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Old 08-05-2010
gcyeaw's Avatar
gcyeaw gcyeaw is offline
Join Date: Dec 2007
Location: Ridgewood
Posts: 8,876
Default Transmission cooler

On my FC BB installed a transmission heat exchanger between the lower radiator hose and the water pump. It is a heat exchanger separate from the radiator, in-line with the lower 2 inch hose. The tranny cooling section of the radiator is not used and the connections are plugged.

It has been suggested that I route the transmission fluid through the radiator heat exchanger section as well to better cool the tranny. For the present I don;t intend to make any changes, but down the road it might be a good thing to do?

I guess there is a give and take balance between the engine cooling and tranny cooling. Can I lower the tranny temp without raising the engine temp? I don't know the BTU load of the transmission compared to the engine, so I don't have a way of calculating the final result.

I am sure there is a lot of good information and experience out there, lets hear your opinion/experience on this subject.
Gardner Yeaw
86 PT-36 Golden Memories
78 FC 33 Happinest (Sold)
Ridgewood, New Jersey
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Old 08-05-2010
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warbucks13477 warbucks13477 is offline
Join Date: Jun 2008
Location: Vernon Center
Posts: 1,302

Gardener the rule of thumb is to always either put the tranny cooler totally separate from the radiator or in parallel with it. If you read other explanations on the internet they will tell you to put the cooler in series with the radiator cooler but they are not operating under the same conditions as you and I in the winter and summer. Putting it in line with the radiator could possibly subject it to to cool a temp in cold weather and to hot in the summer. The optimim temp for most trannys is 160 degrees for best operation and longevity.

My choice would be to keep the tranny cooler separate from the radiator and ensure you get a cooler actually built for the allison tranny. There are lots of them. http://tinyurl.com/29zm5tl

I think you are asking for trouble plumbing your tranny like you are doing but again that is only my preference. Some day I am going to install a separate allison cooler from the site I fave and plumb it separate. Good luck in any case with it
Tom Warner
Vernon Center,NY
1985 PT40 SOLD
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Old 08-05-2010
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rwoody rwoody is offline
Join Date: Dec 2007
Location: Pleasanton
Posts: 5,611

every us car built with v/8 auto has rad cooler in rad......

???????????? are they all wrong or am i reading this wrong>

my sp has he fire truck rad and tran cooler in 1 unit --turn on fan trans and rad cools off .
2003 LX Single Slide Hauling Wood 5
2 miles east from Texas Highway 37-97
Just a Hobby Shop now-Bird Repairs upon Appt and my Schedule -USAR Retired
Pleasanton Tx 78064
Do not use PM... please use

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Old 08-05-2010
Stephen Stephen is offline
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Join Date: Dec 2007
Location: Niagara Falls
Posts: 3,346

Give Mike Hohnstein a call and ask his advice he has been rebuilding and building Transmissions for a long time and is probably the most knowledgeable member on this forum on this matter
Have a Great Day
Stephen Birtles

Email stephen@dupreeproducts.com
Telephone 888 668 4288
cell 716 220 1267
Please use email or telephone to contact
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Old 08-05-2010
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Mike Hohnstein Mike Hohnstein is online now
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Join Date: Jan 2008
Location: NW Milwaukee Metro Area
Posts: 4,449

Thanks Cape. I like the temp monitored in the sump. Heat exchanger in Gards bus is no different from same located in radiator tank, other than there is a good chance it's more efficient. My FC is radiator cooled with a large air to air cooler located next to the generator. Keeps trans cool and oil red. On that subject I prefer Dextron, not a fan of syn oil due to budgetary concernes.
Mike Hohnstein
FMCA 97824
Germantown, WI/Myrtle ID

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Old 08-05-2010
Posts: n/a


I'd start with the assumption that your coolant system is engineered to handle the cooling needs of your engine and that it's in good working order, which means it has some factor of overhead beyond what's required for the engine. Most designs allow for some clogging of the radiator both on the air side and on the fluid side while still sinking the heat power required. I'd also assume that your Telma braking system handles its own heat sinking needs and doesn't appreciably add to the heat build up in your transmission.

