Revmax 68RFE Transmission Cooler Thermostatic Bypass

[ShinerBock]

My re-builder recommended this upgrade when I had my custom trans built. I have been running it for several months now and finally had a chance to tow with it in. All I can say is wow. This is a must have mod for those who tow with their 68RFE even with a completely stock truck if you are seeing high trans temps.

Those that own a 68RFE know that it is notorious for running hotter than most other transmissions. I used to think that it was due to design until I replaced my thermostatic bypass valve with this Revmax one which is advertised to improve flow to the cooler by over 80% and decreases trans temps by as much as 50F. I can attest that it decreased the temps by as much as 30F empty and 20F towing in my truck.

The stock one purposely reduced flow to the cooler in order get the trans temps hotter quicker in colder weather. The problem is that many times it gets stuck or partially stuck in this closed position causing temps to spike inside the trans. Not only that, but the stock valve itself is a very restrictive bottleneck. My re-builder started recommending these bypass valves after seeing several of them fail causing the transmission to fail.

68RFE Transmission Cooler Thermostatic Bypass Upgrade

[cummins2014]

ShinerBock wrote:

cummins2014 wrote:


How close do you watch it ? Towing mine always seems to be in the 170's. But for $119 , why not ?. Can it hurt ???????

FYI, the install is very easy and takes about 10 minutes or less. Just unscrew the fittings on the trans side and the cooler side has c-clips. You will loose about a half a quart of ATF+4 so be sure to pick up a quart and a small enough funnel to add through the dipstick before install.

I am tempted ,although have not seen anything alarming , but I don't watch it all the time . I do towing slow thru town ,or mountain grades .

Got an Alaska trip planned starting first of June next year for three months , I am sure I will be pulling some pretty good grades with the fifth wheel on that trip. That Cummins will get a good workout 🙂

[C_Schomer]

My 12 had max tow with the welded plate exchanger and t-stat block and it was the worst example of oil cooling I ever saw. I got rid of the stat and welded plate earlier this year and used a fan powered Derale cooler and it is Muy Bueno. Craig

[ShinerBock]

IdaD wrote:


Sometimes I have the temps up on the EVIC in which case I keep a pretty close eye on it, other times I have the speedo up on the EVIC. I've towed with it 4+ years so I think I have a pretty good handle on how it behaves at this point. We're typically in the 19-20k lb combined range so I'm not working it as hard as a lot of people.

My brother was the same way until several weeks ago. Never got up that high when towing or even in park for the past few years he has owned it. It just started happening in the past few months.

He has a CTS2 monitor like I do and he has it set to alert him when temps get to 200F. He had the truck in park while hooking up his RV and he said that he heard the alarm go off about five minutes after putting it in park. Got to his truck and the trans was at 200F so he shut the truck off while hooking everything else up. It happened to him again while he was stuck in traffic a few weeks later.

He also said his temps were hotter than normal when towing as well which is why we were thinking that the thermostat in the bypass was broken and remained closed or partially closed. He was going to replace it with another stock unit, but didn't want to take the chance if it failing in a situation where he cannot easily replace it so for $60 more he went with the Revmax unit that has the thermostat deleted.

[IdaD]

cummins2014 wrote:

IdaD wrote:

ShinerBock wrote:

IdaD wrote:
What temps were you seeing when you were towing stock versus now? My truck is totally stock and I've never had problems with the trans overheating.

Mid 180F depending on outside temps. It is not uncommon to get up to 110F here in south Texas. It is also very intermittent too. Sometimes it will go passed 175F and towing the same trailer in the same conditions another day will never even get to 175F which leads me to believe I may have had a valve that was partially stuck every now and then. Now it stays in around 160F when towing.

Interesting. Mine seems to consistently run around 168 towing regardless of outside temperature.

How close do you watch it ? Towing mine always seems to be in the 170's. But for $119 , why not ?. Can it hurt ???????

Sometimes I have the temps up on the EVIC in which case I keep a pretty close eye on it, other times I have the speedo up on the EVIC. I've towed with it 4+ years so I think I have a pretty good handle on how it behaves at this point. We're typically in the 19-20k lb combined range so I'm not working it as hard as a lot of people.

[ShinerBock]

cummins2014 wrote:


How close do you watch it ? Towing mine always seems to be in the 170's. But for $119 , why not ?. Can it hurt ???????

