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On 1/12/2018 at 4:49 PM, AH64ID said:

212°  Is the max allowable temp with a 180° thermostat, from Cummins, so that seems high. 

 

Except our clutches are based on radiator temperature. That radiator needs to start passing heat past the clutch first. 

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Bingo. 3rd gen fan clutch is based off coolant temperature sensed by the ECT sensor. As long as the flow from the thermostat is low the fan will not lock on the 2nd gen. It will cool to fast before the fan senses enough heat to lock even partially on cold winter day.

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2 hours ago, pepsi71ocean said:

 

Except our clutches are based on radiator temperature. That radiator needs to start passing heat past the clutch first. 

 

Exactly, which is a benefit at low loads and why I said just cruising it likely won’t cycle the fan too much more,  but on a hot day where the a/c and intercooler are already heating the radiator up you might get some fan operation while empty.  

 

But when the motor is working hard the hotter the thermostat the hotter the radiator. The radiator is getting fed coolant at least 20° hotter than it was when the fan clutch engange temp was designed, and that’s going to make it engage more often any way you look at it. A partially open 200° thermostat can make for a hotter radiator than a mostly open 180° one.

 

Aside from fuel mileage it shouldn’t hurt anything having the fan work more, I’m simply pointy out that it will operate more often. 

 

Looking at the 1999 FSM the fan will engage when the radiator discharge temp is 160°-179° F. When the radiator is getting fed 180° coolant that will take a while to get to, but at 200° it wont take as long. A 180° thermostat should only be putting 200° coolant into the radiator at very high sustained loads, whereas a 200° thermostat doesn't put coolant any colder than 200° into the radiator. 

 

46 minutes ago, Mopar1973Man said:

 It will cool to fast before the fan senses enough heat to lock even partially on cold winter day.

 

We're talking about year round operation of a 200° thermostat. In the winter it likely won't ever lock on an empty truck without a winter front, but in the summer or with a winter front it might. 

 

I've read about guys, on this forum, having there fan come to life in the winter with a winter front on, and that was likely with a cooler thermostat which means a cooler engine bay. With a 200° engine it doesn't take much radiator heat to get the engine bay to hold 160-179°. 

 

With my MM3 I can watch all of the underhood temp sensors and even in the winter it doesn't take much to warm up that engine bay. As soon as the radiator cracks I get 20°+ warmer IAT's from the radiant heat of the radiator by the intercooler, and the intercooler is ahead of the radiator in the airflow. That's all without a load on the motor. 

Edited by AH64ID

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Never did find a warmer than 180f stat for my 12v first gen, so I swapped in a second gen stat housing and use the appropriate stat.

 

Here ya go -

 

1st gen 12v - 180f 54mm diameter stat (has a tapered seat bypass & 2 jiggle pins) Cummins p/n 5292738.

 

2nd gen 12v - 180f 63mm diameter stat (has a disc bypass & no jiggle pins- uses the one way check valve to purge air) Cummins p/n 3928499/Mopar 05014568AA.

 

2nd gen 24v - 190f 58mm diameter with disc bypass (I initially ground this down to 54mm for my first gen but it's too deep to fit inside the 12v head) Cummins p/n 3946849/Mopar 05015708AC.

 

I use a 63mm NOS Mopar # 3418459 in my second gen stat housing, its 195f, and is identical to the Cummins second gen stat in weight & construction, except that is doesn't have the bypass disc or bypass tapered seat.

 

It's commonly available replacement is NAPA 532090/Stant 13479.

 

I'm not concerned about the bypass function, as its only there to close & prevent flow through the bypass passage when the thermostat is open & provide 100% flow through the rad hoses.

 

The bypass hole is too small to make a difference to cooling flow whether closed or open; its primary function is to ensure a bit of coolant circulation when the thermostat is closed, to help avoid hot spots while warming up.

 

 

 

 

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Posted (edited)

I think the bypass is rather important, especially when you consider that these thermostats spend more time closed than open unless you tow all the time. 

 

Or is it a completatly different setup on a 12V?

 

 

Capture.PNG

Edited by AH64ID

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I believe we are saying the same thing in a slightly different way.

 

It's import to bypass during warmup - the NAPA/Mopar/Stant stat's lack of a bypass disc doesn't prevent the bypass flow - when the stat is closed, the bypass is open, and fulfills it's primary function of circulating a trickle coolant through the head. The engineers should have made the rear freeze plug divert as well, but that's easily fixed. On my setup, I divert the stagnant #5 & #6 coolant to my heater core inlet via a Keating Machine tapped freeze plug insert.

 

When towing, and the stat opens to regulate temperature, the bypass closes - this doesn't really cut down flow through the upper rad hose; I've heat gunned the difference back to back on the same day before & after the stats opened. The tiny bypass doesn't really have a chance at affecting a fully open stat's monster flow that these very stout water pumps facilitate.

 

I'm not concerned about the bypass function, as its only there to

close & prevent flow through the bypass passage when the thermostat is open & provide 100% flow through the rad hoses.

 

The bypass hole is too small to make a difference to cooling flow whether closed or open; its primary function is to ensure a bit of coolant circulation when the thermostat is closed, to help avoid hot spots while warming up.

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