8HP70 Transmission Heater/Thermal Bypass Valve

tsielski · Jul 25, 2017

The transmission heater has come up on several threads over the past few months, particularly by those who feel the 185 degree and higher transmission temperatures are too high. I'm one of them. Without gaining inside engineering data from FCA, it unfortunately will not be possible to find out what the quantifiable efficiency gains FCA realized by heating the transmission fluid, and keeping it hot, versus letting the trans cooler do its job and dropping temperatures to the 150 - 160 range (the range the transmission in my 2011 3500 Cummins saw all the time).

Anyway, I managed to acquire the transmission heater assembly ( Mopar P/N 52014750AD), and started to examine it's operation. The thermal bypass valve (TBV) is located on the lower left side of the assembly (right above the lower mounting bolt), and is held in place by a retaining ring. Two o-ringed tubes provide for ATF flow into and out of the heater. ATF from the trans enters the heater via the lower tube, is simultaneously heated by engine coolant, and also is routed out to the ATF cooler. The TBV lets heated ATF flow back into the trans via the upper tube, and blocks the cooled ATF from flowing back into the transmission.

When the fluid heats to ~180 degrees, the TBV begins to block the flow of heated fluid, and opens to let "cooled" fluid return to the trans. If the fluid cools to less than 180, the TBV again blocks the flow of cooled fluid, letting only heated fluid return. It appears that once everything is heated up, the TBV is letting a combination of heated fluid and "cooled" fluid return to the transmission, the percentage of each determined by the fluid temperature.

So how to disable the TBV? Simply removing the thermostat part of it and replacing the plastic, o-ringed cap would let heated and cooled fluid mix and return to the trans. Not sure that the end result of that would be much different than when the trans is fully heated and the TBV lets a varying mix of heated and cooled fluid return.

Even if the engine coolant lines were disconnected from the assembly (and connected to each other), fluid heated by normal transmission operation would still mix with the cooled fluid. So far I can't come up with a way to remove the thermostat part and block the flow of heated fluid without making that modification permanent , such as driving a precisely sized drift into that portion of the bore to block the return flow of heated fluid. I am still investigating.

Sorry for the long post.

AFMoulton · Jul 25, 2017

Holy crap that's awesome. Could you post some pics maybe? I have some Hydraulic buddies in the AF that are fluid masters and might be able to help.

Sent from my iPhone using Tapatalk

Tach_tech · Jul 25, 2017

There's also the three way coolant valve that's tied in with the transmission. That's another thing to think about that would have to be bypassed/disabled.

Honestly I don't see much of a benefit to try to bypass the system based on some feel it's to hot. The ZF 8speed has been around for awhile now and has shown to be a very reliable and robust transmission. If the transmission has been designed to run at that operating range I don't see an issue. It also does not use conventional ATF like most people are used to, like your Cummins had. Conventional ATF would absolutely have issues operating at temps that the 8 speed does. As an FCA tech until I havent seen/heard of any common failures due to overheating, and I don't really see this as an issue.

You'll also have to remember modifying or bypassing any of it is going to likely void the warranty on the transmission.

Burla · Jul 25, 2017

Very interesting you figured that out, Kudos. Transmission fluid has to be changed so infrequently it pays to put ester based fluid in the tranny, it is like having 60F in the bank. Having said that, I don't know that 180f is a problem, see chart but remember the chart doesn't account for PAO or ester based oils. You can subtract 60 from the number your running at in temperature, and that in effect is how the fluid is "behaving'. Even if you are using PAO, subtract only 30f to be safe, and see you are way under 180F. And for the record it seams like the 8 speed is about the best transmission for fluid I think I've seen. Despite the fluid being thinner then ever, even at 100k miles the fluid is still in good shape in the 8 speed. Look around the forum on guys who are now changing their fluid. It is interesting on the demands of the spec to qualify to be an oil in the 8 speed. Whatever fluid meets the spec, is a true synthetic base, whether it is POA/Group 3 or esters. I don't even know that a mineral oil could meet the spec, which means you worries about degradation of fluid at 180F would be a non issue.

