Transmission thermostat open for flush?

[jacobsen]

I want to do a transmission fluid flush on a 2014 A6 TDI Quattro with the ZF8HP70. With VAG COM software is it possible to manually open the transmission thermostat to the cooler in order to drain the cooler return line with the motor running?

(On the Ram trucks with the 8HP70 the transmission thermostat makes the fluid flush difficult because it only opens when the fluid reaches a certain temperature and it doesn't open full bore, it seems variable.)

[eurospek]

How many miles on your fluid?

[jacobsen]

110,000 miles

[Valpo A7]

Normally its just a dump, replace the filter and refill. Not many actually flush the trans.

[squirrels]

Never,
EVER,
ever,
EVER
Flush an automatic transmission!!!!
EVER!!!!!
ON ANY VEHICLE!!!
This has NEVER been an OEM recommended service procedure!!!!!
Back when I did automatic transmissions, at least 25% of the lower mileage rebuilds I did had been flushed in the past few thousand miles!!!!

Think of your automatic as a delicate ecosystem. It has a fluid medium around it, which soaks into all of the components to some extent - The clutches, the teflon seals, the rubber seals, etc.
When you do the OEM RECOMMENDED fluid exchange at the recommended interval, there is a certain amount of "shock" to this system as you exchange older fluid which has broken down to some extent, an advanced fluid with detergent packages, friction modifiers, etc in it, with new fluid. This is the "intended" amount of shock, and within the factory service interval is statistically very unlikely to cause any trouble, ever. The old/worn compounds are leeched out of the porous transmission components over time and replaced with the newer fluid and additive packages.
Now consider when you do the fluid change at an extended interval - You are over the recommended fluid exchange interval by 1.5x... This is more of a shock to the system, however you are still only exchanging a portion of the fluid (which varies by transmission design) - More of a shock, but as long as you aren't way over this interval, or the transmission has been abused, you are usually still just fine.

Now consider the "flush" - Let's say you do it at the OEM recommended "fluid change" interval - Now you are exchanging nearly 100% of the fluid in the transmission. The first time you heat that thing up, you now have a *significant* deviation (from old to new) of your fluid composition, if you are flushing at "OEM recommended fluid change interval" I would not hesitate to guess that this "flush shock" is easily equivalent to the shock from exceeding your fluid change interval by 2-3x, or more!!

This is why many seasoned transmission techs may tell you something that seems to defy logic, but is recommended for the reasons above - If you do not change your fluid at the OEM recommended interval, and instead end up at more than 2x that interval, you leave that transmission alone and never, ever change the fluid in that transmission again. This is especially true for transmissions with synthetic ATF. I would recommend the same - As a transmission rebuilder/tech, I always asked the service history of the transmission, and if someone wanted a fluid/filter change on something which was between 2-3x service interval, I refused to do it. The risk of subsequent transmission failure was too high. The owner was better off caring for the car, not beating on it, keeping the fluid topped off and driving it until problems occurred. On transmissions with synthetic fluid, this often meant they were still fine until 200k miles. I had a family member with a 4T65E in a GM minivan, they changed the fluid at 60k, and then forgot about it until some time around 150k. I recommended that they leave it alone - The 4T65E of this year had some common issues anyway that were bound to pop up sooner or later. Well, left alone and not abused other than the total lack of fluid changes, that thing lasted until 300k when the entire vehicle was scrapped due to rust.

NEVER flush an automatic transmission!!! This is a gimmick invented by companies who sell to these quick lube places to generate more sales margin on useless services!!! Don't do it!!!!

[jacobsen]

I'm familiar with transmissions and fluids. So no one knows if vcds or vagcom can open the thermostat? I assume it can but I'll have to get it hot driving it to get the thermostat to open so I can drain the cooler return line.

[fastboatster]

somebody can correct me if I'm wrong, but on these zf 8hp transmissions the trans fluid freely circulates through the heat exchanger. It's the flow of the coolant which is regulated. I don't think you need to open the thermostat to get the trans fluid to drain from it. If you really worried though, why won't you just disconnect the trans fluid lines from the heat exchanger and let them all drain?

[sepheroth86]

Never,
EVER,
ever,
EVER
Flush an automatic transmission!!!!
EVER!!!!!
ON ANY VEHICLE!!!
This has NEVER been an OEM recommended service procedure!!!!!
Back when I did automatic transmissions, at least 25% of the lower mileage rebuilds I did had been flushed in the past few thousand miles!!!!

