Hi forum.

I have a 1988 van, 2WD automatic. Here's the numbers:

VIN: JT4YR28V6U5065847
Model: YR29LV-PRBEA
C/tr/ A/tm: 8D7 LJ11 F282 A45D L “B”
Production date JUNE 1988
4Y-EC engine

I have begun to "lose" 4th gear, or overdrive. When the is cold, it works normally, shifting into OD at what seems an appropriate speed and load. However, once the powertrain reaches normal operating temperature, it will inappropriately downshift back to 3rd gear while your cruising at 65 MPH, sending the engine RPM through the roof.

This van is worthless money wise but useful to the family as a cargo hauler. I have maintained it for years, but I'm not a transmission guy. I replaced the fluid last in 2019. It does maybe 2000 miles a year.

I took it Aamco hoping they could possibly just replace the valve body gasket or something simple like that, but they want to pull it out and overhaul it, which for the money doesn't make sense.

So, I would like to try. My plan is to first try to find an actual data plate on the side of the thing that tells me the model/part numbers. Internet research has only gotten me so far. I found this:

Toyota part number : 350102823084
Aisin : 03-72L

I'm not even certain those are right. Once I have proper numbers, I want to order a valve body gasket. I want to try replacing that while also giving the valve body a cleaning.

So, my questions are...

Anybody know where to find the part number stamp on this unit?
Anybody happen to just know the part number right away?
Anybody got a theory about the problem? (losing OD when warm)
Anybody think replacing the valve body gasket is gonna do any good?
Anybody got a anything I'm not thinking of here?

I expect to actually tackle this job in the late fall, so I have some time.

Thanks!