FYI, the install is very easy and takes about 10 minutes or less. Just unscrew the fittings on the trans side and the cooler side has c-clips. You will loose about a half a quart of ATF+4 so be sure to pick up a quart and a small enough funnel to add through the dipstick before install.

[cummins2014]

IdaD wrote:

ShinerBock wrote:

IdaD wrote:
What temps were you seeing when you were towing stock versus now? My truck is totally stock and I've never had problems with the trans overheating.

Mid 180F depending on outside temps. It is not uncommon to get up to 110F here in south Texas. It is also very intermittent too. Sometimes it will go passed 175F and towing the same trailer in the same conditions another day will never even get to 175F which leads me to believe I may have had a valve that was partially stuck every now and then. Now it stays in around 160F when towing.

Interesting. Mine seems to consistently run around 168 towing regardless of outside temperature.

How close do you watch it ? Towing mine always seems to be in the 170's. But for $119 , why not ?. Can it hurt ???????

[BenK]

Personally never liked a by-pass valve on ATF aux radiators. More so if the ATF is also plumbed into the engine coolant radiator, where it normally has more thermal rejection capacity than an ATF AUX radiator

Let the main, engine, coolant radiator take care of the ATF rejection and in extreme cold ambient, heat it enough to vaporize any H2O that condensed into it

The two main sources of heat generation is in the TC (shearing of the fluid) and at the gear faces sliding against each other. Then the pumping & frictional losses

If designed correctly, the gear lube (in this case ATF) will have enough film strength to NOT be wiped off. Theoretically, there will never a metal to metal, but we live in the real world and there will be metal to metal occasionally or often since this is a towing forum. Plus, all lubricants degrade towards their thermal spec limits.

Meaning the shear and film strength lessens as their spec operating limits are approached or exceeded. Why I'll keep my ATF as cool as possible, but stay within it's spec operating temp range


Just referencing a max temp leaves out much data. What is your ATF temp where it starts to break down enough to create varnish? Smoke? These are not the ultimate spec limit for its thermal rating. That stuff happens below that published highest limit

[ShinerBock]

Bionic Man wrote:


Repeating myself, but I find it hard to believe that when EVERY modern vehicle I drive runs 200* normal operating and that is deemed as too hot. I can buy that with the older fluids but with the new synthetics I’m just not convinced.

But that is just it. Just because the sump is reading 200F(which is abnormally high unloaded in all the 68RFE trucks I have driven in the Texas heat) does not mean that it is 200F in hotter parts of the transmission. You may be reading 200F at the sump, but it may be 250-270F at the line going to the cooler. You can check it with a laser temp gun the if you have one.

Also, I do remember seeing this thread over at the Cummins forum where I guys said that his temp unloaded in a 68RFE was 190F and Transengineer stated that that was about 20F higher than normal for unloaded. He also stated that it might be getting that hot due to the thermal bypass valve being stuck.

So if you are seeing 200F unloaded in a cold state like Colorado, I would definitely check that valve. It may be fine when unloaded, but could cause the cooling system to have to work over time to keep it in check when loaded.

[Bionic_Man]

IdaD wrote:

Bionic Man wrote:
In particular, my experience with the 68RFE is that it will also run 200* in normal driving conditions. The person I hear significantly lower temps from also claims 100,000 miles out of a set of Michelins so they either have/had a real special truck or are full of it. Either way, those claims are suspect at best.

Mine consistently runs 168 towing or not. It will climb a bit in some circumstances, most dramatically when I'm climbing a forest service road grade in hot weather at lower speeds, but that's outside the norm. I assume at highway speeds the temp is held in check better because of good airflow. I've never seen it get up near 200 under any circumstances. I don't know if 2012 to 2015 makes a difference or not.

FWIW, I got 20k miles on my stock Firestones and will be lucky to get 35k on my BFG AT/KO2s. Maybe I need to get some of those Michelins you're taking about.

Interesting. Maybe I didn’t pay attention when my truck was newer but it seems that it has always been 200*+. If you look at Shiners original link, it is for a 2013 + 68RFE as they “run hotter than earlier models”.

Repeating myself, but I find it hard to believe that when EVERY modern vehicle I drive runs 200* normal operating and that is deemed as too hot. I can buy that with the older fluids but with the new synthetics I’m just not convinced.

I have no first hand experience running temps as cool as being reported and I drive multiple different cars every week.

