There is no leak under my car. My father in law said something about a modulator valve, but he does not know if my 91 honda accord has one.

It is a 1991 honda accord Auto trans

 

It may be possible to lose some transmission fluid into the cooling system when the fluid passes through the trans ATF cooling loop in the radiator. Check reservoir for any evidence of red ATF.

Suggest having someone w/ a lift perform a close visual inspection of underbody and trans for evidence of leaks.

How much fluid loss have your experienced?

Honda's don't use a vacuum reference modulator valve which may be what your FIL had heard about. These would rupture internally and allow ATF to be pulled into engine.

good luck

 

I have lost enough to cause the gears to slip.

 

"Gear slippage" could be loss of clutch pack holding or torque converter slippage. Both would require loss of several qts probably. Did you measure that much loss on dip stick?

Also slippage can occur for reasons other than loss of ATF. Did you assume that slippage must be due to loss of fluid?

good luck

 

It is losing enough trans fluid to not show on the stick at all. The gears slipping has happened before (How we knew to check things out), and we checked levels to find trans fluid dangerously low. Having a kid required us to put trans fluid in at a time to go to town and get WIC. Ran smooth as butter after that.

I am pretty sure this started happening after we got the AC up and running. So maybe radiator?