Think of your automatic as a delicate ecosystem. It has a fluid medium around it, which soaks into all of the components to some extent - The clutches, the teflon seals, the rubber seals, etc.
When you do the OEM RECOMMENDED fluid exchange at the recommended interval, there is a certain amount of "shock" to this system as you exchange older fluid which has broken down to some extent, an advanced fluid with detergent packages, friction modifiers, etc in it, with new fluid. This is the "intended" amount of shock, and within the factory service interval is statistically very unlikely to cause any trouble, ever. The old/worn compounds are leeched out of the porous transmission components over time and replaced with the newer fluid and additive packages.
Now consider when you do the fluid change at an extended interval - You are over the recommended fluid exchange interval by 1.5x... This is more of a shock to the system, however you are still only exchanging a portion of the fluid (which varies by transmission design) - More of a shock, but as long as you aren't way over this interval, or the transmission has been abused, you are usually still just fine.

Now consider the "flush" - Let's say you do it at the OEM recommended "fluid change" interval - Now you are exchanging nearly 100% of the fluid in the transmission. The first time you heat that thing up, you now have a *significant* deviation (from old to new) of your fluid composition, if you are flushing at "OEM recommended fluid change interval" I would not hesitate to guess that this "flush shock" is easily equivalent to the shock from exceeding your fluid change interval by 2-3x, or more!!

This is why many seasoned transmission techs may tell you something that seems to defy logic, but is recommended for the reasons above - If you do not change your fluid at the OEM recommended interval, and instead end up at more than 2x that interval, you leave that transmission alone and never, ever change the fluid in that transmission again. This is especially true for transmissions with synthetic ATF. I would recommend the same - As a transmission rebuilder/tech, I always asked the service history of the transmission, and if someone wanted a fluid/filter change on something which was between 2-3x service interval, I refused to do it. The risk of subsequent transmission failure was too high. The owner was better off caring for the car, not beating on it, keeping the fluid topped off and driving it until problems occurred. On transmissions with synthetic fluid, this often meant they were still fine until 200k miles. I had a family member with a 4T65E in a GM minivan, they changed the fluid at 60k, and then forgot about it until some time around 150k. I recommended that they leave it alone - The 4T65E of this year had some common issues anyway that were bound to pop up sooner or later. Well, left alone and not abused other than the total lack of fluid changes, that thing lasted until 300k when the entire vehicle was scrapped due to rust.

NEVER flush an automatic transmission!!! This is a gimmick invented by companies who sell to these quick lube places to generate more sales margin on useless services!!! Don't do it!!!!

Getting any 4T65E to last is some sort of a miracle. 😆

Remember when someone at GM thought it was a good idea to mount an LS transverse and strap it to a 4T65E? Good times!

I have always advised the same as you had, either drain and fill as recommended, or never change it.

[jacobsen]

somebody can correct me if I'm wrong, but on these zf 8hp transmissions the trans fluid freely circulates through the heat exchanger. It's the flow of the coolant which is regulated. I don't think you need to open the thermostat to get the trans fluid to drain from it. If you really worried though, why won't you just disconnect the trans fluid lines from the heat exchanger and let them all drain?

This transmission will not circulate the fluid through the cooler unless the thermostat is open. the fluid must be hot enough to trigger the thermostat to open for the fluid to flow.

[fastboatster]

This transmission will not circulate the fluid through the cooler unless the thermostat is open. the fluid must be hot enough to trigger the thermostat to open for the fluid to flow.

I only see a thermostat solenoid in the coolant heat exchanger circuit in the parts diagram. There’re clutch lube/cooling solenoids in the valve body as you probably know this better than me but I’m not aware of any zf 8hp variants with fluid thermostat solenoid/valve built in the valve body. Maybe I’m looking at the gas a6 transmission diagram. From what I could gather, trans fluid temp is regulated by managing coolant flow to the heat exchanger, not transmission fluid. And again, just disconnect the trans hoses, you can probably spend a few bucks on the new orings to make sure everything drains out.
Or perhaps just do what zf suggests:
https://aftermarket.zf.com/remotemed...p-50130-en.pdf

[squirrels]

Getting any 4T65E to last is some sort of a miracle. 😆

Remember when someone at GM thought it was a good idea to mount and LS transverse and strap it to a 4T65E? Good times!

I have always advised the same as you had, either drain and fill as recommended, or never change it.