Basically to qualify as an oil used in the 8 speed, the oil cannot shear down much. If the oil shears down like the oils you can use in the 6 speed, the oil will not spec out for the 8 speed. So if the oil doesn't shear down in the normal operation of the 8 speed, then 180 degrees is nothing. It is like they built a transmission around an oil, as opposed to building an oil around a transmission. The only way to have a fluid start at that low of viscosity, and stay in that viscosity rating, it has to be of the highest quality base oil. I'm just trying to set your mind at ease, these 8 speeds are easy on the oil that meets the spec. The factory oil and replacements that are 25 bucks a quart are PAO, or you can opt for Redline d6 which is a better oil as in it wont drop to 5.5 visc, and as a bonus is much cheaper. Here is what redline says about the spec, so no john doe oil is going to work.

Synthetic D6 ATF is a lower viscosity version of the D4ATF and is designed for better fuel efficiency in CAFE testing... Requires a different approach to a conventional ATF formulation. Rather than beginning with a 7.5 cSt fluid and allowing a viscosity loss in use to drop to 5.5 cSt, the fluid requires a starting viscosity of less than 6.4 and a final drop to no less than 5.5 cSt. Red Line D6 will drop to no less than 6.1 cSt. ...The balanced frictional characteristics provide smooth and consistent shifts for extended drain intervals. The superior stability compared to petroleum ATFs allows high-temperature operation without varnishing valves and clutches which leads to transmission failure.

Mopar ZF atf or Redline D6 are no joke, two of the best transmission oils ever made. One PAO base, one ester base, neither will be an issue at 180F. That is why Ram doesn't even recommend you change it, they say it is a lifetime fluid.

tsielski · Jul 25, 2017

Understand everyone's comments.
There are members on this forum who have contacted ZF specifically about the temperatures we routinely see. The responses which were relayed that I have read were extremely non-committal. The ZF rep spoken to, as one forum member relayed, also took no responsibility for the "end user designed, thermal management devices", and refused to comment on their efficacy. Kind of raises a red flag, at least to me.

The higher temperatures the 8-speed has been forced to operate at by FCA, at least, has been done in FCA's words, in the name of efficiency, or increased fuel mileage. How much of a mileage improvement by running @ 185 degrees and higher, vs say 160 degrees is unknown. Clearly the fluid can cope with those temps. How much of a TBO (time before overhaul) hit do those higher temperatures cause to the transmission itself is also unknown. There's no question about the ZF 8-speed being a great transmission. Would be nice to get the most from it before it needs clutch packs & solenoids.

BTW, the three way coolant valve wouldn't need to be touched. That valve directs coolant to the trans heater, or to cabin heat, based on a number of variables. Connecting the coolant lines to the trans heater to each other, would simply take the trans heater out of that circuit and cause no issues with coolant flow IMO.

Wild one · Jul 25, 2017

Do the other car manufactures that use the 8sp ZF heat the oil in their version of the tranny.I think BMW and Mercedes use the ZF,and I wonder how they do their coolers.

shaggn85 · Jul 12, 2018

tsielski said:
The transmission heater has come up on several threads over the past few months, particularly by those who feel the 185 degree and higher transmission temperatures are too high. I'm one of them. Without gaining inside engineering data from FCA, it unfortunately will not be possible to find out what the quantifiable efficiency gains FCA realized by heating the transmission fluid, and keeping it hot, versus letting the trans cooler do its job and dropping temperatures to the 150 - 160 range (the range the transmission in my 2011 3500 Cummins saw all the time).

Anyway, I managed to acquire the transmission heater assembly ( Mopar P/N 52014750AD), and started to examine it's operation. The thermal bypass valve (TBV) is located on the lower left side of the assembly (right above the lower mounting bolt), and is held in place by a retaining ring. Two o-ringed tubes provide for ATF flow into and out of the heater. ATF from the trans enters the heater via the lower tube, is simultaneously heated by engine coolant, and also is routed out to the ATF cooler. The TBV lets heated ATF flow back into the trans via the upper tube, and blocks the cooled ATF from flowing back into the transmission.

