 for that matter if in reasonable health. Was the engine really lugging/struggling?

An auxiliary trans cooler is never a bad idea, be sure flush or replace the transmission cooler lines and cooler(s) when you swap the transmission, not doing so will contaminate the new transmission with the old fluid.

A TH475 should have straight cut gears, but I believe the ratios are the same as a Th400. @NickTransmissions might be able to shed more information on the 475.

 

[CountKrunk]

What rpms where ya when you pulled 5,000 pounds?

And yes, dig into your rig and figure out your gearing and tire size. You can take those numbers to chat gpt or other calculators to calc rpm tow capacity etc.

+1 an aux trans cooler is never a bad idea. My truck was set up to tow a 5th wheel horse trailer and it has a aux trans cooler. I towed a golf cart on a double axle trailer with it and you wouldnt know if you didn't look behind you. Truck behaved the same, except was MUCH slower up a steep grade haha.

 

[TotalyHucked]

I'm gonna go out on a limb here and say there was something wrong with the trans, a TH400 shouldn't even break a sweat with that kinda light load. Also, since yours is a half ton, you need to figure out what gear ratios you have. If you have like 2.73s or 2.56s, any trans is gonna hate life. An aux cooler would definitely be on my list no matter what route you go. That's a good thing for all transmissions

 

[nvrenuf]

A TH475 should have straight cut gears, but I believe the ratios are the same as a TH400.

This ^^ is correct.

 

[xm20k]

All you would need to do is unbolt the yoke, take the tail housing off, then take a Dremel or bur and take these two rings off were the o ring goes, and a slip yoke will fit right on it without tearing both transmissions down. I've done this several times.

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[NickTransmissions]

I have a 1983 K10 shortbed with a 305. The truck originally had a 700R4 but the previous owner swapped it with a TH400 with a conversion kit from advanced adapters I believe. The kit came with a new mount and output shaft among other things.

Recently I decided to pull a 5000lb trailer and smoked the Th400 in it (I have my suspicions that the 400 had problems already and this was the tipping point), fortunately I found what I believe to be a TH475 that came out of a mobile home. My plan is to take the output shaft out of my 400 and put it in the 475 and run the 475. My concern is that with an underpowered engine it’s going to do the same thing. I’ve heard the 475 has lower first and second gear which might help. It does have a deeper pan which will help. What other steps should I take to keep this new transmission from over heating? Auxiliary trans cooler? Eventually I would like to swap the engine for a 350, but unfortunately I can’t make it work right now.

The 475s have the same ratios as the 400s, as @legopnuematic stated; only diff is the straight cut gear set. Sounds like the TH400 was on its last legs. 5k is nothing for those units. 700R4s and 4L60Es are rated for 5k towing/hauling. If you plan to take the output shaft out, you are rebuilding the transmission as literally everything has to come out of the case to get to it. Cutting the shaft at the rear splines isnt recommended. Plus the 475 shafts are journaled for roller bearings whereas the regular th400s are not.

 

[Matt69olds]

Since you are going to have both transmissions apart, definitely do the dual feed modifications and roller bearing at the back of the case. Drill the spacer plate, omit the spring from the 2-3 accumulator (if the accumulator piston is plastic, find a old school aluminum piston that matches what you currently have) use 2 horseshoe shims in the pressure regulator valve (with the stock blue spring) and reassemble. Pay attention to end play and clutch clearances.

I’m pretty sure Nick has covered these modifications in his thread. These will give a quick firm shift, without knocking the dash out of the truck. Perfect for a work vehicle.

 

[lil catching fox]

The truck wasn’t lugging, it was having trouble going up steep hills and it probably didn’t help that I was trying to push it faster than it should have been going as I was on the interstate with semis zooming by at 80 mph.

It has the radiator transmission cooler, but I’ve done research and sounds like typically people install aux tranny coolers in line with the radiator. From the sounds of it they do this to help warm tranny fluid in the winter and not “cool the tranny too much”, I don’t drive the truck that much in the winter so should I just do an aux cooler or aux cooler and radiator? I also want to add a temp sensor so I can monitor the temp of the transmission in the future. What’s a good auxiliary cooler?

I’m not sure what the gear ratio is I’m guessing still stock, probably 3.08. The truck itself has a 4in lift and sits on 33x12.50R15. It’s not exactly setup to pull with but figured it would hold up. Like @TotalyHucked said I’m pretty sure the tranny was on its last leg. This was its first and only time since I’ve owned it that I’ve pulled something, I’ve really only used it as a cruiser around town.

Unfortunately I don’t think modifying the 475 output shaft will work as I believe the 400 is 32 spline and the original transfer case in the truck (which I believe also needs rebuilt as the speedometer no longer works) is a 27 spline. When they installed the 700R4 conversion kit they installed a new output shaft in the 400 that fits up to the factory transfer case.

 

[Rustisbest]

I run this cooler mounted to the core support and some brackets made out of aluminum angle. Plumbed in after running through the radiator. Dont have a temp gauge but shooting the cast aluminum pan with a temp gun it stays around 150 degrees. I'm guessing the fluid is probably 10-15 degrees hotter than that.

https://www.summitracing.com/parts/...wad84ZLQUB0mz5fpI0_ei4JfzAhkKGVAaAr3OEALw_wcB

https://www.haydenauto.com/media/10981/8p468_en_ada.pdf

 

[HotRodPC]

What tire size and gears does your truck have?

Just the factory in radiator transmission cooler or any auxiliary cooler?

Pulling 5000lbs should not kill a TH400, or even a TH350 for that matter if in reasonable health. Was the engine really lugging/struggling?

An auxiliary trans cooler is never a bad idea, be sure flush or replace the transmission cooler lines and cooler(s) when you swap the transmission, not doing so will contaminate the new transmission with the old fluid.

A TH475 should have straight cut gears, but I believe the ratios are the same as a Th400. @NickTransmissions might be able to shed more information on the 475.

Yes, the gear ratios are the same and the straight cut planetaries in the back is about the only difference. When you mobile home, I'll assume you mean motor home or RV? This transmission doesn't have the Park Drum brake on the back of it?

 

[fast 99]

Heat is the killer of automatics. We had a couple of them in SB tow trucks. Trannys lasted about 100k between rebuilds. If your not familiar the trucks alone weighed around 8500lbs. Add a normal sized vehicle GVW could easily exceed 12k.

I installed 2 aux coolers on one truck with a temp gauge. Pulling hills the oil temp climbs rapidly. Depending on several factors a 50 degree increase from 150 was normal. Unless in a very cold climate don't believe adding one or more coolers could ever hurt durability.

 

[CountKrunk]

To clarify, my trans aux cooler is in line with the radiator. So double cooled. Sorta.

 

[xm20k]

My Bad didn't realize you were dealing with a conversion shaft.

Well, any ways anyone who needs a quick and dirty way to mount either a slip yoke or a transfer case that takes a 400 now knows a way to get around tearing it apart. There is also a yoke that fits over that without the O-ring but I've only ever used a standard yoke and just ground the rings down so it would slide over that section and bottom out in the back of the yoke. Did it once for a trans case that took the 400 shaft but that's been years ago.

 

[HotRodPC]

Yes, Heat for sure is the #1 killer of auto transmissions. 2nd is abuse, and I do mean abuse, not working it hard. I see stupid idiots doing Reverse roll burnouts. That's ABUSE !!! Waiting to 150,000 miles for a fluid and filter change, that's ABUSE.