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Thread: A0 3116 coolant flow diagram?

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    Default A0 3116 coolant flow diagram?

    Does anyone have a coolant flow diagram or know where to find one? TIA

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    4 Star General Guyfang's Avatar
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    What is a AO 3116?

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    4 Star General coachgeo's Avatar
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    Quote Originally Posted by Guyfang View Post
    What is a AO 3116?
    a 3116 CAT found in an A0 (pre A1) FMTV
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    I will see if I can find one, I don't think there is a very simple one in the manual, I think I pieced it together from several drawings and just looking at the system. It is easy enough to describe though.

    Warm coolant exits the head into the thermostat block. This engine uses a bypass thermostat so when cold the coolant exits the bottom of the thermostat block and runs across the front of the engine then down in front of the air compressor to the transmission oil cooler(on a civi 3116 it would go right back into the water pump). It flows thru the trans cooler and returns to the water pump inlet. There are 2 additional lines off of the thermostat block, one line runs to the cabin heater the other line runs to the coolant expansion tank. Both of these devices have return lines back to the water pump inlet. The water pump outputs its coolant directly into the front of the engine block and thru the internal oil cooler, it then flows back thru the block and up into the head where it collects heat and exits back into the thermostat block.

    When the engine gets warm enough to open the thermostat, it diverts the bypass flow from the transmission cooler and sends all the coolant thru a large pipe to the top of the radiator. Coolant passes down thru the rad where it is cooled and exits at the bottom. From there it passes back into the transmission oil cooler and on to the water pump inlet.

    There is one additional line from the top of the radiator to the coolant expansion tank to purge air to the tank/highest point...

    The bypass line down to the transmission cooler is why these beasts are so cold blooded. On a normal cat the warm bypass coolant is recirculated right back into the top of the water pump and back thru the engine. They keep recircing the coolant untill it is warm enough to open the thermostat so they warm up much easier. You are basically robbing all the engine heat to warm the transmission, until you can make enough heat to overcome all that loss...
    Last edited by Ronmar; 05-16-2019 at 14:47.

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    I missed a loop... There is a 3rd line off of the thermostat block that runs to the air compressor head and back to the water pump inlet...

    I also Have not come across a single cooling system diagram. If you look at the cooling system parts in the 24P manual and read thru chapter 6 in the -20 manual, you can get a pretty good idea of how it works. Thats how I learned it
    Last edited by Ronmar; 05-15-2019 at 21:04.

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    The system shunts coolant to the lower rad piping/trans cooler in a stat closed situation to have a + coolant flow around the trans cooler so the trans doesn't overheat. Warm climates not so much, but cooler climates it is possible to have the stat closed most of the time and in turn, overheat the trans. This isn't unique to the FMTV, it is common in most diesel engines. There is also a diverter disc somewhere in the system in the block loop that meters and forces the coolant into the lower radiator piping with the stat closed.
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    That disc is part of the thermostat and interlinked with the main thermostat disc. As the main thermostat disc progressively opens the main port to the radiator, the bypass disc progressively closes the bypass port to the trans cooler. This design combines the two systems under control of the single engine thermostat. Not a bad idea, but when cold, the transmission acts more like a radiator until it is worked hard enough to become a heat source. Warms the trans oil, but prolongs the cold idle for the diesel... The cold warm-up procedure has you put the trans in gear to help turn it into a heat source...

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