I just bought a 1991 4Runner 4x4 with the wonderful 3.0 and automatic transmission. I have been fixing all of the problems the previous owner tried to repair. Anyways thats a whole other story. Problem is overdrive did not work unless it was 90+ outside and I was driving below 60 mph. So I did the following:
Flushed and replaced trans filter
Flushed cooling system
new thermostat
coolant temp sensor
electric fans (fan clutch went out and the fan blade was broken).
Doing all this I can now get the overdrive to work perfect for 2-4 miles then it kicks back into 3rd gear. I have to come to a complete stop and start again for it to work for another 2-4 miles. I am at a lost and getting tired of working on this. This is my first Toyota and has been a major pain so far. I am down to either transmission is bad or the overdrive solenoid is bad. Can any one please shed some light on this problem please.

 

did you went with a OEM thermostat from Toyota dealer?

 

Sounds like you have a temp sensor issue somewhere to me. Trans doesn't think it's warm enough to be in OD or thinks it's getting too warm now and kicking you out of gears to get you to slow down. Chrysler auto's will do that when they overheat, you'll lose OD, and if it stays too hot they'll lose 3rd too to try and get the temp back down.

 

No I did not use the OEM thermostat. I have adjusted the cooling fan to come on at operating temp. Also I have turned the fan off and set it at 160 degrees and the same thing happens. I have even tried a 195 degree thermostat and that did not work either. I don't think the engine or trans are running to hot. The temp gauge is not reading hot and the trans temp light has not come on.

 

it's sounds more likely that it is your thermostat buy oem. aftermarket thermostats suck and they are never to spec like oem

 

I went out and bought an OEM thermostat from the dealer and it did not help with the overdrive problem. On cold mornings it will never go into overdrive, but on warm afternoons it will go into overdrive for a while. When it kicks out of overdrive I can pull over and let it warm up some more and it will work again for a little while. So I now know it is some kind of cooling problem. The gauge will go down a little while I am driving and thats when it kicks out. I have replace the coolant temp sensor and that did not work either. I was told there is another temp sensor that will tell the transmission when the engine is at operating temp so it can go into overdrive. So from my understanding there is a
coolant temp sensor for the gauge
coolant temp switch for the computer
coolant temp sensor for the transmission
and all of these are next to each other. So which one is which. I have read a few threads but still can not figure it out. Is the sensor with 1 wire for letting the transmission know when it can go into overdrive? If so how does the sensor work? Or is there any way to bypass this and have the transmission go into overdrive anytime? Sorry for all the questions but I would like the overdrive to work because I'm only getting 9 MPG and gas is not cheap anymore.

 

by-pass the ECT with a switch just splice one of the wires that go to the ect extend it into the cab so you can have access to it so you can turn it on in the mornings and off when driving around your cel light will pop out but, as a last resort just go for it and do it

 

you may have a misadjusted throttle control cable.

i dont know if this is the issue like i had. i'm driving in the freeway, OD on and everything is fine.

then suddenly, is if i click off OD and it upshifts.

i brought it to the dealer, and explained the issues. he said there is a fine adjustment that has to be made.

now i dont know if that fixed it or not since i got a new block out of this service call due to the HG thing.

this is over a decade ago.

 

I had a similar problem. When it got to freezing or colder O/D wouldn't engage on the highway.
I installed a 195 degree thermostat and it now works.

 

IT'S FIXED!!!!! Kinda, I went back and started checking all the sensors again. The VAFM was bad, there was no connection between 5&7. This is for the air intake temp. So I replaced it and made a big difference on how it ran, but still problem with OD. Went back and tested the new coolant temp sensor and what do you know the new one is bad. Replaced it with a better one and now the transmission works perfectly except now the engine light is on. I read the code and it is code 22 for the coolant temp sensor. The light was never on before when it was not working and now everything is working fine and the light comes on. I can not figure out how to reset the codes. I took the EFI fuse out over night and the light is still one. I also unhooked both battery cables and still on. How do I get it to go off. I hope the new sensor is not bad.