When the fluid heats to ~180 degrees, the TBV begins to block the flow of heated fluid, and opens to let "cooled" fluid return to the trans. If the fluid cools to less than 180, the TBV again blocks the flow of cooled fluid, letting only heated fluid return. It appears that once everything is heated up, the TBV is letting a combination of heated fluid and "cooled" fluid return to the transmission, the percentage of each determined by the fluid temperature.

So how to disable the TBV? Simply removing the thermostat part of it and replacing the plastic, o-ringed cap would let heated and cooled fluid mix and return to the trans. Not sure that the end result of that would be much different than when the trans is fully heated and the TBV lets a varying mix of heated and cooled fluid return.

Even if the engine coolant lines were disconnected from the assembly (and connected to each other), fluid heated by normal transmission operation would still mix with the cooled fluid. So far I can't come up with a way to remove the thermostat part and block the flow of heated fluid without making that modification permanent , such as driving a precisely sized drift into that portion of the bore to block the return flow of heated fluid. I am still investigating.

Sorry for the long post.

thanks for the info on the bypass valve. very helpful...

one thing i would say though is that your assessment of the proper operating temp for this transmission is problematic at best. This is NOT the same as your cummins trans and this unit has a stellar reputation and does not fail often.

this is all new technology with newer materials. you are just stating you dont like the temp with no factual information behind ur statement.

i caution anyone thinking that removing this valve or eliminating the thermal management fuctionality is asking for trouble. ZF is not stupid and they know a lot more about these units than probably all of us.

quit looking for a problem to the solution you found .

shaggn85 · Jul 12, 2018

tsielski said:
Understand everyone's comments.
There are members on this forum who have contacted ZF specifically about the temperatures we routinely see. The responses which were relayed that I have read were extremely non-committal. The ZF rep spoken to, as one forum member relayed, also took no responsibility for the "end user designed, thermal management devices", and refused to comment on their efficacy. Kind of raises a red flag, at least to me.

The higher temperatures the 8-speed has been forced to operate at by FCA, at least, has been done in FCA's words, in the name of efficiency, or increased fuel mileage. How much of a mileage improvement by running @ 185 degrees and higher, vs say 160 degrees is unknown. Clearly the fluid can cope with those temps. How much of a TBO (time before overhaul) hit do those higher temperatures cause to the transmission itself is also unknown. There's no question about the ZF 8-speed being a great transmission. Would be nice to get the most from it before it needs clutch packs & solenoids.

BTW, the three way coolant valve wouldn't need to be touched. That valve directs coolant to the trans heater, or to cabin heat, based on a number of variables. Connecting the coolant lines to the trans heater to each other, would simply take the trans heater out of that circuit and cause no issues with coolant flow IMO.

ZF is likely under contractual obligation to no disclose this information. Keep in mind, the 8HP70 is NOT cheap. FCA does not want to replace it. My father had a 5 series with the exact same transmission. Similar temps. Dollars to donuts your transmission will outlast your truck. service it and enjoy it.

RedSRT4Me · Jul 13, 2018

shaggn85 said:
ZF is likely under contractual obligation to no disclose this information. Keep in mind, the 8HP70 is NOT cheap. FCA does not want to replace it. My father had a 5 series with the exact same transmission. Similar temps. Dollars to donuts your transmission will outlast your truck. service it and enjoy it.

Agreed. No sense in modifying this part if it isn't broken. They engineered this tranny to run like this.

Wild one · Jul 13, 2018

I'm pretty sure the A8 cars like the Challenger/Charger don't have the heater. I know my wifes 2016 Challenger takes way longer for the transmission to get up to temp then my truck does.

guru1ofatl · Jul 13, 2018

The loading on a truck transmission is expected to be higher than that of a car so differences in thermal management between the two are to be expected. Having the ability to rapidly bring the fluid up to temperature during cold start and operation can have benefits for both the fluid as well as the system.

