Starting a trans repair, would like your help!

[Halfcent]

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!

[MarkH]

Hi forum.

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.

If your '88 is anything like my 1st gen (1985) van, the overdrive should work the opposite of what you are experiencing. The overdrive should NOT engage when the engine is cold. There is a temperature switch on the engine that will only send power to the overdrive solenoid when the engine is warm. If you are getting overdrive when the engine is cold, it sounds like the circuit has been messed with. I was having a problem with my overdrive where it wouldn't engage at all. All the components tested fine as per the factory service manual. The solenoid had 12v when it was supposed to. I even removed the solenoid and bench tested it with a battery and it opened and closed. I ended up replacing the solenoid with a blanking plate available online which just allows overdrive to be available all the time. Now it works fine. I assume the solenoid passages weren't allowing enough fluid through to build up enough pressure to engage the overdrive. The one I used was like this one:https://www.amazon.ca/MTC-VM967-1239...91248818&psc=1 I don't know if the one in the link is the exact one. I just found it with a quick Google search, but it looks identical to the one I bought. It could solve your problem for cheap without having to get into the transmission. If I were you though I'd still be curious as to why your overdrive system is behaving the opposite to what it's supposed to. Some people unplug the temperature switch on the engine to make the overdrive engage when cold. Maybe that's what someone did to yours.

[MarkH]

Here are some links to older threads with similar problems. They have some wiring diagrams showing the various components in the circuit. Basically, if any of them are not functioning properly or functioning intermittently (loose connection/corroded wiring/etc.), the overdrive will have problems. You could test them all individually or do what I did and just replace the solenoid with the blanking plate, which removes all the other components out of the equation.https://www.toyotavantech.com/forum/...ight=overdrive
https://www.toyotavantech.com/forum/...ight=overdrive
https://www.toyotavantech.com/forum/...ight=overdrive
There are many more threads, but basically they result in replacing the overdrive solenoid (easy to do since it's on the outside of the transmission with 2 bolts) with a volvo one that is the same or replacing it with the volvo blanking plate like I did. Either way, you shouldn't have to open up the transmission or replace the valve body gasket or get a transmission shop involved.

[Halfcent]

Thanks. This is perfect. I will start with disconnecting the temp sensor. Hopefully it's that simple. I'll report back.

[Halfcent]

I started with the troubleshooting step of disconnecting the temperature sensor lead. This did nothing but render my temperature gauge on the cluster inoperative. OD still behaved as before; worked when cold, not when hot.

When you say disconnect the OD thermo switch are you talking about the connector I have circled in red here?

[Burntboot]

The one circled in Red is what you need to disconnect, not the sender for the gauge.

[Halfcent]

Okay, that seems to have fixed it.

So a bad, internally sorted temperature switch.

What's the consensus here? Just leave it unplugged? Is this switch even available anymore for replacement?