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HI guys...I'm new here. I have a 2000 f250 7.3/4R100. My stock 7.3 trans cooler was not keeping up towing my 10K trailer through desert and climbing up and over a mountain pass in SoCal. The bipass on the side of the trans was not functioning properly either. I deleted the bipass and installed a mishimoto trans cooler. My trans temps run from 100-150 now depending on ambient temps. When it's cold outside and I'm on the flat or downhill it will drop to 100* or even a little below sometimes...even with a load. Driving around town with no load it's always around 100*. My question is, is that too low? Will it damage the trans and should I install an aftermarket thermostatic bipass? Improved racing makes a really nice one for around $200. I also have the Mishimoto radiator that has the trans cooler built in but I am not using it. Will running the trans fluid through the radiator bring the temps up a little around town?


thanks,
~john
 

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It should be fine. Mine used to be about 180 hauling until I added a tru cool in line with the stock one. This time of year running empty my gauge doesnt even move. Mine has been like this for about 8 years now with no issues.

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Discussion Starter #3
It should be fine. Mine used to be about 180 hauling until I added a tru cool in line with the stock one. This time of year running empty my gauge doesnt even move. Mine has been like this for about 8 years now with no issues.

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Thanks Kyle!
 

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The bipass on the side of the trans was not functioning properly either. I deleted the bipass and installed a mishimoto trans cooler.
I recommend rebuilding the bypass (Summit has a kit for around $25) and reinstalling it.

My question is, is that too low? Will it damage the trans and should I install an aftermarket thermostatic bipass? Improved racing makes a really nice one for around $200.
I've seen some thermostats that are restrictive and limit high load cooling. If this one isn't restrictive it would be a good add.

The problem with running too low is that moisture can accumulate in the trans. That can cause rust in the trans. Even worse, if there is moisture in the trans the next time it gets above 212°F it can flash to steam. Suddenly the transmission will be greatly overfilled as the steam expands. This can force a lot of fluid out of the vent and the dipstick tube. I've seen a vehicle burn to the ground when this happened.

I also have the Mishimoto radiator that has the trans cooler built in but I am not using it. Will running the trans fluid through the radiator bring the temps up a little around town?
You should use the cooler in the radiator. While it will lower your temps in ALL conditions, it will really lower them in high load situations. It will NEVER warm the ATF.
 

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IMHO, we all think we know more than the Ford engineers who designed the tranny, but I would suggest that the temperature controlled bypass on the 4R100 serves a purpose otherwise they would not have included it. If you get into cold weather (even northern CA and AZ can be very cold in the winter) the bypass ensures that the tranny fluid will always have a specific minimum temperature. Any external transcooler tries to limit the maximum oil temperature and if large enough will tend to force the fluid temperature to be equal to the ambient temperature. I tow a fair amount and have been in the CA and AZ deserts in summer as well as ID, MN and WY in the winter.
 

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I don't think that is correct, I think the bypass is only there in case of a restriction in the cooling circuit in which case the bypass still allows fluid to flow instead of destroying the transmission. I don't believe it is temperature controlled at all.
 

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Here is a quote from our friends over at Dieselsite: "From 1999-2003 the Ford 7.3L Automatic Transmission (4R100) had an external cooler bypass tube that redirected transmission fluid from the pump output back to the return line of the transmission. The bypassing of the cooler was supposed to happen until the fluid reached operating temps. Unfortunately, these bypass tubes commonly fail preventing transmission fluid flow to the cooler. Another design flaw is even when operating perfectly, the bypass never fully closes, meaning not all fluid is ever sent to the cooler, ever."
 

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It is not thermostaticslly controlled. It is simply a circuit that redirects fluid when theres a blockage in the cooler.

Look inside of one, theres no thermostat or anything, how could it?
 

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You may be correct as here is the description by ATSG: "COOLER BYPASS VALVE - Similar to the Cooler Bypass Valve on the E4OD that provides lubrication to the transmission in case of blocked or partially blocked coolers. We have given you OEM part numbers for both and both bypass valves are illustrated in Figure 7." There sure seems to be some confusion with all of the aftermarket delete kits!
 

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Project Shamu
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Yeah, E4OD/4R100's have been out for 26 years and yet people still dont fully understand them...
 

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Right! If you look at the Sonnax illustration it is obvious the flow through the bypass tube will only occur if the cooler line is blocked and the pressure exceeds the spring pressure.

FYI, I run a Royal Purple oil filter in series with the large cooler from a 6.0.
 
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