Wild one · Jul 13, 2018

guru1ofatl said:
The loading on a truck transmission is expected to be higher than that of a car so differences in thermal management between the two are to be expected. Having the ability to rapidly bring the fluid up to temperature during cold start and operation can have benefits for both the fluid as well as the system.

By your anology the trucks shouldn't have the heater,and the cars should. The heater is there strictly to bring coporate milege numbers up on the trucks,if Ram didn't have to meet the mininium standards,i highly doubt you'd see the heater assembly on the trucks either. But even if you bypass the heater,you still have to deal with the thermostat in the assembly itself.The heater is easily bypassed by just running the coolant hoses together,which would in essence give you the same system as the cars have. I just put a brand new ZF in my truck and had the heater assembly off the transmission to flush ,and you can bypass the coolant hoses easily,but i'm not sure how you'd go about jamming the thermostat open to where it'd flow full time ,it's down in the bottom passage of the heater assembly.

RedSRT4Me · Jul 13, 2018

Wild one said:
By your anology the trucks shouldn't have the heater,and the cars should. The heater is there strictly to bring coporate milege numbers up on the trucks,if Ram didn't have to meet the mininium standards,i highly doubt you'd see the heater assembly on the trucks either. But even if you bypass the heater,you still have to deal with the thermostat in the assembly itself.The heater is easily bypassed by just running the coolant hoses together,which would in essence give you the same system as the cars have. I just put a brand new ZF in my truck and had the heater assembly off the transmission to flush ,and you can bypass the coolant hoses easily,but i'm not sure how you'd go about jamming the thermostat open to where it'd flow full time ,it's down in the bottom passage of the heater assembly.

The big question would be why??? What benefits are seen going through this trouble?

Tach_tech · Jul 13, 2018

Thermostats in transmissions are nothing new. Ram has been using them on the trucks for quite awhile, and not just the 8 speeds. The fluid works best at certain temps, how much of it is for fuel mileage and how much is for proper operation is hard to say. However a transmission with ambient temp fluid is not going to provide the best shift characteristics, especially the 8 speed.

For the average daily driver and truck with some mods I really fail to see how much benefit if any, removing the heater assembly is going to do. I would advise against trying to remove the thermostat, especially if you’re driving your truck in cold temperatures.

Wild one · Jul 13, 2018

Tach_tech said:
Thermostats in transmissions are nothing new. Ram has been using them on the trucks for quite awhile, and not just the 8 speeds. The fluid works best at certain temps, how much of it is for fuel mileage and how much is for proper operation is hard to say. However a transmission with ambient temp fluid is not going to provide the best shift characteristics, especially the 8 speed.

For the average daily driver and truck with some mods I really fail to see how much benefit if any, removing the heater assembly is going to do. I would advise against trying to remove the thermostat, especially if you’re driving your truck in cold temperatures.

The fluid is like water Dan,i'm not sure you'd see much difference in the tranny's operation whether it's at 160F or 185F,lol. I was tempted to bypass the coolant heater when I replaced my tranny,but then decided I wanted to keep some of my high dollar new trannies warrenty,lol.In Canada or the northern States where it gets cold,i think the heater does a good job,but where it doesn't get cold,i'm not so sure the heater is needed.The cars drive year round up here with-out the heater,and it doesn't seem to affect their cold weather operation,so you probably could get away with bypassing the coolant heater up here to,it'll still have the thermostat to regulate the temp after warm-up. My way of thinking is,if the 5.7 and 6.4 cars don't use the coolant heater,the trucks can probably get away with-out it to. The transmission will still get to operating temp as the thermostat is still in place,it'll just take longer to get there. I'm not advocating removing the coolant heater,all i'm saying is I think it can be bypassed,with no real affects to the trucks operation or the transmissions life expectancy.

atom13 · Jul 13, 2018

This entire endeavor reminds me of diy home owners/renovators who demolished walls and questions why architects are so dumb to place a wall where one isn't needed. They get rid of the wall and pat themselves on the back because the house doesn't automatically collapse. Its true that it may take years or tens of years before the house begins to resettle. But the damage is done either way.