It gets better than that! Volvo decided to use it in the XC90 series, and it was so bad that the resulting lawsuit forced them to cover transmission replacement under some extended warranty campaign! They actually stuck a PTO on the end where the extension housing/findal drive cover normally goes and turned it into an AWD transmission

[sepheroth86]

It gets better than that! Volvo decided to use it in the XC90 series, and it was so bad that the resulting lawsuit forced them to cover transmission replacement under some extended warranty campaign! They actually stuck a PTO on the end where the extension housing/findal drive cover normally goes and turned it into an AWD transmission

I had no idea about the Volvo thing. Ugh. The 4T65E is an awful gearbox.

ZZP can make you a stout unit though, to the tune of $5 or $6k.

[Alabama]

I have always advised the same as you had, either drain and fill as recommended, or never change it.

Naive question but just to be sure: drain and fill as recommended is preferred to never change it?

[sepheroth86]

Naive question but just to be sure: drain and fill as recommended is preferred to never change it?

For example: you purchase a used car with 150k on it and an unknown service history (i/e trans fluid has not been changed). Do not change the fluid, simply add as needed.

If you know it has been changed regularly, as recommended, continue to do so.

[jacobsen]

I only see a thermostat solenoid in the coolant heat exchanger circuit in the parts diagram. ThereÂ’re clutch lube/cooling solenoids in the valve body as you probably know this better than me but IÂ’m not aware of any zf 8hp variants with fluid thermostat solenoid/valve built in the valve body. Maybe IÂ’m looking at the gas a6 transmission diagram. From what I could gather, trans fluid temp is regulated by managing coolant flow to the heat exchanger, not transmission fluid. And again, just disconnect the trans hoses, you can probably spend a few bucks on the new orings to make sure everything drains out.
Or perhaps just do what zf suggests:
https://aftermarket.zf.com/remotemed...p-50130-en.pdf

There is an electronic solenoid activated thermostat in the side of the transmission that stops fluid flowing out or back into the transmission. So you must have the fluid up to temperature and you must fill by the transmission fill plug not the return line from the cooler. I'm sure the software can open it but I dont have it so I do it manually. Old school systems circulateed constantly without thermostatic control.

[woolb2]

The selanoid circulates coolant not transmission fluid.

Sent from my SM-G998U1 using Audizine Forum mobile app

[jacobsen]

The selanoid circulates coolant not transmission fluid.

Sent from my SM-G998U1 using Audizine Forum mobile app

No, that's very incorrect.

[woolb2]

N509 valve Screenshot_20220319-160931_Gallery.jpgScreenshot_20220319-160948_Gallery.jpg

Sent from my SM-G998U1 using Audizine Forum mobile app

[Valpo A7]

Woobl2 is correct. The N509 valve is outside of the transmission and is nothing but a coolant flow control valve. It shuts off coolant flow to the transmission oil cooler. It’s actually not really on the side of the transmission line the diagram shows. Some books call it a heater bypass valve.

[jacobsen]

N509 valve Screenshot_20220319-160931_Gallery.jpgScreenshot_20220319-160948_Gallery.jpg

Sent from my SM-G998U1 using Audizine Forum mobile app

It is my fault for assuming then. On the Ram EcoDiesel that has the identical transmission there is a thermostatically controlled valve built on to the side of the transmission that controls the flow of fluid to the cooler and back to the transmission. If the coolant is not warm enough it will not open and allow coolant to flow through the lines up to the cooler or back to the transmission. I assumed it was the same setup and a part specific to the transmission not the vehicle it's installed on.

If this does not exist on the Audi and the coolant simply flows at all times then swapping the fluid should be easy. I haven't looked at it yet but I will soon enough.

[jacobsen]

I see from the picture diagram that in the Audi coolant is brought to the transmission where the cooler is attached to the transmission. On the truck transmission fluid is brought to the radiator. So they're opposites.

[Alabama]

Woobl2 is correct. The N509 valve is outside of the transmission and is nothing but a coolant flow control valve. It shuts off coolant flow to the transmission oil cooler. It’s actually not really on the side of the transmission line the diagram shows. Some books call it a heater bypass valve.

So Post #7 below was correct about how cooling is controlled:

somebody can correct me if I'm wrong, but on these zf 8hp transmissions the trans fluid freely circulates through the heat exchanger. It's the flow of the coolant which is regulated. I don't think you need to open the thermostat to get the trans fluid to drain from it. If you really worried though, why won't you just disconnect the trans fluid lines from the heat exchanger and let them all drain?

[Valpo A7]

N509 location
N509 LOCATION.jpg

Trans cooler
TRANS COOLER LOCATION.jpg


Pictures of the N509 and trans cooler locations on the Audi 3.0t. NO you don't have to remove the engine to access the N509. LOL

I had yanked the engine for timing chains and looked at my old pics to help show these locations on the Audi A6/7