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Old 05-19-2018, 02:02 PM   #1
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1984 27' Airstream 270
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Two TH400 questions

After I installed the new aluminum radiator, I noticed on the first trip the transmission fluid temp made it up to 300 deg. F.

The blue line is the transmission fluid line temperature and the top line is 300 Deg. F.

To avoid huge images here, I only put up thumbnails of the charts.
Full size charts are at this link:
http://theouterlimits.ws/projects/ai...tml#Trans-temp


It was mentioned that it could be transmission slippage causing the heat.


The first chart was the uphill run with 2K pounds in tow. The second is downhill without the 2K in tow.
The charts are ordered from highest speed to lowest. The black lines are the average RPMs from the red lines and the blue lines are the speed at that RPM.


The RPMs are definitely 250 to 300 higher under load in the first chart compared to the second chart.

Questions:
What is the normal transmission fluid temperature under load?
How much slippage is normal on a TH400 transmission under these conditions?
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Old 05-19-2018, 10:49 PM   #2
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Reposted From TCI Website

There's More To Coolers Than You Think:It's no secret that heat kills automatic transmissions. And in high-performance street-strip applications, the problem is especially acute. Small diameter torque converters coupled with stop-and-go traffic greatly increase the heat level in an automatic transmission. In most cases, the extra performance heat under the hood can have the same effect as heavy loads, trailer towing and desert conditions.

How hot is too hot? The ideal operating temperature for automatic transmission fluid is between 175 and 225 F. At approximately 240 F, important additives in the ATF begin to cook. The result is the formation of varnish inside the transmission. At approximately 260 F, internal transmission seals (which are typically manufactured from a polyacrylate material) begin to harden. The end results are leaks, both internal and external, simply because the seals lose their elasticity. At approximately 295 F, transmission clutch plates begin to slip because the oil is breaking down further. At approximately 315 F, seals and clutches effectively burn out. Carbon forms in the oil and for all intents and purposes, the transmission is junk. Just for your information, a typical transmission will die within 2000 miles if subjected to 300 F+ heat.

I have a deep pan on the TH475 with a temperature sending unit that reads the pan oil temp. I try to keep under the 225 deg. Unless long grades it stays under 200 deg. I also have a big trans cooler and also use the radiator cooling coil first then the separate cooler second. Also have larger lines from the transmission to the radiators.
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Old 05-20-2018, 01:31 AM   #3
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Thanks, that info has me be a bit more worried.
Maybe it is time to have someone take a look inside the transmission and maybe put in a new converter?
Looking back on other runs the transmission fluid did get high but seldom over 250.
My lines are 5/16 I wonder if 3/8 would be better for cooling if the new cooler is not good enough?

When I installed the cooler, I caught some of the fluid and noticed it was not the bright red like out of the bottle, more of a faded red with a bit of cloudiness. I saved a sample for Blackstone Labs.

Looking back through my records, a PO had replaced the transmission about 20K miles/ 20 years ago

Quote:
7/16/1998 84732 Replace transmission with rebuilt #401-23HDM by Shift Dynamics $1,737.00 $898.40
$2,560.23 Ritter Precision Automotive Gunnison, CO
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Old 05-20-2018, 09:51 PM   #4
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I also use Mobile 1 ATF. I have switched the lines to AN-8, witch is a 7/16 ID stainless braided line similar to PROFLEX HOSE. The deep pan is cast aluminum with fins on the bottom to add extra help to the cooling.
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Old 05-21-2018, 01:27 AM   #5
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Mine has the deep steel pan, over 2" high. I am going to do another test run up the same hill today. I wont be loaded heavy, but it will be closer to 80F outside. I will be monitoring Transmission fluid temperature in and out of the transmission.


I started looking into what may be the best transmission fluid of the TH400, and according to may opinions, modern Dexron VI is not the best for our older gear.
They recommend either finding Dexron III or Type F fluid.
The claim is that the newer fluid does not give enough "grip" on the plates.
Any thoughts on this?
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Old 05-22-2018, 12:01 AM   #6
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I have run Mobile atf for over 10 years on our 310. This was also in the time frame that I was trying to get the temps. down on the trans. In the over 10 year time frame, have not had any problems with any slip for trans issues. Take a look at a Mobile 1 site. I have run this in all my cars with no problems so far.
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