Looking for fluid input for my rebuilt 727 - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 12 (permalink) Old 03-31-2013, 02:45 PM Thread Starter
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Looking for fluid input for my rebuilt 727

I am going to rebuild the 727 in my 72 D200 real soon. I use to just grab a middle quality fluid off the shelf for my servicings. But now, there are a metric butt ton of fluid options. I spent a lot of money getting good parts for this tranny and I want them to last. I do not want to spend money just for the sake of spending it though. So, I wanted to get some input from others who have some experience with these trannies and the different fluid options out there.
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post #2 of 12 (permalink) Old 03-31-2013, 07:06 PM Thread Starter
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Oh, I have a deeper than stock pan and an external spin on filter. So I am packing a lot more fluid this time around.
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post #3 of 12 (permalink) Old 03-31-2013, 08:19 PM
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Oh, I have a deeper than stock pan and an external spin on filter. So I am packing a lot more fluid this time around.
I've had a lot of 727's through the years and never have used anything but Dexron, or Dexron II in them. The largest thing to making that trans last is keep it serviced, bands adjusted, and cool via an external coil.
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post #4 of 12 (permalink) Old 03-31-2013, 08:37 PM Thread Starter
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That is the other things I added. Thsi truck did not have an external cooler other than the one in the radiator.
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post #5 of 12 (permalink) Old 03-31-2013, 09:55 PM
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That is the other things I added. Thsi truck did not have an external cooler other than the one in the radiator.
I've always done away with the one in the radiator when using an external cooler. The fluid movement throught the transmission builds heat and the radiator is removing heat from the engine. No need to blend the two together so let each cooler do it's respective job.
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post #6 of 12 (permalink) Old 03-31-2013, 10:00 PM Thread Starter
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I have heard good arguments on both side when it comes to using rad coolers. I am on the fence though.
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post #7 of 12 (permalink) Old 03-31-2013, 10:04 PM
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I have heard good arguments on both side when it comes to using rad coolers. I am on the fence though.

Where you live there is no reason to put the engine heat into the transmission when cold. The UP of Michigan, (for instance) it's a different story.
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post #8 of 12 (permalink) Old 03-31-2013, 10:38 PM Thread Starter
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Now I understand the different arguments. I did not think of location and climate.
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post #9 of 12 (permalink) Old 03-31-2013, 11:10 PM
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Now I understand the different arguments. I did not think of location and climate.
Heat is the greatest enemy of any fluid based hydraulic system. I feel the OEM's have used the radiator cooling method rather than a separate cooler as both a cost saving measure, and a way to warm the fluid to operating temperature must faster in cold climates. A transmission cooler coil in a radiator is usually a single pass of very little dwell time though the coil resulting in very little heat rejection as compared to a coil operating with ram air affect over it. If you think about it, the input to a radiator mounted automatic transmission cooler is in nearer the top than bottom of the radiator core. This is because the coolant returning from the engine is hotter in the top than bottom due to cooling air flowing across the core. The transmission fluid enters the coil, makes a sharp downward turn to near the bottom, then exits the core through what is typically much cooler fluid to be recirculated to the engine. Some heat of the transmission fluid is rejected by this method but not near as much as a small separate cooler coil for the transmission will provide. There are mechanical thermostat operated bypass valves that can be used also to bypass the coil and return the fluid to the transmission unit the thermostat is satisfied also. These allow the transmission to come to operating temperature very quickly by not allowing any external cooling and then open allowing fluid to run through the coil once satisfied. Kind of expensive up front for passenger vehicle use however.
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post #10 of 12 (permalink) Old 03-31-2013, 11:30 PM Thread Starter
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That is a very good explanation of how it works. It has me rethinking a few things now.
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