Transmissions old to new designs works on fluid dynamics. That's why you only are able to check the correct amount after its warmed and the car is in neutral and idling. Most transmissions that I have owned states the best temperature to run is 180-200 degrees. Ford, chevy, honda Toyota. Most mechanics will tell you it's better to not idle and warm the engine slowly. Its best to start and drive slowly to bring Temps up faster. The most wear an engine or trans sees is during cold startup and cold Temps. This is also why they make block heaters for colder climates.

One reason I chose to buy my ram is because of the technology and fore thought that's been engineered into the truck verses the other manufacturers. Theirs alot of European influence in our trucks. I've noticed that my truck comes up to temp faster than my wife's previous Toyota Venza; also a v6. I assume that the grill shutters remain closed to speed up warmup time. If you follow ecomodders or hypermilers they have been trying to incorporate active shutters for a very long time.

I think mpg and Cafe standards are the new whipping boy. Yes they really are crappy for diesels. But look at how little progress was made before the standards. It wasn't because the technology wasn't available it was because they didn't have to give us a truck that can deliver decent mpg. My pentastar gets better mileage than my wife's 2016 honda odyssey. I didn't even think that was possible.

Tl;Dr. Made you look.

Wild one · Jul 13, 2018

atom13 said:
This entire endeavor reminds me of diy home owners/renovators who demolished walls and questions why architects are so dumb to place a wall where one isn't needed. They get rid of the wall and pat themselves on the back because the house doesn't automatically collapse. Its true that it may take years or tens of years before the house begins to resettle. But the damage is done either way.

Transmissions old to new designs works on fluid dynamics. That's why you only are able to check the correct amount after its warmed and the car is in neutral and idling. Most transmissions that I have owned states the best temperature to run is 180-200 degrees. Ford, chevy, honda Toyota. Most mechanics will tell you it's better to not idle and warm the engine slowly. Its best to start and drive slowly to bring Temps up faster. The most wear an engine or trans sees is during cold startup and cold Temps. This is also why they make block heaters for colder climates.

One reason I chose to buy my ram is because of the technology and fore thought that's been engineered into the truck verses the other manufacturers. Theirs alot of European influence in our trucks. I've noticed that my truck comes up to temp faster than my wife's previous Toyota Venza; also a v6. I assume that the grill shutters remain closed to speed up warmup time. If you follow ecomodders or hypermilers they have been trying to incorporate active shutters for a very long time.

I think mpg and Cafe standards are the new whipping boy. Yes they really are crappy for diesels. But look at how little progress was made before the standards. It wasn't because the technology wasn't available it was because they didn't have to give us a truck that can deliver decent mpg. My pentastar gets better mileage than my wife's 2016 honda odyssey. I didn't even think that was possible.

Tl;Dr. Made you look.

Better do some reading,you check the 8 speeds fluid level at 85F to at most 122F,long ways off warmed up to operating temp. I think Paramount sells a 160 thermostat for the cars,and i'm guessing they know more about the transmission,then all the so-called experts on this thread,lol. If they recommend a 160 tranny thermostat for their high horsepower 8 speeds,they're probably more on the ball about the tranny then the DIY'ers on here,lol.

Wild one · Jul 13, 2018

http://www.paramountperformanceprod...rmostatic-valve-by-paramount-performance.aspx

Wild one · Jul 13, 2018

Imagine the benefit of leaving the starting line at 160F compared to 195F! This will help your transmission fluid life. It is not sold separately, only available with the Paramount A8.

atom13 · Jul 13, 2018

Wild one said:
Imagine the benefit of leaving the starting line at 160F compared to 195F! This will help your transmission fluid life. It is not sold separately, only available with the Paramount A8.

I guess we found a definitive answer guys.

8HP70 Transmission Heater/Thermal Bypass Valve

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