First of all, don't say that I don't know what I'm talking about. I know quite a bit about automatic transmissions. I've rebuilt my share of them over the years, and I completely understand how they work. I also take pride in my profession as an auto technician, and I don't mislead people or spout off things that I don't understand. If I tell you how something works, or why something works, then I'm telling you from my own understanding of it, not just rehashing something that I heard somewhere else. If you want to disagree with something I have to say, that's fine, but don't try to discredit me as "obviously knowing nothing about transmissions". Instead you should explain why you think I am incorrect about something. You haven't done that, you are just repeating the things you have heard from a bunch of other people. You have told him "just use what it says in your owners manual", and talked about "incompatible fluids" and "shocking the transmission", both of which sound to me like the things people are usually told by a shop that doesn't want to do a trans service on a trans that is already slipping. Maybe I'm wrong and you know more than I do about the design of transmissions and fluids. Maybe you know even more than the engineer who designed the transmission, but you haven't come off that way, and the impression I got is that you are repeating as fact things you have heard other people say. Seeing that, I feel an obligation to inform this person who is seeking advice exactly what I think, and exactly why I think it. If you think I'm wrong, and you want to help him, you should do the same and explain to him the benefits of doing it your way and precisely why I am wrong. What you should not do is announce that I don't know anything about transmissions. First, that simply isn't true, and second, it doesn't give understanding of anything to this person who asked a question and presumably want to understand why the answer is what it is. Now that that's out of the way...
I have never heard anywhere before anything from ford saying you can't mix merc5 and merc3. If that is the case, then why on every bottle of merc5 does it say that it can be used as a replacement for merc3? Why does the ford TSB say that merc5 is the new replacement for merc3? I think what you are thinking of is the old typeF fluid, which was not compatible with the dexron/mercon, and would swell all the seals in the trans and cause it to fail. That doesn't happen mixing merc5 and merc3. In fact, the only difference between the 2 fluids is that 5 is a synthetic version of 3, and it has some additional friction additives.
Also, like I said, if there is no damage to the trans at this point, then changing all the fluid sooner will be a good thing. The only way changing all the fluid would be a bad thing is if the trans is already damaged from the old fluid, and if that's the case, changing all the fluid will only expedite the inevitable. There isn't going to be any "shocking the transmission" by changing the fluid. Either the clutches are wiped out and the friction material floating around in the fluid is keeping the car moving, or the clutches aren't worn out in which case putting in all fresh fluid, with the proper additives for those friction materials and that transmission, could only help. If the clutches are worn out, then slowly changing the fluid isn't going to help you because once all the old fluid is eventually out, your trans will be just as bad as it would have been if you drained all the fluid out in the first place. It might actually be worse because for that 6 months while you had half the new fluid and half the old fluid, all the additives and friction modifiers in the old fluid would have been used up already, which could cause the friction discs in the trans to wear out faster.
This concept of changing half the fluid exists for one reason and one reason only, and that is that it isn't convenient on most cars to change all the fluid. 4R70s have a drain plug in the torque converter, but most transmissions don't. In a case like that, people don't want to do work twice, so they either drop the pan and change the filter with the 4 or so quarts that come out that way, or they leave the pan and filter alone and just do a flush. The best way to do it is to change the filter and all the fluid. It is no different than doing an oil change. As the oil goes through the engine, the additives break down and the oil carries dirt and other particles through the filter. If you change the oil and don't change the filter, the new oil will pick up all that filth that is stuck in the filter. If you change the filter and don't change the oil, you won't have the proper additives in there, and the new filter will get clogged up faster because the dirty old oil is going through it right away instead of clean new oil. The transmission fluid and filter are no different. Additives in trans fluid break down. As they do so, the fluid becomes less able to prevent wear in the trans. The fluid also gets "dirty" with the friction material that wears off the clutches. If you change the filter and 1/3 of the fluid, then you still have all that fluid without the proper additives, and the new filter is going to get dirty much faster because the particles being carried through the dirty fluid will get caught in it instead of starting off with all clean new fresh fluid. Similarly, if you do a flush and change all the fluid, but you leave the old filter, all that new fluid is going to get contaminated when it goes through the old dirty filter. That's why the right way is to change all the fluid and the filter at the same time.
nsjuice, if you have any more questions about this, feel free to PM me.
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