[ShinerBock]

I will also say that the thermostat inside this valve(which is similar to a coolant thermostat) can fail or not open all the way like my brother's valve seem to do when we heated it up past 170F. There is no thermostat in the Revmax valve to fail just like there is no actuator on my fixed geometry S364/80 turbo to fail versus OE.

[IdaD]

Bionic Man wrote:
In particular, my experience with the 68RFE is that it will also run 200* in normal driving conditions. The person I hear significantly lower temps from also claims 100,000 miles out of a set of Michelins so they either have/had a real special truck or are full of it. Either way, those claims are suspect at best.

Mine consistently runs 168 towing or not. It will climb a bit in some circumstances, most dramatically when I'm climbing a forest service road grade in hot weather at lower speeds, but that's outside the norm. I assume at highway speeds the temp is held in check better because of good airflow. I've never seen it get up near 200 under any circumstances. I don't know if 2012 to 2015 makes a difference or not.

FWIW, I got 20k miles on my stock Firestones and will be lucky to get 35k on my BFG AT/KO2s. Maybe I need to get some of those Michelins you're taking about.

[ShinerBock]

Bionic Man wrote:

Shiner, can you link to a document that states expected or acceptable operating temps for the 68 (or any modern transmission)? I’m sincerely interested.

Turbo Diesel Register issue 90, pages 86-87 covers temps of the 68RFE.

There is also this article form ETE.... 68RFE Transmission Equipped Dodge Trucks

My main concern is not really what sump temps are, which is what the gauge reads, but rather other areas that get much hotter. I wish Ram would have put the sensor somewhere between the transmission and cooler since that would be a better place to read the highest temp. If the sump is reading 200F, then there is no doubt that this line is much hotter.

I can see how someone up north may not be that concerned since outside temps do not get that hot as it does here and most places that do get that hot have a rather low humidity(like the desert) which isn't so bad. However, with south Texas high temps and high humidity making 100F(which is normal here) feel much hotter, things start to snowball down hill once truck temps start to get hot because the cooler/radiators can only reduce the heat so many degrees from what the outside temperature is.

One way to combat this is to not let them get that hot in the first place which is what this valve is suppose to do. The stock valve purposely closes the loop to the cooler to purposely get the temps hotter quickly which is mainly for minimal fuel gains and/or to get keep the fluid hot in very cold climates from what TransEngineer has stated before. These trucks are generically made for many different conditions/situations and most people who live in areas where it hardly ever gets to 90F and if it does then it is a low 15% humidity will probably never need such a mod so this thread kind of does not apply to them.

[Huntindog]

Bionic Man wrote:
I looked everywhere I could to find normal/acceptable operating temp of the Dexron in my Yukon without success. Two GM shops told me that 250* was “to be expected” with what I was doing. When I added the new cooler and had the fluid changed two weeks ago the old transmission fluid that had been over 250* at least a couple of times looked fine.

In particular, my experience with the 68RFE is that it will also run 200* in normal driving conditions. The person I hear significantly lower temps from also claims 100,000 miles out of a set of Michelins so they either have/had a real special truck or are full of it. Either way, those claims are suspect at best.

Not saying running cooler isn’t better, but I’m also far from convinced that temps are a weak link in this particular transmission.

Shiner, can you link to a document that states expected or acceptable operating temps for the 68 (or any modern transmission)? I’m sincerely interested.

With synthetic fluid, higher temps such as 250 won't hurt the fluid... It will however hurt the seals in the tranny. length of time at high temps, and frequency have a bearing on this... But best to avoid it.

[Bionic_Man]

I looked everywhere I could to find normal/acceptable operating temp of the Dexron in my Yukon without success. Two GM shops told me that 250* was “to be expected” with what I was doing. When I added the new cooler and had the fluid changed two weeks ago the old transmission fluid that had been over 250* at least a couple of times looked fine.

In particular, my experience with the 68RFE is that it will also run 200* in normal driving conditions. The person I hear significantly lower temps from also claims 100,000 miles out of a set of Michelins so they either have/had a real special truck or are full of it. Either way, those claims are suspect at best.

Not saying running cooler isn’t better, but I’m also far from convinced that temps are a weak link in this particular transmission.

Shiner, can you link to a document that states expected or acceptable operating temps for the 68 (or any modern transmission)? I’m sincerely interested.