Transmission fluid: change?

[Goldstar]

I got my ‘72 Mark about 6 months ago and don’t know much of it’s history other than the previous 2 female owners didn’t drive it much, just 36k miles, so it must have sat for much of its life. Transmission works fine and I initially thought it best to at least change the fluid. I find, however, some feel I should leave it alone or risk loosening up junk that could harm it. I’m still leaning hard to replacing the fluid but thought I’d run it by you guys first.
Also, there is a small leak from the pan which hopefully could be addressed at the same time. What do you think?
Peter

[TonyC]

Well, Peter, if you have spotted a leak and have confirmed its location to be at the pan, around the gasket, then that would be a good reason to replace the old fluid, as the fluid needs to come out anyway when you remove the pan. If you want to really double-down on fluid change, then you can remove the drain plug from the torque converter and drain that as well. But, when you acquire new fluid, it is imperative that you get Type F. Do not use any Dexron-based fluid; mixing the two formulas (and there will be mixture just from the residue left behind) will eventually destroy your transmission. Since transmissions are the worst mechanical problem one can have in a car, you don't want to let that happen.

There is one thing you need to keep in mind about the fluid used in your car's transmission. Type F was formulated to be used indefinitely inside a transmission; it did not require nearly so many routine changes as contemporary Dexron formulas did. Therefore, when you do change the fluid, don't expect to have to do it again any time soon. It's very likely that, since the car was not driven heavily by its previous owners, the transmission hasn't gotten a lot of wear from use. On the other hand, too little use over too much time can be equally as destructive as too much wear over too little time; that's the second law of thermodynamics for you. I say the leak is a good reason to change the fluid, as an incidental service while repairing the leak.

---Tony

[thegermanfan]

In case you are not experienced with these transmissions, I would strongly suggest getting a good quality transmission gasket. summitracing.com has a "lubelocker" gasket which is pricey, but will provide a troublefree installation without any fluid seeping afterwards. I was very satisfied with this pan gasket. Mechanics with more experience than I have, will deem such an expense overkill, but for the novice this is the way to go. The OEM cork/rubber gaskets easily leak, if the pan bolts are tightened too strong or too weak. The oil pan itself can be distorted with relative ease, leading to neverending oil wetness on the pan. There are replacement pans with thicker gauge steel on the market. These thicker oil pans can also be used, but the steel reinforced pan gasket is really the deciding factor.
And of course only Type F fluid can be used.

[Goldstar]

Thanks for the replies. I’ll change the fluid and get the better gasket.
Peter

[action]

Drain the torque converter too.
The torque converter will hold 10 12 quarts

Then drain the transmission fluid every 36,000 miles, alon with coolant drain and refill and wheel bearing repack.

Action

[Goldstar]

My original owners manual says 12.7 quarts including the converter, I’m going with that.

[TonyC]

I doubt you'll need to change the fluid that often, unless the transmission has a malfunction in it (there may be potential for one, a design flaw of the C-6s that I was told about 8 years ago). Like I said, Type F in a Ford transmission was formulated to last a very long time, unlike the Dexron formulas used back then. I'd say 50,000 miles would be a good minimum interval; just remember to check it on a regular basis, for drops in level or for changes of color. If the latter occurs, if it looks to be turning brown, then do a change at a shorter interval.

---Tony