Transmission fluid change

1990 Supra comp with pcm 351 Ford.

Recently got fed up paying 120 an hour for minor maintenance. I’m not sure what is in the tranny, but last year my mechanic had charged $550 for impeller, plugs, wires, cap, points, trans fluid, oil change. He ended up telling me my transmission fluid was fine and my impeller as well and because things took longer than expected still charged me $570 without doing the impeller or the trans fluid. I took over this year and am using a marine filter for oil, 25-40 or 25-50 mercury semi synthetic marine oil, and I’m pretty sure I have 20w in the trans because the fluid is clear. It’s still clean, but I want to change it. I can’t find straight 20w anywhere and seeing as I’ve never done a trans fluid change before I don’t trust myself to get all the oil out to switch to atf. I use a mighty vac fluid transfer system for extraction. I’ve heard running the drive is bad out of water and I know I’d need several flushes. I’m either going to pay a shop to do the trans fluid switch or stick with an acceptable oil. I would prefer not to change the type of fluid that’s in there unless it poses a serious risk. I’ve got up around 800 hours on the boat. What is your take on the transmission fluid? Can you give me any tips of what not to do? Will anything substitute for the 20w? I can get 0w20 synthetic or sae 30

Marine mechanics are very expensive and rarely do work that up to par with the bill. Your best bet is to educate yourself and do your own maintenance. Your Comp is one of the simplest boat to work on. I would suck out as much trans fluid as you can and replace it with Mercon/Dexron III run it with a hose on the trailer and replace it again you should be fine. For engine oil I use Mobil 1 15w50 with a Motorcraft FL1A filter they can be found at Walmart under 30.00. Where are you located?

Thanks for the reply! I was using rotella 15-40 woth fl1a motor craft filter. Last winter freeze came before my mechanic had time so i winterized myself and he told me I used the wrong oil and filter, that they had to be specifically marine oils so that’s why I ended up doing this. He also changed oil when winterizing and first thing in the spring. Seems like oil change would make more sense in the spring, why put fresh oil in just for the boat to sit all winter? As far as changing the trans fluid, how do I know I got my suction hose to the bottom? Is there anything I can screw up if I twist the hose wrong and it goes somewhere other than where it’s supposed to? I live in Angels camp, ca and spend time on new melones and don pedro lakes. Thank you for your help!

Changing the oil before layup at the end of the season is best as the oil is fresh. As you run the engine, the oil builds acidity and that then sits all winter eating at the engine. I would much rather layup with fresh clean oil for an extended sit.

+1 on what jetlink said. You need to suck the trans fluid out the dipstick hole with a 1/4'' plastic hose. Be careful some have gotten the hose stuck in the trans . I have been doing this for 30 years never had a problem. I think you need a new mechanic What he said about the oil and filter is BS. Why would you change the oil at the end of the season and again at the start of the new season?? How did he determine the impeller was good unless he removed it from the pump and if he had it out why would it take any more time to just replace it?? Your boat is very easy to work on and the maintenance is not rocket science. Once you do it you will see how easy it really is.

The sucker hose is the easy way.

The correct way (by the manual) is to undo the 2 bolts at the bottom of the tranny, which will drop a small plate, seal, and allow you to pull the screen to clean and re-install. The down side is there is no room for a catch can between the tranny and bilge...you have to wrap a plastic bag around the tranny to catch the fluid. It's not quite 2 quarts total.