Probably a dumb ATF question

[FLFrank35]

I have a Ford 4R70W (auto). I thought it needed Dexron III/Mercon so that's what I put in. Then I learned the correct fluid is Mercon V. I also recently discovered my trans oil pan gasket is leaking and I need to replace it. Since I will be draining the Dexron III/Mercon, can I just replace it with Mercon V or do I need to flush the system?

Thanks,
Frank

[NAZ]

First off, how do you "flush the system" in an automatic trans. The friction plates have been soaked in whatever fluid they ran in and the torque converter holds a significant quantity of fluid that does not drain during an oil change. Simply filling with new fluid only dilutes what's left in the converter and what has soaked into the friction materials.

But I believe your situation is not doom and gloom as Dexron III and Mercon V are very close in their formulation. Some application charts claim they are compatible and can be substituted for one another. However, you should do your own research on this to satisfy yourself that this is indeed correct. For me, I'd likely continue using the Dexron III but also would not loose any sleep over draining and refilling with Mercon V. But it's not my transmission so due diligence on your part is required, don't just take my word for it or anyone else's that may respond. You're getting free advice with a money back guarantee which is worth exactly $zero.

[FLFrank35]

First off, how do you "flush the system" in an automatic trans. The friction plates have been soaked in whatever fluid they ran in and the torque converter holds a significant quantity of fluid that does not drain during an oil change. Simply filling with new fluid only dilutes what's left in the converter and what has soaked into the friction materials.

But I believe your situation is not doom and gloom as Dexron III and Mercon V are very close in their formulation. Some application charts claim they are compatible and can be substituted for one another. However, you should do your own research on this to satisfy yourself that this is indeed correct. For me, I'd likely continue using the Dexron III but also would not loose any sleep over draining and refilling with Mercon V. But it's not my transmission so due diligence on your part is required, don't just take my word for it or anyone else's that may respond. You're getting free advice with a money back guarantee which is worth exactly $zero.

Well, I don't know anything about flushing the system. In the articles I read, some people recommended a type of pressure flush to get all of the old fluid out but didn't specify manual or auto. And I don't know enough about transmissions to know whether you can/should flush an automatic or not, which is why I posted the question. My fluid isn't old since I've only just put it in and put, maybe, a mile or 2 on it. I have read that Dexron III/Mercon is acceptable but Mercon V is preferable, but I didn't know if mixing the two would cause any negative side-effects.

Thanks for the tip!
Frank

[RuffShod]

First off, how do you "flush the system" in an automatic trans. The friction plates have been soaked in whatever fluid they ran in and the torque converter holds a significant quantity of fluid that does not drain during an oil change. Simply filling with new fluid only dilutes what's left in the converter and what has soaked into the friction materials.

But I believe your situation is not doom and gloom as Dexron III and Mercon V are very close in their formulation. Some application charts claim they are compatible and can be substituted for one another. However, you should do your own research on this to satisfy yourself that this is indeed correct. For me, I'd likely continue using the Dexron III but also would not loose any sleep over draining and refilling with Mercon V. But it's not my transmission so due diligence on your part is required, don't just take my word for it or anyone else's that may respond. You're getting free advice with a money back guarantee which is worth exactly $zero.

What he said. Just rebuilt the same trans in my F-150. Flushing the system would be a trick! I wouldn't mix D3 with M5.