89 F150 transmission fluid low / no leaks
 

1989 F150 Lariat XLT 5.0 2x4 here. I've had to top off the transmission fluid a few times so I took it to the shop assuming I had a leak. No leak could be found. Not at the pan, or behind the inspection plate, or any of the seals. They said it was good. So where is it going???

Change your modulator...about 25 bucks at any auto parts store.

This is the AOD not the C6 so there is no vacuum modulator.

Your truck has a transmission cooler mounted in the bottom tank of the radiator. Sometimes the cooler leaks into the cooling system. Check the coolant for signs of contamination.

I flushed the coolant maybe 2 years ago and it was fine. And the shop said they drained a little coolant and it was fine. But I have noticed the temperature gauge will spike every so slightly every now and again and then go back to normal. I wonder if this could be a sign? I'm also going to drain the coolant myself this weekend when I do an oil change. The only thing is I swear I've seen red fluid under the truck but they couldn't find a leak after 2 days of looking at it. This is very odd.

I've got cardboard under the truck for now and I'm going to meticulously clean everything and get to the bottom of this leak.

Good plan.

So I've had cardboard under it and no leaks. But I'm still using transmission fluid. And the radiator coolant is bright green and looks fine. I'm stumped....