So where is the heat coming from. Your engine has it's requirements which are well spelled out by the manufacturer. Maybe 40% of the fuel energy content is converted into mechanical energy, somewhere around 30% is sunk by the exhaust system, 25% is sunk by the cooling system, and 5% is sunk by other means (oil system, radiation, etc). What you're proposing is to sink some additional mechanical energy into the cooling system. Can the cooling system handle the additional heat power load? The most likely time when the transmission temps rise is when ascending a grade, and unfortunately this is exactly when the engine needs all the cooling it can get. So if my assumptions are correct, then your cooling system should be capable of sinking some small additional amount of heat power. If not, then while ascending a hill, do you find yourself backing off cause your engine temps are rising? If so, then I don't think I'd ask the cooling system to do more work until the engine needs are met (rodding, new core, etc). If you find yourself backing off cause your transmission temps are rising, then adding addition cooling power via the radiator seems reasonable.

Last edited by davidmbrady; 08-06-2010 at 07:12 AM. Reason: Fixed grammar.
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Old 08-10-2010
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maustin maustin is offline
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Join Date: Dec 2008
Location: Redmond
Posts: 233

I just got back from a 900 mile trip through western Washington with 5000 +/- in tow. I too am concerned about transmission temps and have chimed in on several threads.

After studying the Allison manual and operating the bus on several mountain passes, I'm not convinced that there is anything wrong with high temperature readings on the trans temp gauge. That gauge measures the fluid temp as it is leaving the torque converter and when the converter isn't locked it creates a LOT of heat.

I took Mr. Hohnsteins advice and installed a seperate gauge in the pan of the transmission. This gauge has never exceeded 220 +/- a little even on long 2nd gear pulls when the dash gauge was showing somewhere between the 230 and 320 graduations, which are very close together making it hard to pinpoint the true temperature. The Allison book calls out a maximum of 300' out of the converter. On these pulls the engine coolant remained below 195 with the fan on.

When replacing all engine hoses this spring, I inspected the coolant side of the transmission cooler and found it very clean, I think the heat transfer is as good as it can be. I would hate to see a guy spend a bunch of money to try and fix something that isn't broken. If you have a substantial difference 30-50 degrees between the leaving and return temps of your transmission fluid, I'm inclined to think there is nothing wrong with the cooler.

My .02
M. Austin
Redmond, OR
1982 FC33
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Old 08-10-2010
Jim Harvie Jim Harvie is offline
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Join Date: Jul 2009
Location: Lancaster
Posts: 726

When I made the nozzles for my "Mister", I had transmission cooling in mind, as well.
Rather than have nozzles mounted at the top. I used 5/16 brake line, and zig zaged it inbetween the flats of the grill, and bored holes all the way down, so the water is as cold at the bottom, as it is at the top, and will cool down the Transmission cooler, as well.
Using a short piece of rubber hose, mounted next to the windshield washer hose, it remains mounted to the grill, and is connected with the rubber hose.
Jim Harvie
Lancaster MA.
1982 35 FT. FC, SB
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Old 08-10-2010
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Ernest Ekberg Ernest Ekberg is offline
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Join Date: May 2009
Location: Weatherford, Tx
Posts: 5,940

my transmission cooler on my FC was mounted under forward compartment, curb side.
Ernie Ekberg
Prevost Liberty 45 XL
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Old 08-10-2010
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Randy Dupree Randy Dupree is offline
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Join Date: Dec 2007
Location: Port St Joe,Fl
Posts: 35,908

i always wondered about running the drain hoses from the roof top AC units down onto the radiator on the pushers,free water from the sky.
email me at randy@randydupree.com only.

Randy Dupree
2000 LXI 43
Port St Joe Fl.

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